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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

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Presentation Transcript
  • Introduction – Specific Issues with LEDs
  • IES LM-79-08
  • Current CIE Activities
what s good
What’s good?
  • Long lifetime
  • Robust
  • “Tuneable” colours
  • (Becoming) highly energy efficient
what s not so good
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
Photometric Challenges
  • Quasi-monochromatic spectra means good quality photocells are more important than ever …
photometric challenges7
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 challenges8
Photometric Challenges
  • Directionality of light output of LEDs can cause inverse-square law to fail at shorter test distances …
inverse square law
Inverse-Square Law

Eg: Divergent LEDs on a linear luminaire

inverse square law10
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 challenges11
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
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
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 photometry15
Integrating Sphere Photometry

LM-79 says:

  • Two geometries (also specified by CIE 84):
    • 4 (full sphere)
    • 2 (hemisphere)
integrating sphere photometry16
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 photometry17
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 photometry18
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
  • A goniophotometer measures luminous intensity distribution and chromaticity distribution
  • Can derive luminous flux etc.
  • Has advantage of being absolute measurement

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
  • 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
Spatial non-uniformity of chromaticity
  • Deviation of chromaticity from spatial avg
spatial non uniformity of chromaticity23
Spatial non-uniformity of chromaticity
  • Deviation of chromaticity from spatial avg

Spatially averaged colour temperature = 5870K

spatial non uniformity of chromaticity24
Spatial non-uniformity of chromaticity
  • Deviation of chromaticity from spatial avg

Spatially averaged coordinates: u’ = 0.2051, v’ = 0.4716

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
  • 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



Thank youfor your kind attention