Chapter 4 - Optical sensors:. Optical sensors Optical sensors are those sensors that detect electromagnetic radiation in the broad optical range – from

Chapter 4 - Optical sensors:Chapter 4 - Optical sensors:

Optical sensorsOptical sensors

Optical sensors are those sensors that detect electromagnetic radiation in the broad optical range – from far infrared to ultraviolet

Approximate range of wavelengths from 1mm (3x1011 Hz or far infrared) to 1 nm (3x1017 Hz or upper range of the ultraviolet range).

Direct methods of transduction from light to electrical quantities (photovoltaic or photoconducting sensors)

Indirect methods such as conversion first into temperature variation and then into electrical quantities (PIR sensors).

Optical sensors are those sensors that detect electromagnetic radiation in the broad optical range – from far infrared to ultraviolet

Approximate range of wavelengths from 1mm (3x1011 Hz or far infrared) to 1 nm (3x1017 Hz or upper range of the ultraviolet range).

Direct methods of transduction from light to electrical quantities (photovoltaic or photoconducting sensors)

Indirect methods such as conversion first into temperature variation and then into electrical quantities (PIR sensors).

Spectrum of “optical” radiationSpectrum of “optical” radiation Nomenclature:

Visible light Infrared radiation (not infrared “light”) Ultraviolet radiation (not UV “light”)

Ranges shown are approximate and somewhat arbitrary

Nomenclature: Visible light Infrared radiation (not infrared “light”) Ultraviolet radiation (not UV “light”)

Ranges shown are approximate and somewhat arbitrary

Infrared radiationInfrared radiation

Approximate spectrum 1mm (300 GHz) to 700nm (430 THz)

Meaning: below redNear infrared (closer to visible light)Far infrared (closer to microwaves) Invisible radiation, usually understood as

“thermal” radiation1nm=109m 1GHz=109 Hz, 1THz=1015 Hz

Approximate spectrum 1mm (300 GHz) to 700nm (430 THz)

Meaning: below redNear infrared (closer to visible light)Far infrared (closer to microwaves) Invisible radiation, usually understood as

“thermal” radiation1nm=109m 1GHz=109 Hz, 1THz=1015 Hz

Visible lightVisible light

Approximate spectrum 700nm (430 THz) to 400nm (750 THz)

Based on our eye’s responseFrom red (low frequency, long wavelength)To violet (high frequency, short wavelength)Our eye is most sensitive in the middle (green

to yellow)Optical sensors may cover the whole range,

may extend beyond it or may be narrower

Approximate spectrum 700nm (430 THz) to 400nm (750 THz)

Based on our eye’s responseFrom red (low frequency, long wavelength)To violet (high frequency, short wavelength)Our eye is most sensitive in the middle (green

to yellow)Optical sensors may cover the whole range,

may extend beyond it or may be narrower

Ultraviolet (UV) radiationUltraviolet (UV) radiation

Approximate spectrum 400nm (750 THz) to 400pm (300 PHz)

Meaning - above violetUnderstood as “penetrating” radiationOnly the lower end of the UV spectrum

is usually sensedExceptions: radiation sensors based on

ionization (chapter 9)

Approximate spectrum 400nm (750 THz) to 400pm (300 PHz)

Meaning - above violetUnderstood as “penetrating” radiationOnly the lower end of the UV spectrum

is usually sensedExceptions: radiation sensors based on

ionization (chapter 9)

A word on unitsA word on units

SI units include: meter, kg, second, ampere, candela, temperature kelvin and the mole

All other units are derived unitsCandela “is the luminous intensity, in a

given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiation intensity of 1/683 watt per steradian”

SI units include: meter, kg, second, ampere, candela, temperature kelvin and the mole

All other units are derived unitsCandela “is the luminous intensity, in a

given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiation intensity of 1/683 watt per steradian”

Units of luminosityUnits of luminosity

Units of illuminanceUnits of illuminance

MaterialsMaterials

Optical sensingOptical sensing

Based on two principles Thermal effects of radiation Quantum effects of radiation

Thermal effects: absorption of radiation of the medium through increased motion in atoms. This may release electrons (heating)

Quantum effects: photon interaction with the atoms and the resulting effects, including release of electrons.

Based on two principles Thermal effects of radiation Quantum effects of radiation

Thermal effects: absorption of radiation of the medium through increased motion in atoms. This may release electrons (heating)

Quantum effects: photon interaction with the atoms and the resulting effects, including release of electrons.

The photoelectric effectThe photoelectric effect

Planck’s equation:e=hf [ev]

h = 6.6262x10 [joule.second] (Planck’s constant)

f = frequencye = energy of a photon at radiation frequency f.

This is called the quantum of energy Higher for higher frequency Can be imparted to electrons as kinetic energy

Note: this energy is also called ionization energy and is used to distinguish between “dangerous” and “benign” radiation

Planck’s equation:e=hf [ev]

h = 6.6262x10 [joule.second] (Planck’s constant)

f = frequencye = energy of a photon at radiation frequency f.

This is called the quantum of energy Higher for higher frequency Can be imparted to electrons as kinetic energy

Note: this energy is also called ionization energy and is used to distinguish between “dangerous” and “benign” radiation


Photons collide with electrons at the surface of a material

The electrons acquire energy and this energy allows the electron to: Release themtselves from the surface of

the material by overcoming the work function of the substance.

Excess energy imparts the electrons kinetic energy.

Photons collide with electrons at the surface of a material

The electrons acquire energy and this energy allows the electron to: Release themtselves from the surface of

the material by overcoming the work function of the substance.

Excess energy imparts the electrons kinetic energy.


This theory was first postulated by Einstein in his photon theory (photoelectric effect) in 1905 (for which he received the Nobel Prize):

hf e0 = k

e0 is called the work function (energy required to leave the surface of the material)

k represents the maximum kinetic energy the electron may have outside the material. Energy is “quantized”

This theory was first postulated by Einstein in his photon theory (photoelectric effect) in 1905 (for which he received the Nobel Prize):

hf e0 = k

e0 is called the work function (energy required to leave the surface of the material)

k represents the maximum kinetic energy the electron may have outside the material. Energy is “quantized”


For electrons to be released, the photon energy must be higher than the work function of the material. Frequency must be sufficiently high or: Work function must be low

Frequency at which the photon energy equals the work function is called a cutoff frequency Below it no quantum effects may be observed (only thermal

effects) Above it, thermal and quantum effects are present. At higher frequencies (UV radiation) quantum effects

dominate.

For electrons to be released, the photon energy must be higher than the work function of the material. Frequency must be sufficiently high or: Work function must be low

Frequency at which the photon energy equals the work function is called a cutoff frequency Below it no quantum effects may be observed (only thermal

effects) Above it, thermal and quantum effects are present. At higher frequencies (UV radiation) quantum effects

dominate.

