Sphere Standards and Standard Spheres

8/3/2019 Sphere Standards and Standard Spheres

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Optronic Laboratories, Inc.

Sphere Standards andStandard SpheresDr. Richard Young



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Sphere ApplicationsStandard Spheres

Total flux measurements All the light from a lamp is measured

Irradiance measurements Light at a surface is measured

Sphere standards

Large uniform source is used for calibrationof instruments


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Total FluxAn integrating sphere has several

interesting properties:

Any part of the sphere surface sees allother parts of the sphere surface equally. This means a detector at any point on the surface can measure the total

power in the entire sphere.

Reflections from the sphere wall add to the

lamp power, giving more power inside thesphere than the lamp is generating.


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Total FluxThe lamp is placedin the center.

A baffle preventsdirect light hitting

the detector.

The sphere wallsand baffle are

highly reflective.

Lamp

Baffle

Detector


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Total FluxLight from thelamp hits the

sphere wall equallyin almost all

directions


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Total Flux

Light from thelamp hits thesphere wall equallyin almost all

directions ...but there are

variations insphere response.

Shadow

area

Partial Shadow

area


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Total Flux

In these shadowareas, the firststrike (lightdirectly from the

lamp) is not fullymeasured.

A sphere cannothave PERFECTresponse.

Shadow

area

Partial Shadow

area



Total FluxAlthough perfect response is not

attainable with this design, practicalspheres can come very close.

How close they come depends onattention to small details of design.

An expert is someone who knows some of the

worst mistakes that can be made in his subjectand how to avoid them.

- W. Heisenberg



Total Flux

Response is bestviewed on a radargraph.

Response varies

with sphere sizeand reflectivity.

If a reflectivity of

95% is used

0

0.2

0.4

0.6

0.8

1

0.5 m sphere



Total Flux


Response varies



95% is used

0

0.2

0.4

0.6

0.8

1

1.0 m sphere



Total Flux


Response varies



95% is used

0

0.2

0.4

0.6

0.8

1

2.0 m sphere



Total Flux

A reflectivity of98% or more ismore common forUS manufactured

spheres.0

0.2

0.4

0.6

0.8

1



Total Flux

Some Europeanstandardsrecommend 80%reflectivity.

But this giveslarge geometricerrors.

0

0.2

0.4

0.6

0.8

1

Note the higher

response in places



Total Flux

This high responseis caused byreflections fromthe detector side

of the baffle.



Total Flux

This high responseis caused byreflections fromthe detector side

of the baffle. It is present in all

spheres, but someare much worsethan others.



Total Flux

All the prior sphereresponses had onething in common:

The detector had a

cosine collector on it. If we remove the

cosine collector

Radial Response Map

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1

2 3 45 6 7

89 1 01 1 1 2

1 31 4 1 5

1 61 7

1 81 9

2 02 1

2 2

2 32 42 5

2 6

2 72 8

2 9

3 03 1

3 2

3 3

3 43 5

3 6

3 7

3 83 9

4 0

4 1

4 2

4 3

4 4

4 5

4 6

4 7

4 8

4 9

5 0

5 1

5 25 3

5 4

5 5

5 6

5 7

5 8

5 9

6 0

6 1

6 2

6 3

6 4

6 5

6 6

6 76 8

6 9

7 0

7 1

7 2

7 3

7 4

7 5

7 6

7 7

7 8

7 9

8 0

8 1

8 28 3

8 4

8 5

8 6

8 7

8 8

8 9

9 0

9 1

9 2

9 3

9 4

9 5

9 6

9 79 8

9 9

1 00

1 01

1 02

1 03

1 04

1 05

1 06

1 07

1 08

1 09

1 1 0

1 1 1

1 1 21 1 3

1 1 4

1 1 5

1 1 6

1 1 7

1 1 8

1 1 9

1 2 0

1 2 1

1 2 2

1 2 3

1 2 4

1 2 5

1 2 6

1 2 71 2 8

1 2 9

1 3 0

1 3 1

1 3 2

1 3 3

1 3 4

1 3 5

1 3 6

1 3 7

1 3 8

1 3 9

1 4 0

1 4 1

1 4 21 4 3

1 4 4

1 4 5

1 4 6

1 4 7

1 4 8

1 4 9

1 5 0

1 5 1

1 5 2

1 5 3

1 5 4

1 5 5

1 5 61 5 71 5 8

1 5 9

1 6 01 6 1

1 6 2

1 6 3

1 6 41 6 5

1 6 6

1 6 71 6 8

1 6 91 7 0

1 7 11 7 2

1 7 31 7 4

1 7 51 7 6

1 7 71 7 8

1 7 91 8 01 8 1

1 8 21 8 31 8 4

1 8 51 8 61 8 71 8 81 8 91 9 0

1 9 11 9 21 9 3

1 9 4

1 9 51 9 61 9 71 9 81 9 92 00

2 012 022 032 042 05

2 062 072 08

2 092 1 0

2 1 12 1 2

2 1 32 1 4

2 1 5

2 1 62 1 72 1 8

2 1 9

2 2 02 2 1

2 2 2

2 2 32 2 4

2 2 5

2 2 6

2 2 72 2 8

2 2 9

2 3 02 3 12 3 2

2 3 3

2 3 4

2 3 5

2 3 6

2 3 7

2 3 8

2 3 9

2 4 0

2 4 1

2 4 2

2 4 3

2 4 4

2 4 52 4 6

2 4 7

2 4 8

2 4 9

2 5 0

2 5 1

2 5 2

2 5 3

2 5 4

2 5 5

2 5 6

2 5 7

2 5 8

2 5 9

2 6 02 6 1

2 6 2

2 6 3

2 6 4

2 6 5

2 6 6

2 6 7

2 6 8

2 6 9

2 7 0

2 7 1

2 7 2

2 7 3

2 7 4

2 7 52 7 6

2 7 7

2 7 8

2 7 9

2 8 0

2 8 1

2 8 2

2 8 3

2 8 4

2 8 5

2 8 6

2 8 7

2 8 8

2 8 9

2 9 02 9 1

2 9 2

2 9 3

2 9 4

2 9 5

2 9 6

2 9 7

2 9 8

2 9 9

3 00

3 01

3 02

3 03

3 04

3 053 06

3 07

3 08

3 09

3 1 0

3 1 1

3 1 2

3 1 3

3 1 4

3 1 5

3 1 6

3 1 7

3 1 8

3 1 9

3 2 03 2 1

3 2 2

3 2 3

3 2 4

3 2 5

3 2 6

3 2 7

3 2 8

3 2 9

3 3 0

3 3 1

3 3 2

3 3 3

3 3 4

3 3 53 3 6

3 3 7

3 3 8

3 3 9

3 4 0

3 4 1

3 4 2

3 4 3

3 4 4

3 4 5

3 4 6

3 4 7

3 4 8

3 4 93 5 0

3 5 1

3 5 2

3 5 33 5 4

3 5 5

3 5 6

3 5 73 5 8

3 5 9

3 6 03 6 1

3 6 23 6 3

3 6 43 6 5

3 6 63 6 7

3 6 83 6 9

3 7 03 7 1

3 7 23 7 33 7 4

3 7 53 7 63 7 7

3 7 83 7 93 8 03 8 1 3 8 23 8 3

3 8 43 8 53 8 6

Cosine collectorremoved

Equatorial

Angle

...the response ofeven the bestsphere is destroyed.



Total FluxMany Sources are highly directional

Fluorescent Lamps



Total FluxMany Sources are highly directional

LEDs



Total Flux

A sphere has areasof uniformresponse (green).

And non-uniform

areas (red). If the source is

highly directional,it should be

pointed at a greenarea for the bestresults.



Total Flux

The green area isbigger for largerspheres.

The red area is

bigger for largerbaffles.



Total FluxGeometrically, the highest

accuracies are obtained by orientinglamps so the maximum output isdirected at areas of uniformresponse.

Highly reflective coatings give muchlower geometrical errors, regardlessof orientation, than less reflectivecoatings.



Total FluxHowever,Anything placed inside spheres,

including lamps, holders, sockets

and cables, can absorb light andchange the sphere throughput.

The higher the reflectivity of the

sphere, the bigger the change tothroughput when something isplaced inside.



Total Flux

0.01%

0.10%

1.00%

10.00%

100.00%

0.00001% 0.0001% 0.001% 0.01% 0.1%

Object in Sphere as % of Sphere Volume

%C

hangeinSphereT

hroughput

99%

98%

97%

95%

90%

80%

Coating

Reflectivity

Here, the effective reflectivity is

changed by just 0.25%


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Total FluxThis example is for a black sphericalobject in the center of the sphere.

Actual changes will depend on the

objects reflectance, shape andposition in the sphere, and can belarger than shown.


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Total FluxThe lamp used in calibration and thelamp to be measured are rarely the

same.

