Photometry of LED
Lighting Devices
Tony Bergen
Contents
• Introduction – Specific Issues with LEDs
• IES LM-79-08
• Current CIE Activities
Introduction –
Specific Issues with LEDs*
* And solid-state lighting devices in general
What’s good?
•
•
•
•
Long lifetime
Robust
“Tuneable” colours
(Becoming) highly energy efficient
What’s not so good?
•
•
•
•
Output is very temperature dependant
Poor design gives shorter life
Issues with luminance/glare
Good photometry is harder
Photometric Challenges
• Quasi-monochromatic spectra means
good quality photocells are more
important than ever …
Photometric Challenges
• Pulse-width modulated light causes
timing and measurement issues
• Long stabilisation time
• Ambient temperature sensitivity
• Absolute photometry instead of Relative
(cd/klm)
Photometric Challenges
• Directionality of light output of LEDs can
cause inverse-square law to fail at
shorter test distances …
Inverse-Square Law
Eg: Divergent LEDs on a linear luminaire
Inverse-Square Law
Consider a 1200 mm luminaire
measured at 6 metres (5 : 1)
•Beam incorrectly measured
•Inverse square law doesn’t apply

I  E x d2
Photometric Challenges
• Sometimes need to use CIE
recommendations* for floodlight
photometry to calculate required test
distance
* CIE Publication no. 43 “Photometry of Floodlights”
IES LM-79-08
Electrical and Photometric Measurements of
Solid-State Lighting Products
IES LM-79-08
• Specification released in 2008
• Extra-special consideration given to:
– Ambient (environmental) conditions
– Spectral properties
– Thermal characteristics
• Gives guidelines for measurement in
integrating sphere and goniophotometer
Integrating Sphere Photometry
• Sphere with inside diffuse, high reflectance white
• Light output from test lamp is compared with light
output from reference (known) lamp
• Measure luminous flux, luminous efficacy and
spatially-averaged chromaticity
Integrating Sphere Photometry
LM-79 says:
• Two geometries (also specified by CIE 84):
– 4 (full sphere)
– 2 (hemisphere)
Integrating Sphere Photometry
• For 2 geometry, plug the gap or have a
darkened room behind
• If plugging the gap, make sure that the cover
disk doesn’t extract heat from the device
Integrating Sphere Photometry
• LM-79 suggests two methods of
measurement:
– Sphere-photometer uses a traditional
photocell and picoammeter or equivalent
(beware spectral mismatch)
– Sphere-spectroradiometer uses a spectro
to measure both flux and chromaticity
(recommended method)
Integrating Sphere Photometry
• Match reference lamp and test lamp as
closely as possible
• Make sure the internal temperature is within
25° ± 1°C
• Calculate spectral mismatch correction factors
if necessary
• LM-79 slightly more relaxed on sample size
for given sphere size than CIE 84
Goniophotometry
• A goniophotometer measures luminous intensity
distribution and chromaticity distribution
• Can derive luminous flux etc.
• Has advantage of being absolute measurement
Goniophotometry
LM-79 says:
• Make sure test distance is sufficiently long so
that the inverse square law applies
• Make sure test angle increments are
sufficiently small to make measurement
accurate
• Keep room temperature within 25° ± 1°C
• Calculate spectral mismatch correction factors
if necessary
Goniophotometry
• Measure chromaticity:
– In steps of 10° in elevation angle
– In two orthogonal C-planes 0° and 90°
• Calculate spatially-averaged
chromaticity, weighted by:
– Luminous intensity in each direction
– Solid angle
Spatial non-uniformity of chromaticity
• Deviation of chromaticity from spatial avg
Spatial non-uniformity of chromaticity
Colour Temperature (K)
• Deviation of chromaticity from spatial avg
6400
6200
6000
5800
5600
5400
5200
0
10
20
30
40
50
60
70
80
Elevation Angle (°)
Spatially averaged colour temperature = 5870K
90
Spatial non-uniformity of chromaticity
• Deviation of chromaticity from spatial avg
Spatially averaged coordinates: u’ = 0.2051, v’ = 0.4716
Current CIE
Division 2 Activities
TC2-50
• Measurement of the Optical Properties of LED
Clusters and Arrays
• This is the main standard that we want to see
completed
• It will cover similar aspects to the IES LM-79-08
• Has been held up in the past due to arguments over
definitions and changed chair twice
• From Budapest meeting 2009 we now have a
promising way forward
TC2-58
• Measurement of LED Radiance and
Luminance
• This is a difficult area of measurement
because LEDs are small and directional
• Some similarities with laser safety
TC2-63
• Optical measurement of High-Power
LEDs
• CIE 127 “Measurements of LEDs”
already covered low power LEDs
• This standard will look at measurement
of individual high power LEDs, as
opposed to LED clusters and luminaires
TC2-64
• High speed testing methods for LEDs
• Looking into test methods for
production-line testing of LEDs
• Want to make measurements consistent
and comparable between labs
TC2-66
• Terminology of LEDs and LED
Assemblies
• This TC is looking in to terminology for
different types of LEDs and LED
packages
• Will be used to create appendices for
the TC2-50
TC2-65
• Photometric measurements in the
mesopic range
• This is important for photometry of street
lighting luminaires where their application will
often be in the mesopic range
• The mesopic range favours white LED
sources compared with traditional HPS
streetlights
Reporterships
• R2-42 Measurement for LED
Luminaries
• R2-43 Measurement of Integrated
LED Light Sources
• R2-44 Photometric Characterisation of
Large Area Flat Sources used for
Lighting
Thank you
for your kind attention
Tony Bergen
Technical Director
Photometric Solutions International
Factory Two, 21-29 Railway Avenue
Huntingdale, Vic, 3166, Australia
Tel: +61 3 9568 1879
Fax: +61 3 9568 4667
Email: tonyb@photometricsolutions.com
Web: www.photometricsolutions.com