Work function tableWork function tableTable 4.1. Work functions for selected materials given in [eV]Material Work FunctionAluminum 3.38Bismuth 4.17Cadmium 4.0Cobalt 4.21Copper 4.46Germanium 4.5Gold 4.46Iron 4.4Nickel 4.96Platinum 5.56Potasium 1.6Silicon 4.2Silver 4.44Tungsten 4.38Zinc 3.78

Table 4.1. Work functions for selected materials given in [eV]Material Work FunctionAluminum 3.38Bismuth 4.17Cadmium 4.0Cobalt 4.21Copper 4.46Germanium 4.5Gold 4.46Iron 4.4Nickel 4.96Platinum 5.56Potasium 1.6Silicon 4.2Silver 4.44Tungsten 4.38Zinc 3.78

Some notes:Some notes:

Thermoelectric effect is a surface effect Most notable in conductors Group 1 (Alkalis) has lowest work function

values - often used in thermoelectric cells (later)

The amount of electrons released becomes a measure of radiation intensity

Electrons may be emitted by thermionic emission - a totally different issue based on thermal effect

Thermoelectric effect is a surface effect Most notable in conductors Group 1 (Alkalis) has lowest work function

values - often used in thermoelectric cells (later)

The amount of electrons released becomes a measure of radiation intensity

Electrons may be emitted by thermionic emission - a totally different issue based on thermal effect

The photoconducting effectThe photoconducting effect

A solid state (volume) effectMost notable in semiconductorsBased on displacement of valence

and/or covalence electronsValence electrons: bound to individual

atoms in outer layersCovalence electrons: bound but shared

between neighboring atoms in the crystal

A solid state (volume) effectMost notable in semiconductorsBased on displacement of valence

and/or covalence electronsValence electrons: bound to individual

atoms in outer layersCovalence electrons: bound but shared

between neighboring atoms in the crystal

Model: photoconducting effectModel: photoconducting effect

Photons collide with electronsElectrons must acquire sufficient energy to:

Leave the valence band Move into the conduction band Minimum energy required: band gap energy

Photons collide with electronsElectrons must acquire sufficient energy to:

Leave the valence band Move into the conduction band Minimum energy required: band gap energy


In the conduction band, electrons are mobile and free to move as a current.

When electrons leave their sites, they leave behind a “hole” which is simply a positive charge carrier.

This hole may be taken by a neighboring electron with little additional energy (recombination)

Net current is due to electrons and holes. Manifested as a change in concentration of

carriers (electrons and holes) in the conduction band and therefore in conductivity of the medium

In the conduction band, electrons are mobile and free to move as a current.

When electrons leave their sites, they leave behind a “hole” which is simply a positive charge carrier.

This hole may be taken by a neighboring electron with little additional energy (recombination)

Net current is due to electrons and holes. Manifested as a change in concentration of

carriers (electrons and holes) in the conduction band and therefore in conductivity of the medium


Conductivity of the medium is: e - charge of electron e - mobility of electrons [m2/Vs] p - mobility of holes [m2/Vs] n - concentration (density) of electrons

[/m3] p - concentration (density) of holes [/m3] - conductivity of the medium

Conductivity is temperature dependent (mobility and concentrations are temperature dependent)

Conductivity of the medium is: e - charge of electron e - mobility of electrons [m2/Vs] p - mobility of holes [m2/Vs] n - concentration (density) of electrons

[/m3] p - concentration (density) of holes [/m3] - conductivity of the medium

Conductivity is temperature dependent (mobility and concentrations are temperature dependent)

σ = eμen + μpp

photoconducting effectphotoconducting effect This change in conductivity or the resulting change in current is

the a direct measure of radiation intensity. The photoconducting effect is most common in semiconductors

because the band gaps are relatively small. It exists in insulators as well but there the band gaps are very

high and therefore it is difficult to release electrons except at very high energies.

In conductors, most electrons are free to move (they are in the conduction band and hence far above the band gap in energy) which indicates that photons will have minimal or no effect on the conductivity of the medium.

Semiconductors are the obvious choice for sensors based on the photoconducting effect while conductors will most often be used in sensors based on the photoelectric effect

This change in conductivity or the resulting change in current is the a direct measure of radiation intensity.

The photoconducting effect is most common in semiconductors because the band gaps are relatively small.

It exists in insulators as well but there the band gaps are very high and therefore it is difficult to release electrons except at very high energies.

In conductors, most electrons are free to move (they are in the conduction band and hence far above the band gap in energy) which indicates that photons will have minimal or no effect on the conductivity of the medium.

Semiconductors are the obvious choice for sensors based on the photoconducting effect while conductors will most often be used in sensors based on the photoelectric effect

Photoconducting effectPhotoconducting effect

Conductivity results from the charge, mobilities of electrons and holes and the concentrations of electrons, n and p from whatever source.

In the absence of light, the material exhibits what is called dark conductivity, which in turn results in a dark current.

Depending on construction and materials, the resistance of the device may be very high (a few MegaOhms (M) or a few k.

When the sensor is illuminated, its conductivity changes depending on the change in carrier concentrations (excess carrier concentrations).

Conductivity results from the charge, mobilities of electrons and holes and the concentrations of electrons, n and p from whatever source.

In the absence of light, the material exhibits what is called dark conductivity, which in turn results in a dark current.

Depending on construction and materials, the resistance of the device may be very high (a few MegaOhms (M) or a few k.

When the sensor is illuminated, its conductivity changes depending on the change in carrier concentrations (excess carrier concentrations).


This change in conductivity is Carriers are generated at a certain

generation rate They also recombine at a recombination

rate typical for the material, wavelength, carrier lifetime, etc.

Generation and recombination exist simultaneously

Under a given illumination a steady state is obtained when these are equal.

Under this condition, the change in conductivity is (p,n - lifetimes, f - # of carriers generated per second per volume

This change in conductivity is Carriers are generated at a certain

generation rate They also recombine at a recombination

rate typical for the material, wavelength, carrier lifetime, etc.

Generation and recombination exist simultaneously

Under a given illumination a steady state is obtained when these are equal.

Under this condition, the change in conductivity is (p,n - lifetimes, f - # of carriers generated per second per volume

Δσ = eμeΔn + μpΔp

Δσ = efμnτn + μpτp


If p - type carriers dominate - p-type photoconductor

If n - type carriers dominate - n type photoconductor

Opposite type carrier concentrations are negligible

A particular type is obtained by doping (see chapter 3)

If p - type carriers dominate - p-type photoconductor

If n - type carriers dominate - n type photoconductor

Opposite type carrier concentrations are negligible

A particular type is obtained by doping (see chapter 3)

Photoconducting effect - sensitivity

Photoconducting effect - sensitivity

Sensitivity to radiation (efficiency) L is the length of the sensor (distance

between electrodes) and V the voltage across the sensor.

Sensitivity: the number of carriers generated per photon of the input radiation.

To increase sensitivity materials with high carrier lifetimes keep the length of the photoresistor small the latter is typically achieved through the

meander construction shown below

Sensitivity to radiation (efficiency) L is the length of the sensor (distance

between electrodes) and V the voltage across the sensor.

Sensitivity: the number of carriers generated per photon of the input radiation.

To increase sensitivity materials with high carrier lifetimes keep the length of the photoresistor small the latter is typically achieved through the

meander construction shown below

G = VL2

μnτn + μpτp

Photoconductor - structurePhotoconductor - structure

photoconducting effectphotoconducting effect

Properties vary among semiconductorsThe lower the band gap the more effective

the semiconductor will be at detection at low frequencies (long wavelengths).

The longest wavelength specified for the material is called the maximum useful wavelength, above which the effect is negligible.

