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 Types of Measurement

• Techniques
• Photometry colorimetry
• Radiometry
• Spectroradiometry

• Types
• Total Flux
• Angular Intensity
• At a surface
• At the source

4
Techniques Types of Measurement

• Techniques
• 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

• Techniques
• 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

• Techniques
• 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

• Techniques
• Photometry colorimetry
• Radiometry
• Spectroradiometry

• Types
• Total Flux

Light emitted in ALL directions
Quantities end with flux
8
Techniques Types of Measurement

• Techniques
• Photometry colorimetry
• Radiometry
• Spectroradiometry

• Types
• Angular Intensity

Light emitted in SPECIFIED directions and angles
Quantities end with intensity
9
Techniques Types of Measurement

• Techniques
• 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

• Techniques
• 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
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
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.
AND many commercial products for 2? Flux ignore
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
Although it is not focused, we can clearly see
the cup/die structure on the screen.
We can also see the light is not uniform at the
surface, so results depend on the size and
position of the measurement area.
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
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 reflex mirror lets the user see what is being
measured
The sectional drawing shows what happens inside
the solid housing.
Light from the source
is focussed by the telescope.
If the mirror is flipped out of the way
42
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

The mirror and aperture are combined so the area
being measured is viewed directly.
44
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.