LED Measurement

1. LED Measurement Dr. Richard Young Optronic Laboratories, Inc.

2. Techniques & Types of Measurement Several types of light measurement are possible. These define WHAT you measure. For each type of measurement, there are several possible techniques. These define HOW you measure.

3. Techniques Photometry & colorimetry Radiometry Spectroradiometry Types Total Flux Angular Intensity At a surface At the source Techniques & Types of Measurement

4. Techniques Photometry & colorimetry Types Total Flux Angular Intensity At a surface At the source Techniques & Types of Measurement How does it look to humans? Quantities start with “photopic” or “luminous”

5. Techniques Radiometry Types Total Flux Angular Intensity At a surface At the source Techniques & Types of Measurement How much energy is produced? Quantities start with “radiometric” or “radiant”

6. Techniques Spectroradiometry Types Total Flux Angular Intensity At a surface At the source Techniques & Types of Measurement How is the energy distributed? Quantities start with “spectral” or “spectroradiometric”

7. Techniques Photometry & colorimetry Radiometry Spectroradiometry Types Total Flux Techniques & Types of Measurement Light emitted in ALL directions Quantities end with “flux”

8. Techniques Photometry & colorimetry Radiometry Spectroradiometry Types Angular Intensity Techniques & Types of Measurement Light emitted in SPECIFIED directions and angles Quantities end with “intensity”

9. Techniques Photometry & colorimetry Radiometry Spectroradiometry Types At a surface Techniques & Types of Measurement Light falling onto areas of an object Quantities end with “irradiance” or “illuminance”

10. Techniques Photometry & colorimetry Radiometry Spectroradiometry Types At the source Techniques & Types of Measurement Light emitted from areas within the source Quantities end with “radiance” or “luminance”

11. Techniques & Types of Measurement Photometry + Total Flux = Total Luminous Flux unit: lumens Radiometry + Total Flux = Total Radiant Flux unit: Watts Spectroradiometry + Total Flux = Total Spectral Flux unit: Watts/nm

12. Techniques & Types of Measurement Photometry + Angular Intensity = Luminous Intensity unit: candelas = lumen/sr Radiometry + Angular Intensity = Radiant Intensity unit: Watts/sr Spectroradiometry + Angular Intensity = Spectroradiometric Intensity unit: Watts/(sr nm)

13. Techniques & Types of Measurement Photometry + at a surface = Illuminance unit: lux = lumen/m² Radiometry + at a surface = Irradiance unit: Watts/m² Spectroradiometry + at a surface = Spectral Irradiance unit: Watts/(m² nm)

14. Techniques & Types of Measurement Photometry + at a source = Luminance unit: candelas/m² = lumen/(sr m²) Radiometry + at a source = Radiance unit: Watts/(sr m²) Spectroradiometry + at a source = Spectral Radiance unit: Watts/(sr m² nm)

15. LEDs General Considerations for all measurements • Emission from LEDs generally depends critically on temperature. • Ambient temperature affects results. • Heat-sinking, which includes how and where electrical connections are made, affects results. • Emission from LEDs also depends on supplied current. • Use current regulated rather than voltage regulated supplies where possible.

16. LEDs • LED chips are virtually ideal light sources. • Very small, almost point sources • Reasonably uniform • Lambertian, except at high angles • Almost monochromatic in most cases • All types and techniques of measurement are easily employed.

17. LEDs • LED packages are very useful, but... • They do not behave like small sources. • They are generally non-uniform. • They have highly angular emission. • They are almost monochromatic in most cases. • Unique difficulties are found with most types and techniques of measurement. • Standard conditions are required for agreement between laboratories.