Availability of electrons is temperature dependent - each semiconductor has a maximum useful temperature (see table)

Properties vary among semiconductorsThe lower the band gap the more effective

the semiconductor will be at detection at low frequencies (long wavelengths).

The longest wavelength specified for the material is called the maximum useful wavelength, above which the effect is negligible.

Availability of electrons is temperature dependent - each semiconductor has a maximum useful temperature (see table)

Photoconductive properties of semiconductors

Photoconductive properties of semiconductors

Table 4.2. Band gap energies, longest wavelength and working temperatures forselected semiconductorsMaterial Band gap [eV] Longest wavelength

λmax [ ]mWorki ngtemperature[°K]

ZnS 3.6 0.35 300CdS 2. 0.52 300CdSe .8 0.69 300CdTe .5 0.83 300Si .2 .2 300Ge 0.67 .8 300PbS 0.37 3.35InAs 0.35 3.5 77PbTe 0.3 .3PbSe 0.27 .58InSb 0.8 6.5 77Ge:Cu 30 8Hg/CdTe 8- 77Pb/SnTe 8- 77InP .35 0.95 300GaP 2.26 0.55 300Note: properties of semiconductor s va ry wit h doping and othe r impurities. The values shownshould be viewed a s representati ve only.

Table 4.2. Band gap energies, longest wavelength and working temperatures forselected semiconductorsMaterial Band gap [eV] Longest wavelength

λmax [ ]mWorki ngtemperature[°K]

ZnS 3.6 0.35 300CdS 2. 0.52 300CdSe .8 0.69 300CdTe .5 0.83 300Si .2 .2 300Ge 0.67 .8 300PbS 0.37 3.35InAs 0.35 3.5 77PbTe 0.3 .3PbSe 0.27 .58InSb 0.8 6.5 77Ge:Cu 30 8Hg/CdTe 8- 77Pb/SnTe 8- 77InP .35 0.95 300GaP 2.26 0.55 300Note: properties of semiconductor s va ry wit h doping and othe r impurities. The values shownshould be viewed a s representati ve only.

Photoconductive propertiesPhotoconductive properties Example: InSb (Indium Antimony):

maximum wavelength of 5.5 m sensitive in the near infrared range band gap is very low - very sensitive. but electrons can be easily released by thermal

sources totally useless for sensing at room temperatures

(300K) (most electrons are in the conduction band) These carriers serve as a thermal background

noise for the photon generated carriers. it is often necessary to cool these long wavelength

sensors to make them useful by reducing the thermal noise.

Example: InSb (Indium Antimony): maximum wavelength of 5.5 m sensitive in the near infrared range band gap is very low - very sensitive. but electrons can be easily released by thermal

sources totally useless for sensing at room temperatures

(300K) (most electrons are in the conduction band) These carriers serve as a thermal background

noise for the photon generated carriers. it is often necessary to cool these long wavelength

sensors to make them useful by reducing the thermal noise.

SemiconductorsSemiconductors

Various photoconductors (photoresistors)

Various photoconductors (photoresistors)

PhotodiodesPhotodiodes

Semiconducting diode exposed to radiation Excess carriers due to photons add to the

existing charges in the conduction band exactly in the same fashion as for a pure semiconductor.

The diode itself may be reverse biased, forward biased or unbiased

Forward biased mode is not useful as a photosensor Number of carrier in conducting mode is large Number of carrier added by radiation small Sensitivity is very low

Semiconducting diode exposed to radiation Excess carriers due to photons add to the

existing charges in the conduction band exactly in the same fashion as for a pure semiconductor.

The diode itself may be reverse biased, forward biased or unbiased

Forward biased mode is not useful as a photosensor Number of carrier in conducting mode is large Number of carrier added by radiation small Sensitivity is very low

Biasing of a diodeBiasing of a diode

I-V charactersitsics of a diodeI-V charactersitsics of a diode

Photodiode - two modesPhotodiode - two modes

Two modes of operation as photodiode1. Photoconductive mode

Diode is in reverse bias Operates similarly to a photoconductor

2. Photovoltaic mode Diode is not biased Operates as a source (solar cell for

example)

Two modes of operation as photodiode1. Photoconductive mode

Diode is in reverse bias Operates similarly to a photoconductor

2. Photovoltaic mode Diode is not biased Operates as a source (solar cell for

example)

In dark mode there are very few carriers flowing Photons release electrons from the valence

band either on the p on n side of the junction. These electrons and the resulting holes flow

towards the respective polarities (electrons towards the positive pole, holes towards the negative pole)

A photocurrent, which in the absence of a current in the diode constitute the only current (a small leakage current exists - see equivalent circuit).

In dark mode there are very few carriers flowing Photons release electrons from the valence

band either on the p on n side of the junction. These electrons and the resulting holes flow

towards the respective polarities (electrons towards the positive pole, holes towards the negative pole)

A photocurrent, which in the absence of a current in the diode constitute the only current (a small leakage current exists - see equivalent circuit).

Photoconducting modePhotoconducting mode

Photoconductive mode - additional effect

Photoconductive mode - additional effect

The large inverse bias accelerates the electrons

Electrons can collide with other electrons and release them across the band gap,

This is called an avalanche effect it results in multiplication of the carriers available. Sensors that operate in this mode are called

photomultiplier sensors

The large inverse bias accelerates the electrons

Electrons can collide with other electrons and release them across the band gap,

This is called an avalanche effect it results in multiplication of the carriers available. Sensors that operate in this mode are called

photomultiplier sensors

Photoconductive mode - equivalent circuit

Photoconductive mode - equivalent circuit

It is the total current in the load Due to photons plus other sources

ThermalLeakageCapacitances, etc.

It is the total current in the load Due to photons plus other sources

ThermalLeakageCapacitances, etc.

Photoconductive diode - operation

Photoconductive diode - operation

Current in reverse biased mode is: I0 is the leakage current, Vd is the voltage across the junction, k=8.62x10-5 eV/K (Boltzman’s const.) T is the absolute temperature

Current due to photons is: P is the radiation power density (W/m2) f is frequency is called the quantum absorption

efficiency A is the area of the diode exposed (PA =

power absorbed by the junction) h is Planck’s constant

Current in reverse biased mode is: I0 is the leakage current, Vd is the voltage across the junction, k=8.62x10-5 eV/K (Boltzman’s const.) T is the absolute temperature

Current due to photons is: P is the radiation power density (W/m2) f is frequency is called the quantum absorption

efficiency A is the area of the diode exposed (PA =

power absorbed by the junction) h is Planck’s constant

Id = I0 eeVd/KT − 1

Ip = ηPAe

hf

Photoconductive diode - operation (cont.)


Total external current is I0 is typically small (negligible) 10 nA or less

Neglecting I0, the total external current is This current gives a direct

reading of the power absorbed by the diode

It is not constant since the relation depends on frequency and the power absorbed itself is frequency dependent.

Total external current is I0 is typically small (negligible) 10 nA or less

Neglecting I0, the total external current is This current gives a direct

reading of the power absorbed by the diode

It is not constant since the relation depends on frequency and the power absorbed itself is frequency dependent.

