1. Dr. Richard Young Optronic Laboratories, Inc.
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 & Types of Measurement
Photometry & colorimetry
Radiometry
Spectroradiometry
Types
Total Flux
Angular Intensity
At a surface
At the source

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

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

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

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

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

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

10. Techniques & Types of Measurement
Photometry & colorimetry
Radiometry
Spectroradiometry
Types
At the source
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 Emission from LEDs generally depends critically on temperature.
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 Total luminous flux, Total radiant flux, Total spectral flux
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 Total luminous flux, Total radiant flux, Total spectral flux
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 = 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 Total luminous flux, Total radiant flux, Total spectral flux
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. 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.
Adding up the values for all directions gives the total flux.
We can map the angular properties of a source by measuring at all values of  and . 

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

32. 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 Flux exclude an auxiliary lamp, giving large errors.
How far the LED extends into the sphere can affect results
AND many commercial products for 2 Flux ignore cosine collection at the detector, giving large errors.

33. Here is an example of LEDs measured in
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 Total luminous flux, Total radiant flux, Total spectral flux
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 Total luminous flux, Total radiant flux, Total spectral flux
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.
An aperture then isolates the part of the image to be measured.
The telescope refocuses it to give an image.

40. Solid Collection angle
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. The image is directed onto the aperture for measurement .
Luminance /Radiance
Two main types of telescope exist for this application
Reflex Telescopes
The image is directed onto the aperture for measurement .

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

44. Direct Viewing Telescope
Luminance /Radiance
Reflex Telescope
Direct Viewing Telescope
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

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.