The Top Ten Errors (and Misconceptions) in LED Photometry · the sphere won’t integrate properly, giving large errors. Rules of thumb: For 4 measurements of 2D/3D luminaires, sphere

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The Top Ten Errors (and Misconceptions) in LED Photometry

LED Specifications: the Truth, the Whole Truth

and Nothing but the Truth

Robert Yeo, Pro-Lite Technology

Dr Gareth John, Photometric & Optical Testing Services

Lighting Fixture Design Conference Wednesday 5th June 2013


Can We Trust LED Specifications?

Many people distrust LED and SSL specifications. Why?

We’ll consider how LED specifications are determined

The importance of “absolute photometry”

The common errors made in photometry (some are LARGE!)

Understanding why CCT is a bad metric


We tested an off-the-shelf

GU10 LED replacement:

Claimed Actual

Luminous Flux (lumens)

320 252

CCT (Kelvin) 3,000 2,900

CRI (RA) N/S 72

Power (Watts) 5 5

Beam Angle 45° 45°

The luminous flux was tested in a 1m integrating

sphere at 25.5°C after a 20 minute stabilisation

time

Call me an Old Cynic .....


In the Good Old Days .....

“Relative Photometry” ruled:

Fluorescent lamp

output is constant

regardless of what

fitting it is put into

Multiply lamp flux by

fitting LOR to calculate

flux of luminaire

Photometric data

formatted in units of

candelas per 1000

lumens


Relative photometry is no

longer applicable to SSL:

LED output depends

upon operating

temperature

In turn it depends

upon the thermal

design of the fitting

For most SSL we have

to perform absolute

photometry

However with SSL ...


How Are LED Specifications Tested?

LED specifications – fact or fiction?

LED specifications are determined under idealised laboratory

conditions: short flash of current, junction temperature of 25°C

An LED will typically operate at >60°C in a luminaire

If you use ten 100 lumen LEDs in a luminaire, you must not

expect to generate 1,000 lumens from the luminaire, you will

probably get 750-800 lm in practice

As LEDs get hotter:

Luminous flux DECREASES

CCT INCREASES

CRI changes


LED Output Versus Temperature


Rendering Versus Temperature


Dr John’s Top Tips

#1: Test the complete fitting – don’t

extrapolate from rated LED flux

#2: Test the fitting after it has reached

thermal equilibrium – measure “hot

lumens” and not “cold lumens”

#3: Measure the CCT and CRI of the

complete fitting – thermal and optical

effects significantly shift the rated

colour temperature and colour

rendering of LEDs


Measuring Total Luminous Flux

Luminous flux – in lumens – is usually

measured in an integrating sphere.

However .....

Not all spheres are created equal

Beware SHADOWS!

Size IS important!

You might be suffering from self

absorption


Shadowing Effects in Spheres

The perfect sphere is perfectly spherical, has no holes and is painted with a

100% reflective, perfectly Lambertian coating. The aim is to achieve a

perfectly uniform luminance distribution over the surface of the sphere


Low Reflectance = High Errors

Recent studies have shown

that low reflectance coatings

introduce a significant

directionality to integrating

sphere readings:

The lower the

reflectance, the more

directionally sensitive

Significant errors with

spotlights

+15% to -22% in a

sphere painted with an

80% coating


Sphere Size Does Matter

The perfect sphere is perfectly

impossible! There is no “one-size-fits-

all” policy when it comes to integrating

spheres. If the light source is too big,

the sphere won’t integrate properly,

giving large errors. Rules of thumb:

For 4 measurements of 2D/3D

luminaires, sphere diameter should

be 10x source size or 1.5x for

linear lamps (3% surface area per

IES LM79-08)

For external 2 measurements,

sphere diameter should be 3x

source size Credit: DW Windsor Ltd


Are You Self Absorbed?

The light source placed

inside a sphere both

emits light and absorbs

some of that light. This

results in the test lamp

flux being measured

LOW (usually by some

tens of percent – a

significant source of

error). Also effects CCT

& CRI. These errors

are simply corrected

using an auxiliary lamp.


#4: Use a high quality integrating sphere –

a wooden box or other geodesic

shape painted with Dulux white vinyl

matt will NEVER give good results!