Il

= Id

− Ip

= I0

ee V

d/ KT

− −

PAe

hf

Il

= Id

− Ip

= I0

ee V

d/ KT

− −

PAe

hf

Il ≈ ηPAe

hf



As the input power increases the characteristic curve of the diode changes as shown, resulting in an increase in reverse current

This current represents the sensed quantity

As the input power increases the characteristic curve of the diode changes as shown, resulting in an increase in reverse current

This current represents the sensed quantity

Photodiode - constructionPhotodiode - construction

Any diode can serve as a photodiode if: n region, p region or pn junction are exposed to

radiation Usually exposure is through a transparent window

or a lens Sometimes opaque materials are used (IR, UV)

Specific structures have been developed to improve one or more of the characteristics The most important improvement is in the dark

current

Any diode can serve as a photodiode if: n region, p region or pn junction are exposed to

radiation Usually exposure is through a transparent window

or a lens Sometimes opaque materials are used (IR, UV)

Specific structures have been developed to improve one or more of the characteristics The most important improvement is in the dark

current

Structures of planar photodiodes

Structures of planar photodiodes

Photodiodes - constructionPhotodiodes - construction

A - Oxide layer increases resistivity - reduced dark current

B. - PIN diodeAddition of the intrinsic p layer increases

resistanceReduces dark current

C. - pnn+ diode - a layer of conducting n+ added Reduces resistance Improves response to low wavelengths

A - Oxide layer increases resistivity - reduced dark current

B. - PIN diodeAddition of the intrinsic p layer increases

resistanceReduces dark current

C. - pnn+ diode - a layer of conducting n+ added Reduces resistance Improves response to low wavelengths


D - A combination of B and C Addition of the intrinsic p layer increases resistance Reduces dark current and improves low wavelength

response

E. - Schotky diode (metal-semiconductor junction) Improved infrared (high wavelength) response Metal layer (hold) must be transparent (very thin layer

F. - npp+ diode - as in B

D - A combination of B and C Addition of the intrinsic p layer increases resistance Reduces dark current and improves low wavelength

response

E. - Schotky diode (metal-semiconductor junction) Improved infrared (high wavelength) response Metal layer (hold) must be transparent (very thin layer

F. - npp+ diode - as in B


Available in various packages and for various applications

Individual diodes in cans with lensesSurface mount diodes used in infrared

remote controlsArrays (linear) of various sizes for scanners Infrared and UV diodes for sensing and

control

Available in various packages and for various applications

Individual diodes in cans with lensesSurface mount diodes used in infrared

remote controlsArrays (linear) of various sizes for scanners Infrared and UV diodes for sensing and

control

Photodiodes Photodiodes

Photodiode as used in Photodiode array used in a CD player a scanner

Photodiode as used in Photodiode array used in a CD player a scanner

Photovoltaic diodesPhotovoltaic diodes

The diode is not biasedServes as a generator

Carriers generated by radiation create a potential difference across the junction

Any photodiode can operate in this mode Solar cells are especially large-surface

photodiodes

The diode is not biasedServes as a generator

Carriers generated by radiation create a potential difference across the junction

Any photodiode can operate in this mode Solar cells are especially large-surface

photodiodes

Photovoltaic modePhotovoltaic mode

Equivalent circuit of photodiode in photovoltaic mode Capacitance is usually large Leakage current is small Response of solar cells is slow due to very large

capacitance

Equivalent circuit of photodiode in photovoltaic mode Capacitance is usually large Leakage current is small Response of solar cells is slow due to very large

capacitance

Solar cellsSolar cells

The phototransistorThe phototransistor

Two junctionsOne forward, one reverse biased

Two junctionsOne forward, one reverse biased

The phototransistors The phototransistors

With the bias shown, the upper diode (the collector-base junction) is reverse biased while the lower (base-emitter) junction is forward biased.

In a regular transistor, a current IB injected into the base is amplified by the amplification factor of the transistor

With the bias shown, the upper diode (the collector-base junction) is reverse biased while the lower (base-emitter) junction is forward biased.

In a regular transistor, a current IB injected into the base is amplified by the amplification factor of the transistor

The phototransistorThe phototransistor

In a regular transistor: = amplification Ib = base current Ic = collector current

Emitter current: In phototransistor, the base is

eliminated. A dark current exists: I0 = leakage current

In a regular transistor: = amplification Ib = base current Ic = collector current

Emitter current: In phototransistor, the base is

eliminated. A dark current exists: I0 = leakage current

IC = βIb

IE = Ib β + 1

IC = I0β, IE = I0 β + 1

The phototransistor (cont.)The phototransistor (cont.)

When the junction is illuminated: Collector current: Emitter current:

(leakage current is neglected) Operation of the phototransistor is

identical to that of the photodiode except for the amplification provided by the transistor structure.

When the junction is illuminated: Collector current: Emitter current:

(leakage current is neglected) Operation of the phototransistor is

identical to that of the photodiode except for the amplification provided by the transistor structure.

IB

= Ip

=

PAe

hf

IC = Ipβ = βηPAe

hf

IE = β + 1ηPAe

hf

Phototransistor (cont.)Phototransistor (cont.)

for even the simplest transistors is of the order of 100 (and can be much higher),

Amplification is linear in most of the operation range

The phototransistor is a very useful device and commonly used for detection and sensing

for even the simplest transistors is of the order of 100 (and can be much higher),

Amplification is linear in most of the operation range

The phototransistor is a very useful device and commonly used for detection and sensing

Phototransistor - generalPhototransistor - general

The high amplification allows phototransistors to operate at low illumination levels

They are typically much smaller than photodiodes.

Thermal noise can be a bigger problem. In many cases, a simple lens is also provided

to concentrate the light on the junction, which for transistors is very small.

The high amplification allows phototransistors to operate at low illumination levels

They are typically much smaller than photodiodes.

Thermal noise can be a bigger problem. In many cases, a simple lens is also provided

to concentrate the light on the junction, which for transistors is very small.

A typical phototransistorA typical phototransistor

Photoelectric sensors, Photomultipliers

Photoelectric sensors, Photomultipliers

Based on the photoelectric effectMetal electrodesEvacuated tubesSome of the oldest optical sensorsUses:

Presence detection, counting, security Sensing very weak sources, night vision

(photomultipliers)

Based on the photoelectric effectMetal electrodesEvacuated tubesSome of the oldest optical sensorsUses:

Presence detection, counting, security Sensing very weak sources, night vision

(photomultipliers)

Photoelectric sensorsPhotoelectric sensors

Sometimes called photoelectric cellsMade of a photocathode, photoanode in an

evacuated tubePhotocathode - made of a low work function

material (usually alkali coated)Electrons are accelerated towards the

photoanodeCurrent through the device is a measure of

radiation intensity

Sometimes called photoelectric cellsMade of a photocathode, photoanode in an

evacuated tubePhotocathode - made of a low work function

material (usually alkali coated)Electrons are accelerated towards the

photoanodeCurrent through the device is a measure of

radiation intensity

The alkali columnThe alkali column

The photoelectric sensorThe photoelectric sensor

“light” represents radiationThe voltage is usually a few hundred voltsThe photoanode and photocathode are

usually shaped for best prformance

“light” represents radiationThe voltage is usually a few hundred voltsThe photoanode and photocathode are

usually shaped for best prformance


The number of emitted electrons per photon is the quantum efficiency of the sensor or Gain (or sensitivity) and depends to a large extent on the material used for the photocathode (its work function)