18. LEDs Here is a list of measurements that might be required: • Total luminous flux, Total radiant flux, Total spectral flux • Luminous intensity, Radiant intensity, Spectroradiometric intensity • Illuminance, Irradiance, Spectral irradiance • Luminance, Radiance, Spectral radiance

19. LEDs Here is a list of measurements that might be required: • Total luminous flux, Total radiant flux, Total spectral flux • Luminous intensity, Radiant intensity, Spectroradiometric intensity • Illuminance, Irradiance, Spectral irradiance • Luminance, Radiance, Spectral radiance The only measurements with standard conditions are in bold.

20. Averaged LED Intensity Condition A 31.6 cm Mechanical axis 1 cm2 circular aperture d = 0.001 sr Conditions specified in CIE Publication 127

21. Averaged LED Intensity Condition B CIE committee TC2-46 is currently working on acceptable tolerances in recommended conditions, with the aim of creating an ISO/CIE standard for this type of measurement. 10.0 cm Mechanical axis 1 cm2 circular aperture d = 0.01 sr Conditions specified in CIE Publication 127

22. LEDs Here is a list of measurements that might be required: • Total luminous flux, Total radiant flux, Total spectral flux • Luminous intensity, Radiant intensity, Spectroradiometric intensity • Illuminance, Irradiance, Spectral irradiance • Luminance, Radiance, Spectral radiance How should total flux be measured?

23. Total Flux d  We can map the angular properties of a source by measuring at all values of  and . Adding up the values for all directions gives the total flux. A more common method is to use an integrating sphere, which gives the total in all directions with one measurement. 

24. Total Flux LED • The LED is placed in the sphere center. • A baffle prevents direct light hitting the detector. • The sphere walls and baffle are highly reflective. Baffle Cosine Detector

25. Total Flux • A sphere has areas of uniform response (green). • And non-uniform areas (red). • If the source is highly directional, it should be pointed at a green area for the best results.

26. Total Flux • The LED flux is calculated from signals with the LED and with a standard (known) flux source. • But, anything placed in the sphere affects its throughput. • The lamp or LED used in calibration and the LED to be measured are rarely the same. • Changes in throughput between these lamps will mean results will be wrong unless the changes are also measured.

27. Total Flux Auxiliary Lamp • An auxiliary lamp, which is housed permanently in the sphere, is used to measure changes in throughput. • For photometers or radiometers, best results are with an auxiliary lamp the same as the LED to be measured. • For spectroradiometers, a white light source is best.

28. Total Flux Auxiliary Lamp • The auxiliary lamp is powered up while the standard or test lamp is in the sphere. • But not switched on. • The ratio of signals is the change in throughput. • This is part of the calibration procedure.

29. Total Flux • Good total flux measurements require: • A large high reflectivity sphere • Small, well designed, baffles • A cosine collection detector at the sphere wall • An auxiliary lamp • LEDs present no problem to this type of flux measurement.

30. Total Flux • So why do we need standard conditions for measurement? • Another, more common measurement is forward-looking or 2 flux. • Flux is measured with the LED at the sphere wall. • It is NOT the same as total flux. • It is generally confused with total flux.

31. Forward-looking or 2 Flux So what should be measured for 2 luminous flux? But this assumes the LED is a point source and we know this is incorrect. Any light forward of this plane should be OK as a definition.

32. How far the LED extends into the sphere can affect results Forward-looking or 2 Flux AND LED holders can affect results. CIE committee TC2-45 is currently working on recommended conditions for this type of measurement. AND many commercial products for 2 Fluxexclude an auxiliary lamp, giving large errors. AND many commercial products for 2 Fluxignore cosine collection at the detector, giving large errors.

33. Comparing Fluxes This is red epoxy. Here is an example of LEDs measured in “2” flux (without auxiliary lamp) and total flux (with auxiliary lamp) conditions. These are clear epoxy.

34. LEDs Here is a list of measurements that might be required: • Total luminous flux, Total radiant flux, Total spectral flux • Luminous intensity, Radiant intensity, Spectroradiometric intensity • Illuminance, Irradiance, Spectral irradiance • Luminance, Radiance, Spectral radiance How should illuminance/irradiance be measured?

