Photometry of LED Lighting Devices

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Photometry of LED Lighting Devices
Tony Bergen
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Contents

• Introduction Specific Issues with LEDs
• IES LM-79-08
• Current CIE Activities

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Introduction Specific Issues with LEDs And
solid-state lighting devices in general
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Whats good?

• Long lifetime
• Robust
• Tuneable colours
• (Becoming) highly energy efficient

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Whats not so good?

• Output is very temperature dependant
• Poor design gives shorter life
• Issues with luminance/glare
• Good photometry is harder

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Photometric Challenges

• Quasi-monochromatic spectra means good quality
  photocells are more important than ever

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Photometric Challenges

• Pulse-width modulated light causes timing and
  measurement issues
• Long stabilisation time
• Ambient temperature sensitivity
• Absolute photometry instead of Relative (cd/klm)

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Photometric Challenges

• Directionality of light output of LEDs can cause
  inverse-square law to fail at shorter test
  distances

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Inverse-Square Law
Eg Divergent LEDs on a linear luminaire
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Inverse-Square Law
Consider a 1200 mm luminaire measured at 6 metres
(5 1)

• Beam incorrectly measured
• Inverse square law doesnt apply
• ? I ? E x d2

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Photometric Challenges

• Sometimes need to use CIE recommendations for
  floodlight photometry to calculate required test
  distance CIE Publication no. 43 Photometry of
  Floodlights

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IES LM-79-08Electrical and Photometric
Measurements of Solid-State Lighting Products
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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

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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

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Integrating Sphere Photometry

• LM-79 says
• Two geometries (also specified by CIE 84)
• 4? (full sphere)
• 2? (hemisphere)

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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 doesnt extract heat from the device

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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)

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Integrating Sphere Photometry

• Match reference lamp and test lamp as closely as
  possible
• Make sure the internal temperature is within 25
  1C
• Calculate spectral mismatch correction factors if
  necessary
• LM-79 slightly more relaxed on sample size for
  given sphere size than CIE 84

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Goniophotometry

• A goniophotometer measures luminous intensity
  distribution and chromaticity distribution

• Can derive luminous flux etc.
• Has advantage of being absolute measurement

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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 1C
• Calculate spectral mismatch correction factors if
  necessary

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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

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Spatial non-uniformity of chromaticity

• Deviation of chromaticity from spatial avg

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Spatial non-uniformity of chromaticity

• Deviation of chromaticity from spatial avg

Spatially averaged colour temperature 5870K
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Spatial non-uniformity of chromaticity

• Deviation of chromaticity from spatial avg

Spatially averaged coordinates u 0.2051, v
0.4716
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Current CIEDivision 2 Activities
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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

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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

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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

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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

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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

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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

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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

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Thank youfor your kind attention

• Tony Bergen
• Technical Director
• Photometric Solutions International
• Factory Two, 21-29 Railway Avenue
• Huntingdale, Vic, 3166, Australia
• Tel [phone]
• Fax [phone]
• Email tonyb_at_photometricsolutions.com
• Web www.photometricsolutions.com