Photocathodes are made of the alkali group and their alloys

The number of emitted electrons per photon is the quantum efficiency of the sensor or Gain (or sensitivity) and depends to a large extent on the material used for the photocathode (its work function)

Photocathodes are made of the alkali group and their alloys


Photocathodes are made of the alkali group and their alloys - cesium based materials are most common: Low work function Spectral response from IR (1000nm) to UV Evacuated tube or argon filled (to increase

electron production) Older devices used metal cathodes, coated with

alkali compounds (Lithium, Potasium, Sodium or Cesium or a combination of these)

Photocathodes are made of the alkali group and their alloys - cesium based materials are most common: Low work function Spectral response from IR (1000nm) to UV Evacuated tube or argon filled (to increase

electron production) Older devices used metal cathodes, coated with

alkali compounds (Lithium, Potasium, Sodium or Cesium or a combination of these)

Photoelectric sensorsPhotoelectric sensors

Typical gain about 10Newer photoelectric sensors:

NEA (negative electron affinity) surfaces Constructed by evaporation of cesium or

cesium oxide onto a semiconductor’s surface

Operate the same as the older devices but have lower work functions and require lower anode voltages

Typical gain about 10Newer photoelectric sensors:

NEA (negative electron affinity) surfaces Constructed by evaporation of cesium or

cesium oxide onto a semiconductor’s surface

Operate the same as the older devices but have lower work functions and require lower anode voltages

PhotomultipliersPhotomultipliers

A development of photoelectric sensorsThe output (number of electrons) is

multiplied by a large factor Has a photocathode and a photoanode Additional intermediate cathodes, called

dynodes are added between the photocathode and photoanode

A development of photoelectric sensorsThe output (number of electrons) is

multiplied by a large factor Has a photocathode and a photoanode Additional intermediate cathodes, called

dynodes are added between the photocathode and photoanode

Photomultiplier - principlePhotomultiplier - principle

Photomultiplier - biasingPhotomultiplier - biasing

Photomultipliers - operationPhotomultipliers - operation

Cathode and dynodes are made of low work function materials such as Beryllium-Copper (BeCu)

Dynodes are at increasing potentials Creates potential difference to previous

dynode Accelerates the electrons towards the next

dynode

Cathode and dynodes are made of low work function materials such as Beryllium-Copper (BeCu)

Dynodes are at increasing potentials Creates potential difference to previous

dynode Accelerates the electrons towards the next

dynode

Photomultipliers - operationPhotomultipliers - operation

Cathode: Each photon releases n electrons Electrons are accelerated towards 1st dynode

Dynodes: Each incoming electrons releases n electrons Electrons are then accelerated towards the next

dynode Number of dynodes can be large (10 or more)

Cathode: Each photon releases n electrons Electrons are accelerated towards 1st dynode

Dynodes: Each incoming electrons releases n electrons Electrons are then accelerated towards the next

dynode Number of dynodes can be large (10 or more)

Photomultipliers - GainPhotomultipliers - Gain

Multiplication: Given k dynodes: Each dynode releases n

secondary electrons: Gain of the photomultiplier is:

Net effect: a very low light intensity can generate a very large current

Gain can exceed 106.

Multiplication: Given k dynodes: Each dynode releases n

secondary electrons: Gain of the photomultiplier is:

Net effect: a very low light intensity can generate a very large current

Gain can exceed 106.

G = nk

Photomultipliers - GainPhotomultipliers - Gain

Current gain depends on: Construction: Number of dynodes: Inter-dynode voltages:

Additional considerations: electrons must be “forced” to transit between

electrodes at about the same time to avoid distortions in the signal.

To do so, the dynodes are often shaped as curved surfaces which also guides the electrons towards the next dynode

Grids and slats are added – to decrease transit time and improve quality of the signal, (for imaging applications)

Current gain depends on: Construction: Number of dynodes: Inter-dynode voltages:

Additional considerations: electrons must be “forced” to transit between

electrodes at about the same time to avoid distortions in the signal.

To do so, the dynodes are often shaped as curved surfaces which also guides the electrons towards the next dynode

Grids and slats are added – to decrease transit time and improve quality of the signal, (for imaging applications)

Photomultipliers - noisePhotomultipliers - noise

Noise: Noise is critical because of the multiplying effect Dark current due to thermal emission is both

potential and temperature dependent

Noise: Noise is critical because of the multiplying effect Dark current due to thermal emission is both

potential and temperature dependent

a is a constant depending on materialsA area of the emitting cathode T absolute temperature

I0 = aAT2e−E0/kT

Photomultipliers - noisePhotomultipliers - noise

Other sources of noise: Shot noise: due to fluctuations of the current of

discrete electrons multiplication noise due to the statistical spread of

electrons Susceptibility to magnetic fields. Since magnetic

fields apply a force on moving electrons, they can force electrons out of their normal paths reducing their gain and more distorting the signal in imaging applications.

Other sources of noise: Shot noise: due to fluctuations of the current of

discrete electrons multiplication noise due to the statistical spread of

electrons Susceptibility to magnetic fields. Since magnetic

fields apply a force on moving electrons, they can force electrons out of their normal paths reducing their gain and more distorting the signal in imaging applications.

Photomultipliers - applicationsPhotomultipliers - applications

Used for very low light applications such as in night vision systems.

Photomultiplier sensor are placed at the focal point of a telescope to view extremely faint objects in space.

Photomultipliers are part of a broader class of devices called image intensifiers which use various methods (including electrostatic and magnetic lenses) to increase the current.

Have been largely replaced by CCD devices

Used for very low light applications such as in night vision systems.

Photomultiplier sensor are placed at the focal point of a telescope to view extremely faint objects in space.

Photomultipliers are part of a broader class of devices called image intensifiers which use various methods (including electrostatic and magnetic lenses) to increase the current.

Have been largely replaced by CCD devices

CCD sensors and detectorsCCD sensors and detectors

CCD - Coupled Charge DeviceVery common in optical devices

Cameras Video cameras

Have many of the properties of photomultipliers - but simpler, cheaper and higher quality images Low voltage, low radiation intensity Color images, semiconductor construction Very small and fully integrable devices

CCD - Coupled Charge DeviceVery common in optical devices

Cameras Video cameras

Have many of the properties of photomultipliers - but simpler, cheaper and higher quality images Low voltage, low radiation intensity Color images, semiconductor construction Very small and fully integrable devices

CCD - structureCCD - structure Made of a conducting

substrate A p or n type semiconductor

layer is deposited on top. Above it a thin insulating layer

made of Silicon Oxide A transparent conducting layer

above the SiO2 (gate): Allows penetration of photons Can be set at a desired potential

with respect to the substrate This structure is called a Metal

Oxide Semiconductor (MOS)

Made of a conducting substrate

A p or n type semiconductor layer is deposited on top.

Above it a thin insulating layer made of Silicon Oxide

A transparent conducting layer above the SiO2 (gate): Allows penetration of photons Can be set at a desired potential

with respect to the substrate This structure is called a Metal

Oxide Semiconductor (MOS)

CCD - operationCCD - operation

The gate and the substrate form a capacitor. Gate is biased positively with respect to the

substrate. A depletion region in the semiconductor makes this device a very high resistance device.