#5: Make sure your sphere is large

enough – if you can barely squeeze

the fitting inside you’ll get shadowing

errors

#6: Correct for self absorption – if you

don’t, you’ll probably sell yourself very

short!



Goniophotometry & Photometric Data

“Photometric Data” refers to

standardised files containing

the far-field luminous intensity

(candelas) versus angle for a

fitting:

Measured using a

goniophotometer

Generated in .ies or .ldt

formats

Major sources of error

include too coarse an

angular sampling interval

and measuring intensity

in the near-field Credit: Lighting Sciences


Photometric Distance


#7: Use the old 5x fitting size rule at your

peril – placing the photometer at a

distance of 5x the light source’s

luminous aperture (e.g. 7.6m for a 5

foot fitting) was OK for extended,

diffused sources but for narrow beam

angle sources you could be

measuring in the near-field and seeing

very low candela readings

#8: Think about your scan resolution – it’s

no good scanning a 15° spot with a 5°

increment – you’ll only be taking 2-3

slices through the beam (beam shape

will be inaccurate)



CCT is a Really Bad Metric

Correlated colour temperature (CCT, Kelvin) is a simplified metric used

to indicate the colour of white light produced by a lamp. The issue with

CCT is that two light sources can have the same CCT but look

completely different colours. The problem is down to the mathematical

definition of CCT and our high sensitivity to colour difference.


The Definition of CCT

In 1960 CIE UCS, lines of iso-CCT

fall within ± 0.02 duv

Two LEDs with

the same CCT

can have

distinctly different

colours. The

difference can be

as much as

± 0.02 duv which

is about 20 times

greater than the

minimum

perceivable

colour difference


#9: As a luminaire designer, under no

circumstances should you use CCT

as a purchasing metric – to stand any

chance of colour matching LEDs or

LED modules you have to specify

colour in terms of the CIE chromaticity

coordinates, Cx & Cy. And then

negotiate binning tolerances with your

vendor



White Light from Blue - Fluorescence

Differences in the

thickness or optical path

length through an LED

phosphor can lead to

significant differences in

the correlated colour

temperature (CCT) at

different angles from the

LED. When the phosphor

is thinner, less blue light

is converted and the CCT

is higher; when it is

thicker, more blue light is

converted and the CCT is

lower


Colour Shift Through an LED Phosphor

This is the colour variation from a remote phosphor LED spotlight. In the

centre of the beam, the CCT is 7,500 Kelvin. At 30°, the CCT has

dropped to 4,500 Kelvin. An integrating sphere measurement would only

yield the average CCT.


#10: Beware the effect of colour shift from

white LEDs – the rated CCT is

probably based upon a directional

measurement in the centre of the

beam or an average value if

performed using an integrating

sphere. Ask to see evidence of the

colour shift with angle from the LED

or LED module.



Photometric Measurements

Don’t Have to be Testing


AA About Pro-Lite & Photometric Testing

Photometric measurements don’t have to

be testing with light measurement

equipment from Pro-Lite. Measure the

amount of light, its colour temperature, its

rendering quality and its spatial or angular

distribution. Products include:

• Integrating spheres from Labsphere

& SphereOptics

• Goniophotometers from Radiant

Zemax and UL/Lighting Science

• Photometers & colorimeters from

Konica Minolta

• S:P lux meter from Solar Light Co

• Training in photometry, colorimetry

and photobiological safety

v

Photometric Testing provides the

measure of confidence in independent

LED, lamp & luminaire testing. From what

is one of the most up-to-date lighting test

laboratories in the UK, our range of

services includes:

• S:P ratio per BS 5489-1:2012

• Photometry per BS 13032, IES LM-79

• Standard photometric data (.ies & .ldt)

• Measure lumens, CCT & CRI

• Photobiological safety per BS 62471

• Equipment hire

• On-site measurement services,

including statutory light nuisance


Contact Information

Robert Yeo

[email protected]

Gareth John [email protected]

Documents

The Top Ten Errors (and Misconceptions) in LED Photometry · the sphere won’t integrate properly, giving large errors. Rules of thumb: For 4 measurements of 2D/3D luminaires, sphere