35. Illuminance/Irradiance The total light hitting the area must be measured. • Illuminance and irradiance is the light falling onto an area of surface. • The light can come from any direction and may be from multiple sources.

36. Illuminance/Irradiance With LED packages, the pattern on a screen varies with distance. die cup We can also see the light is not uniform at the surface, so results depend on the size and position of the measurement area. Although it is not focused, we can clearly see the cup/die structure on the screen.

37. Illuminance/Irradiance • Apart from noting that the illuminance depends on measurement area and position, we should note: • Illuminance is not really a property of a LED. • The method of measurement is independent of the position, orientation or distance of the source(s). • Single LEDs are rarely used in general lighting. • The illumination provided by an LED “lamp”, which contains several elements, is likely to be more uniform. • Chip LEDs give fairly uniform illuminance. • Little dependence on area or position.

38. LEDs Here is a list of measurements that might be required: • Total luminous flux, Total radiant flux, Total spectral flux • Luminous intensity, Radiant intensity, Spectroradiometric intensity • Illuminance, Irradiance, Spectral irradiance • Luminance, Radiance, Spectral radiance How should luminance/radiance be measured?

39. An aperture then isolates the part of the image to be measured. Luminance/Radiance The LED emits light. The telescope refocuses it to give an image.

40. Luminance /Radiance Source Solid Collection angle • The size of the lens defines the solid collection angle. • The measurement area corresponds to the aperture at the image of the telescope. • The source MUST be bigger than the measurement area. Measurement area

41. Luminance /Radiance • Two main types of telescope exist for this application • Reflex Telescopes The sectional drawing shows what happens inside the solid housing. The reflex mirror lets the user see what is being measured Light from the source… If the mirror is flipped out of the way… …is focussed by the telescope.

42. Luminance /Radiance • Two main types of telescope exist for this application • Reflex Telescopes The image is directed onto the aperture for measurement .

43. Image appears with a “missing” circular area (the aperture) The “missing” portion is sent to the detector. Object Luminance /Radiance • Two main types of telescope exist for this application • Direct Viewing Telescopes The mirror and aperture are combined so the area being measured is viewed directly.

44. Relatively inexpensive If the viewing optics and aperture are not perfectly equivalent it gives: Alignment errors Parallax errors No cross-checks which aperture is being used Aperture in image plane Costs more Since the image and aperture are viewed together there are: No alignment errors No parallax errors The size of the aperture is seen with the image Aperture at an angle to the image plane Luminance /Radiance Reflex Telescope Direct Viewing Telescope

45. Luminance /Radiance • For large, uniform, Lambertian sources, luminance measurements are generally: • Insensitive to focus of the telescope • Insensitive to position of the measurement area • Insensitive to rotation of the telescope axis • Insensitive to lens or measurement area size • Insensitive to the source/telescope distance • For single LED packages, luminance measurements are just the opposite: • They are extremely sensitive to everything

46. Luminance /Radiance • Chip LEDs are easy to measure, provided a small enough aperture is available. • Package LEDs are very difficult to measure: • Lenses create a co-dependence of measurement collection angle and measurement area. • Almost any value can be obtained, depending on the conditions of measurement.

47. Luminance /Radiance • There are no recommendations for measurement of luminance of LED packages. • Currently, the following are being discussed: • Measure the chip before it is packaged. • Cut and polish the package to give a flat exit surface. • Measure the Condition A averaged LED intensity and divide the result by the chip emission area (excluding any contact areas). • This gives the “effective” luminance, rather than true luminance, but has the advantage of being easy and consistent with other types of measurement.

48. Conclusions • Chip LEDs are relatively easy to measure. • Packaged LEDs can prove difficult to measure. • When comparing results, make sure the same measurement conditions are used. • Where possible, use recommended conditions. • Use well designed measurement equipment.