Optical radiation impinges on the device, penetrates through the gate and oxide layer to release electrons into the depletion layer

Charge density is proportional to radiation intensity. These are attracted to the gate but cannot flow through the oxide layer and are trapped there.

The gate and the substrate form a capacitor. Gate is biased positively with respect to the

substrate. A depletion region in the semiconductor makes this device a very high resistance device.

Optical radiation impinges on the device, penetrates through the gate and oxide layer to release electrons into the depletion layer

Charge density is proportional to radiation intensity. These are attracted to the gate but cannot flow through the oxide layer and are trapped there.

CCD operation (cont.)CCD operation (cont.)

To measure this charge:

Reverse bias the MOS device to discharge the electrons through a resistor

The current through the resistor is a direct measure of light intensity

To measure this charge:

Reverse bias the MOS device to discharge the electrons through a resistor

The current through the resistor is a direct measure of light intensity

CCD - method of sensing charge

CCD - method of sensing charge

CCD - 2-D arraysCCD - 2-D arrays Multiple rows in the two dimensional array. A new image is obtained at the end of each scan. Signal obtained is typically amplified and digitized

and used to produce the image Image can then be displayed on a display array such

as a TV screen or a liquid crystal display. There are many variation of this basic process:

To sense color, filters may be used to separate colors into their basic components (RGB – Red-Green-Blue).

Each color is sensed separately and forms part of the signal. Thus, a color CCD will contain three cells per “pixel” each reacting to one color.

Multiple rows in the two dimensional array. A new image is obtained at the end of each scan. Signal obtained is typically amplified and digitized

and used to produce the image Image can then be displayed on a display array such

as a TV screen or a liquid crystal display. There are many variation of this basic process:

To sense color, filters may be used to separate colors into their basic components (RGB – Red-Green-Blue).

Each color is sensed separately and forms part of the signal. Thus, a color CCD will contain three cells per “pixel” each reacting to one color.

CCD - applicationsCCD - applications

CCD devices are the core of most types of electronic cameras and video recorders

Also used in scanners (where linear arrays are used).

Used for very low light application by cooling the CCDs to low temperatures. Sensitivity is much higher primarily due to reduced

thermal noise. In this mode CCD have successfully displaced

photomultipliers.

CCD devices are the core of most types of electronic cameras and video recorders

Also used in scanners (where linear arrays are used).

Used for very low light application by cooling the CCDs to low temperatures. Sensitivity is much higher primarily due to reduced

thermal noise. In this mode CCD have successfully displaced

photomultipliers.

A CCD array for a video camera (500lines,625pixels,3colors)

A CCD array for a video camera (500lines,625pixels,3colors)

Thermal-Based Optical Sensors

Thermal-Based Optical Sensors

Based on thermal effects of radiation Most pronounced at lower frequencies (longer

wavelengths) Most useful in the infrared and microwave portions

of the spectrum. What is measured is the temperature associated

with radiation. A large variety of sensors exist In many cases, the only option available (such as

direct measurement of power at microwave and IR frequencies)

Based on thermal effects of radiation Most pronounced at lower frequencies (longer

wavelengths) Most useful in the infrared and microwave portions

of the spectrum. What is measured is the temperature associated

with radiation. A large variety of sensors exist In many cases, the only option available (such as

direct measurement of power at microwave and IR frequencies)

Thermal-Based Optical Sensors (cont.)

Thermal-Based Optical Sensors (cont.)

• The sensors based on these principles carry different names, some traditional, some descriptive. Early sensors were known as pyroelectric sensors (from

the greek pur for fire). Bolometers are thermal radiation sensors, which are

essentially thermistors and the name refers mostly to its application in microwave and mm wave measurements.

Others names like the PIR (Passive Infra Red) or AFIR (Active Far Infra Red) are more descriptive but are broader and encompass many types of sensors.

Almost any temperature sensor may be used to measure radiation as long as a mechanism can be found to transform radiation into heat.

• The sensors based on these principles carry different names, some traditional, some descriptive. Early sensors were known as pyroelectric sensors (from

the greek pur for fire). Bolometers are thermal radiation sensors, which are

essentially thermistors and the name refers mostly to its application in microwave and mm wave measurements.

Others names like the PIR (Passive Infra Red) or AFIR (Active Far Infra Red) are more descriptive but are broader and encompass many types of sensors.

Almost any temperature sensor may be used to measure radiation as long as a mechanism can be found to transform radiation into heat.

Types of thermal radiation sensors

Types of thermal radiation sensors

Thermal radiation sensors are divided into two classes – Passive Infrared (PIR) and Active Infrared (AFIR) sensors.

PIR: radiation is absorbed and converted to heat. Temperature rise is measured by a sensing element to

yield an indication of the radiative power. AFIR the device is heated from a power source

Variations of this power due to radiation (for example the current or voltage needed to keep the temperature device constant) give an indication of radiation.

Thermal radiation sensors are divided into two classes – Passive Infrared (PIR) and Active Infrared (AFIR) sensors.

PIR: radiation is absorbed and converted to heat. Temperature rise is measured by a sensing element to

yield an indication of the radiative power. AFIR the device is heated from a power source

Variations of this power due to radiation (for example the current or voltage needed to keep the temperature device constant) give an indication of radiation.

PIR sensors - structurePIR sensors - structurePIR sensor has two basic components;

An absorption section that converts radiation into heat

A proper temperature sensor that converts heat into an electrical signal.

Absorption section must be able to Absorb as much of the incoming radiated

power at the sensor’s surface as possible Respond to changes in radiated power

density quickly.

PIR sensor has two basic components; An absorption section that converts

radiation into heat A proper temperature sensor that converts

heat into an electrical signal.Absorption section must be able to

Absorb as much of the incoming radiated power at the sensor’s surface as possible

Respond to changes in radiated power density quickly.

PIR sensors - structure (cont.)PIR sensors - structure (cont.)• Absorber is made of a metal of good heat conductivity (gold

is a common choice in high quality sensors) • Often blackened to increase absorption. • Volume of the absorber is kept small to allow good response

(quick cooling) to changes in radiation • Absorber and the sensor will be encapsulated or placed in a

gas filled or evacuated hermetic chamber to avoid variations in sensing signals due to air motion

• A transparent (to infrared radiation) window typically made of Silicon but other materials may be used (Germanium, Zinc Selenide, etc.)

• The choice of the sensor dictates to a large extent the sensitivity, spectral response and physical construction of the device.

• Absorber is made of a metal of good heat conductivity (gold is a common choice in high quality sensors)

• Often blackened to increase absorption. • Volume of the absorber is kept small to allow good response

(quick cooling) to changes in radiation • Absorber and the sensor will be encapsulated or placed in a

gas filled or evacuated hermetic chamber to avoid variations in sensing signals due to air motion

• A transparent (to infrared radiation) window typically made of Silicon but other materials may be used (Germanium, Zinc Selenide, etc.)

• The choice of the sensor dictates to a large extent the sensitivity, spectral response and physical construction of the device.

Thermopile PIR sensorThermopile PIR sensor In this device, sensing is done by a

thermopile. A thermopile is made of a number of

thermocouple connected in series electrically but in parallel thermally (that is they are exposed to identical thermal conditions).

The thermopile generates a potential proportional to radiation

The thermopile is connected thermally to the absorber but insulated electrically

In this device, sensing is done by a thermopile.