Different changes in throughputbetween these lamps will meanresults will be wrong unlessthroughput changes are alsomeasured.


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Total Flux

An auxiliary lamp,which is housedpermanently in thesphere, is used to

measure changesin throughput.

Auxiliary

Lamp


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Total FluxThe auxiliary lamp

is powered upwhile the standardor test lamp is inthe sphere. But not switched on.

The ratio of signalsis the change inthroughput. This is part of the

calibrationprocedure.

Auxiliary

Lamp


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Total FluxGood total flux measurementsrequire:

A large high reflectivity sphere.

Small, well designed, baffles.A cosine collection detector at the sphere

wall.

An auxiliary lamp.


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Total FluxIt also helps to have:

Uniform measurement procedures. e.g. keep a constant time between powering up a lamp and measuring it.

Dedicated software to guide the userthrough calibration and measurement.

Accurate power supplies for lamps.

NIST traceable calibration lamps.

Heisenberg experts for support.


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Irradiance

Irradiance is thelight flux fallingonto a surface.

The light can come

from any directionand may be frommultiple sources.

The total lighthitting the surfacemust be measured.


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Irradiance

The apparent area of a surface changeswith angle.

This is called the cosine law.A measurement device that follows the

cosine law is called a cosine collector.


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Irradiance

An integratingsphere with aninput port makesan excellent cosine

collector ...provided certain

design rules arefollowed.


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IrradianceThe first rule is:

If light does notobey the cosine lawwhen entering thesphere, there is little

the sphere can do tocompensate.

This is easier saidthan done because

a sphere has 2sides the insideand the outside.

Magnify


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IrradianceThe first rule is:

If light does notobey the cosine lawwhen entering thesphere, there is little

the sphere can do tocompensate.

This is easier saidthan done because

a sphere has 2sides the insideand the outside.

Inside

Outside


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Irradiance

This creates atube effect thatstops some of thelight at higher

angles entering thesphere directly.

Some

light is

blocked


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Irradiance

So, as the angle is changed, the cosineresponse gets worse and worse.

A circle obeys the cosine law

A tube does not obey the cosine law


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Irradiance

The answer issimple.

Make the outside ofthe sphere flat to

meet the inside atthe port.

Thus eliminatingthe tube.


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Irradiance

So now the lightenters the sphereobeying the cosinelaw.

Rule #2: Add a

baffle to prevent

direct light hitting

the detector.


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Sphere Standards

If we replace thedetector with alamp, this sphereis now a source

instead of acollection optic.


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Sphere Standards

The uniformity ofthe sphere outputdepends on theway the light

enters the sphere.Cosine diffusers at

the input giveuniform output but

throw away a lot oflight.


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Sphere Standards

It is difficult toachieve high lightlevels and gooduniformity.

Especially if theoutput level alsoneeds to be variable.

One option is to

change the design.


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Sphere Standards

By making surethe input light onlyhits the baffle, theoutput light is

randomized. If the input light

level varies, sodoes the output.

To track changes,we add a monitor.


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Sphere StandardsThis monitor is

baffled so it onlysees the outputlight, not the input.

So it can becalibrated to showthe output levels

directly.


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Sphere StandardsThe amount of

light entering thesphere can becontrolled by

varying a slit.


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Sphere StandardsNon-imaging

collection mirrorscan increase thelight intensity.

Provided thebeam isapproximately

uniform and over-fills the largestslit.


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Sphere Standards

Slits can be linear (1D) The width varies

Or area (2D)

The width and height vary together


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Sphere StandardsA 2D slit provides better control over a

wide range of levels than a linear (1D) slit.

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Normalized slit width

Normalizedoutput

1D slit 2D slit

With a 1D slit, the output drops

almost vertically at small slit widths


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Sphere StandardsExpressed another way, the slit resolution

needed to change the output by 1%

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

Normalized output

Norm

alizedchangeinslitfor1%

changeinoutput

1D slit 2D slit

At a resolution of 10-5, the 1D slit cannot

control the output below below 3 decades

2D slits can give

up to 1000 times

better control

than 1D slits

2D slits are clearly

superior for high dynamic

range sources


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Sphere Standards

By careful design and materialselection, one can achieve:

High uniformity of output

High maximum levels of output

Very stable operation

6 or more decades of light level adjustment

Direct reading of the output

Easy adjustability and setting to any level

An almost constant spectrum at all levels


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Sphere Standards andStandard SpheresDr. Richard Young


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Sphere Standards and Standard Spheres