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
www.pro-lite.co.uk
www.photometrictesting.co.uk
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
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Call me an Old Cynic .....
We tested an off-the-shelf
GU10 LED replacement:
Luminous Flux
(lumens)
CCT (Kelvin)
CRI (RA)
Power (Watts)
Beam Angle
Claimed
Actual
320
252
3,000
2,900
N/S
72
5
5
45°
45°
The luminous flux was tested in a 1m integrating
sphere at 25.5°C after a 20 minute stabilisation
time
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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
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However with SSL ...
Relative photometry is no
longer applicable to SSL: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
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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
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LED Output Versus Temperature
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Rendering Versus Temperature
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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
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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
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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
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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
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Sphere Size Does Matter
The perfect sphere is perfectly
impossible! There is no “one-size-fitsall” 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
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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.
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Dr John’s Top Tips
#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!
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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
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Credit: Lighting Sciences
Photometric Distance
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Dr John’s Top Tips
#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)
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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.
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The Definition of CCT
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
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In 1960 CIE UCS, lines of iso-CCT
fall within ± 0.02 duv
Dr John’s Top Tips
#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
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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
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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.
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Dr John’s Top Tips
#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.
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Photometric Measurements
Don’t Have to be Testing
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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
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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
v
Contact Information
Robert Yeo
robert.yeo@pro-lite.co.uk
Gareth John
gareth.john@photometrictesting.co.uk
www.pro-lite.co.uk
www.photometrictesting.co.uk