A thermopile is made of a number of thermocouple connected in series electrically but in parallel thermally (that is they are exposed to identical thermal conditions).

The thermopile generates a potential proportional to radiation

The thermopile is connected thermally to the absorber but insulated electrically

Thermopile PIR sensor (cont.)Thermopile PIR sensor (cont.)Any two materials can be used but

some material combinations produce higher potential differences.

Thermopiles can only measure temperature differences hence the thermopile is made of alternating cold (reference) and hot (sensing) junctions

Any two materials can be used but some material combinations produce higher potential differences.

Thermopiles can only measure temperature differences hence the thermopile is made of alternating cold (reference) and hot (sensing) junctions

Thermopile PIR (cont.)Thermopile PIR (cont.)

Structure of a thermopile PIR with reference temperature sensor

Structure of a thermopile PIR with reference temperature sensor

Thermopile PIR (cont.)Thermopile PIR (cont.)

All “cold” junctions are held at a known lower temperature

All “hot” junctions are held at the sensing temperature.

Cold junctions are placed on a relatively large frame that has high thermal capacity and hence the temperature will fluctuate slowly

Hot junctions are in contact with the absorber which is small and has low heat capacity

All “cold” junctions are held at a known lower temperature

All “hot” junctions are held at the sensing temperature.

Cold junctions are placed on a relatively large frame that has high thermal capacity and hence the temperature will fluctuate slowly

Hot junctions are in contact with the absorber which is small and has low heat capacity

Thermopile PIR (cont.)Thermopile PIR (cont.)The frame may be cooled (or a heat

exchanger may be used), or a reference sensor may be used on the frame so that the temperature difference can be properly monitored and related to the radiated power density at the sensor.

In most PIRs a crystalline or polycrystaline silicon and aluminum are used: Silicon has a very high thermoelectric coefficient Is compatible with other components of the sensor aluminum has a low coefficient and can be easily

deposited on silicon surfaces.

The frame may be cooled (or a heat exchanger may be used), or a reference sensor may be used on the frame so that the temperature difference can be properly monitored and related to the radiated power density at the sensor.

In most PIRs a crystalline or polycrystaline silicon and aluminum are used: Silicon has a very high thermoelectric coefficient Is compatible with other components of the sensor aluminum has a low coefficient and can be easily

deposited on silicon surfaces.

Pyroelectric sensorsPyroelectric sensors

Pyroelectric effect: an electric charge generated in response to heat flow through the body of a crystal (a passive sensor)

Charge is proportional to the change in temperature Heat-flow sensors Pyroelectric sensors are best viewed as sensing

changes in radiation. Used mostly in motion sensing

Pyroelectric effect: an electric charge generated in response to heat flow through the body of a crystal (a passive sensor)

Charge is proportional to the change in temperature Heat-flow sensors Pyroelectric sensors are best viewed as sensing

changes in radiation. Used mostly in motion sensing

Pyroelectric sensors (cont.)Pyroelectric sensors (cont.)

Pyroelectricity was discovered in the 18th century in Tourmaline crystals.

By the end of the 19th century, pyroelectric sensors were made of Rochelle salt.

Currently there are many materials used: Barium Titanate Oxide (BaTiO3) Lead Titanite Oxide (PbTiO3) PZT materials (PbZrO3). PVF (polyvinyl fluoride) PVDF (polyvinylidene fluoride) are also used. Many others

Pyroelectricity was discovered in the 18th century in Tourmaline crystals.

By the end of the 19th century, pyroelectric sensors were made of Rochelle salt.

Currently there are many materials used: Barium Titanate Oxide (BaTiO3) Lead Titanite Oxide (PbTiO3) PZT materials (PbZrO3). PVF (polyvinyl fluoride) PVDF (polyvinylidene fluoride) are also used. Many others

Pyroelectic sensors - theoryPyroelectic sensors - theory

When a pyroelectric material is exposed to temperature change T, a charge Q is generated as: A is the area of the sensor PQ is the pyroelectric charge coefficient

defined as:Ps is the spontaneous polarization of

the material (a property of the material, related to its electric permittivity)

When a pyroelectric material is exposed to temperature change T, a charge Q is generated as: A is the area of the sensor PQ is the pyroelectric charge coefficient

defined as:Ps is the spontaneous polarization of

the material (a property of the material, related to its electric permittivity)

Q = PQ

A Δ T Q = PQ

A Δ T

PQ

=

d Ps

dT


V = PV

h Δ T V = PV

h Δ T

A potential difference V is developed across the sensor as: h is the thickness of the crystal PV its pyroelectric voltage

coefficient: E the electric field across the sensor The two coefficients, (voltage and

charge coefficients) are related as follows:

A potential difference V is developed across the sensor as: h is the thickness of the crystal PV its pyroelectric voltage

coefficient: E the electric field across the sensor The two coefficients, (voltage and

charge coefficients) are related as follows:

PV

=

dE

dT

PQ

PV

=

d Ps

dE

= ε0

εr


By definition, the sensor’s capacitance is:

Or: Change in voltage is proportional

to the change in temperature. Depends strongly on permittivityThin samples provide larger

change (larger capacitance)

By definition, the sensor’s capacitance is:

Or: Change in voltage is proportional

to the change in temperature. Depends strongly on permittivityThin samples provide larger

change (larger capacitance)

C =

Q

V

= ε0

εr

A

h

C =

Q

V

= ε0

εr

A

h

V = PQ

ε0

εr

h

Δ T

Table of pyroelectric materialsTable of pyroelectric materialsTable 4.9 . Pyroelectric materials ans some of their properties.

Material PQ [C/m2K] PV [V/mK] εr .Curie Temp[° ]C

TGS( )single crystal

3.5 0x − .3 0x 6 30 9

LiTaO3

( )single crystal2.0 0x − 0.5 0x 6 5 68

BaTiO3

( )Ceramic.0 0x − 0.05 0x 6 000 20

PZT( )Ceramic

.2 0x − 0.03 0x 6 600 30

PVDF( )polymer

0. 0x − 0. 0x 6 2 205

PbTiO3

( )polycrystalline2.3 0x − 0.3 0x 6 200 70

= TGS TriGlycine Sulfate = ( , ) 3PZT Pb Zr Ti O

Table 4.9 . Pyroelectric materials ans some of their properties.

Material PQ [C/m2K] PV [V/mK] εr .Curie Temp[° ]C

TGS( )single crystal

3.5 0x − .3 0x 6 30 9

LiTaO3

( )single crystal2.0 0x − 0.5 0x 6 5 68

BaTiO3

( )Ceramic.0 0x − 0.05 0x 6 000 20

PZT( )Ceramic

.2 0x − 0.03 0x 6 600 30

PVDF( )polymer

0. 0x − 0. 0x 6 2 205

PbTiO3

( )polycrystalline2.3 0x − 0.3 0x 6 200 70

= TGS TriGlycine Sulfate = ( , ) 3PZT Pb Zr Ti O

Pyroelectric sensors - structure

Pyroelectric sensors - structure

Consists of a thin crystal of a pyroelectric material between two electrodes (like a capacitor)

Some sensors use a dual element The second element can be used as a reference

by, for example, shielding it from radiation and is often used to compensate for common mode effects such as vibrations or very rapid thermal changes which can cause false effects

The two elements are connected in series or in parallel.

Consists of a thin crystal of a pyroelectric material between two electrodes (like a capacitor)

Some sensors use a dual element The second element can be used as a reference

by, for example, shielding it from radiation and is often used to compensate for common mode effects such as vibrations or very rapid thermal changes which can cause false effects

The two elements are connected in series or in parallel.

Pyroelectric sensor - structurePyroelectric sensor - structure

PIR motion detectorPIR motion detector

PIR motion detector - dataPIR motion detector - data

RE200B dual IR sensor designed for motion detection. includes a differential FET amplifier operates at 3-10 V field of view of 138º horizontally (wide dimension

of window) and 125º vertically. optical bandwidth (sensitivity region) between 7

and 14 m (in the near infrared region).

RE200B dual IR sensor designed for motion detection. includes a differential FET amplifier operates at 3-10 V field of view of 138º horizontally (wide dimension

of window) and 125º vertically. optical bandwidth (sensitivity region) between 7

and 14 m (in the near infrared region).

Pyroelectric sensors - application


Motion detection, especially of the human body (sometimes of animals)

The change in temperature of infrared radiation (between 4 and 20 m) causes a change in the voltage across the sensor which then is used to activate a switch or some other type of indication

Motion detection, especially of the human body (sometimes of animals)

The change in temperature of infrared radiation (between 4 and 20 m) causes a change in the voltage across the sensor which then is used to activate a switch or some other type of indication



TGS and Lithium tantalite crystals are most often used for these sensors

Ceramic materials and now the polymeric materials are also very commonly used

Decay time: time needed for the charge on the electrodes to diffuse. Of the order os 1-2 seconds because of the very high

resistance of the materials It also depends on the external connection of the device. This response time is very important in the ability of the

sensors to detect slow motion

TGS and Lithium tantalite crystals are most often used for these sensors

Ceramic materials and now the polymeric materials are also very commonly used

Decay time: time needed for the charge on the electrodes to diffuse. Of the order os 1-2 seconds because of the very high

resistance of the materials It also depends on the external connection of the device. This response time is very important in the ability of the

sensors to detect slow motion

Bolometers Bolometers

Simple radiation power sensor (RMS) over the whole spectrum of electromagnetic radiation

Most commonly used in microwave and far infrared ranges.

Consist of any temperature measuring device but usually of a small RTD or a thermistor.

Usually very small in size to allow local measurements

Simple radiation power sensor (RMS) over the whole spectrum of electromagnetic radiation

Most commonly used in microwave and far infrared ranges.

Consist of any temperature measuring device but usually of a small RTD or a thermistor.

Usually very small in size to allow local measurements

Bolometers (cont.) Bolometers (cont.)

The operation is as follows: Radiation is absorbed by the device directly This causes a change in its temperature. This temperature rise is proportional to the

radiated power density at the location of sensing. This change causes a change in the resistance of

the sensing element which is then related to the power or power density at the location being sensed.

Background temperature must be known or compensated for by a separate measurement.

The operation is as follows: Radiation is absorbed by the device directly This causes a change in its temperature. This temperature rise is proportional to the

radiated power density at the location of sensing. This change causes a change in the resistance of

the sensing element which is then related to the power or power density at the location being sensed.

Background temperature must be known or compensated for by a separate measurement.

Bolometers - sensitivityBolometers - sensitivity Sensitivity is given as:

= (dR/dT)/R is the TCR of the bolometer,

εs its surface emissivity, ZT is the thermal

resistance of the bolometer,

R0 its resistance at the background temperature,

the frequency, the thermal time

constant T the rise in

temperature

Sensitivity is given as: = (dR/dT)/R is the

TCR of the bolometer, εs its surface emissivity, ZT is the thermal

resistance of the bolometer,

R0 its resistance at the background temperature,

the frequency, the thermal time

constant T the rise in

temperature

=

α εs

2

ZT

R0

Δ T

1 + α0

Δ T 1 + ( ωτ )

2

=

α εs

2

ZT

R0

Δ T

1 + α0

Δ T 1 + ( ωτ )

2

For best results, thermal impedance should be high (well insulated sensor) and its resistance should be high as well

Bolometers - constructionBolometers - construction

Bolometers are fabricated as very small thermistors or RTDs,

Usually as individual components or as integrated devices.

It is important to insulate the sensing element from the structure supporting it so that its thermal impedance is high.

This can be done by simple suspension of the sensor by this wires.

Bolometers are fabricated as very small thermistors or RTDs,

Usually as individual components or as integrated devices.

It is important to insulate the sensing element from the structure supporting it so that its thermal impedance is high.

This can be done by simple suspension of the sensor by this wires.

Bolometers - notesBolometers - notes

Bolometers are some of the oldest devices used for radiation sensing

Are beeing used for many applications in the microwave region including: Mapping of antenna radiation patterns, Detection of infrared radiation, Testing of microwave devices and much

more.

Bolometers are some of the oldest devices used for radiation sensing

Are beeing used for many applications in the microwave region including: Mapping of antenna radiation patterns, Detection of infrared radiation, Testing of microwave devices and much

more.

Active Far Infrared (AFIR) Sensors

Active Far Infrared (AFIR) Sensors

Principle (simplistic):A power source heats the sensing element to

a temperature above ambient Temperature is kept constantAdditional heat is provided to the sensors

through radiationPower necessary to keep the temperature

constant is a measure of radiated power

Principle (simplistic):A power source heats the sensing element to

a temperature above ambient Temperature is kept constantAdditional heat is provided to the sensors

through radiationPower necessary to keep the temperature

constant is a measure of radiated power

AFIR - theoryAFIR - theory

Temperature of the sensing element is constant

AFIR sensor can be viewed as being time independent.

Power supplied to the sensor: P is power supplied by an

external source PL is power lost through

conduction is radiation power sensed

Temperature of the sensing element is constant

AFIR sensor can be viewed as being time independent.

Power supplied to the sensor: P is power supplied by an

external source PL is power lost through

conduction is radiation power sensed

P = PL

+ P = PL

+

AFIR - theoryAFIR - theory Power loss is:

s is a loss coefficient or thermal conductivity (which depends on materials and construction),

Ts the sensor’s temperature Ta the ambient temperature

Temperature of the radiating source is: ε is emissivity (total) is electric conductivity Ta is ambient temperature A is area of the sensor

Power loss is: s is a loss coefficient or

thermal conductivity (which depends on materials and construction),

Ts the sensor’s temperature Ta the ambient temperature

Temperature of the radiating source is: ε is emissivity (total) is electric conductivity Ta is ambient temperature A is area of the sensor

PL

= s

Ts

− Ta

PL

= s

Ts

− Ta

Tm

= Ts

4

−

A ε

V2

R

− s

Ts

− Ta

AFIR - application and notesAFIR - application and notes

AFIR are rather complex Require stable power supply and

temperature control circuitryMuch more sensitive than PIRsUsed for low contrast radiation sourceRarely used for motion detection

AFIR are rather complex Require stable power supply and

temperature control circuitryMuch more sensitive than PIRsUsed for low contrast radiation sourceRarely used for motion detection

Documents

Chapter 4 - Optical sensors:. Optical sensors Optical sensors are those sensors that detect electromagnetic radiation in the broad optical range – from