Guidelines / Manual
BTS256-LED Tester / Page 6
About LED Measurement
Version 01.2009-01
Picture 6.1:
White LED Assembled to DUT PCB of Gigahertz-Optik’s
LEDA2 LED Measurement Adapter simulating Application
Conditions During Qualification Measurement
The BTS256-LED tester is a light measurement device es-
pecially designed to analyze the light output of printed cir-
cuit board (PCB) mounted and operated Light Emitting Di-
odes (LED).
Basics of LED Measurements:
LEDs
are semiconductor light sources with a high efficiency
electrical power to light power conversion. As with any
semiconductor device, operating temperature effects
changes in performance referred to as a device’s tempera-
ture coefficient. In connection with LEDs the temperature
coefficient will effect a reduction in light output and a drift in
color. Operation under high junction temperature conditions
may effect lifetime. Certain ambient operating environ-
ments, e.g. high humidity, can impact lifetime and device
specifications as well. Thermal management is of primary
importance to the successful implementation of LEDs.
Sorting or grading of individual LEDs by color differences
caused by tolerances in the semiconductor process is a
common practice offered by most semiconductor manufac-
turers. But due to differing LED manufacturer’s sorting proc-
esses and operating conditions, the LED processing indus-
try accepts the need for in-house measurements. These
measurements should be made with the LED device in its
actual operating state in the application.
The most common
light measurement quantity
used in
LED testing is luminous flux measured in lumen. This quan-
tity corresponds to LED efficiency by correlation of the total
light output to the electrical power. Measurement of the total
light output in lm instead of luminous intensity in cd pro-
duces much better reproducibility because it is independent
of spatial light distribution (picture 6.2) which may be influ-
enced by temperature, humidity, distance, different viewing
angles, misalignment and other experimental error.
Measurement of luminous flux with a
goniometric pho-
tometer
is the most precise method of measurement. Here
a summation of the spatial luminous intensity distribution
within the hemisphere in front of the LED is performed.
However this is a time and cost intensive method typically
applied in high level R&D and Quality laboratories.
In industry a light meter with an
integrating sphere
(picture
6.3) are the most common measurement devices em-
ployed. This approach offers easy and fast operation as
well as cost effectiveness. The integrating sphere acts as
light integrator for spatially emitting light sources. The inte-
gration effect is the result of multiple diffuse reflections of
the light on the diffuse reflecting surface of the hollow
sphere which results in a uniform light distribution at the
sphere surface. The illuminance measured at any position
on the integrating sphere surface is therefore an indicator of
the total flux generated by a light source inside or outside of
the sphere. As with any other measurement device integrat-
ing spheres exhibit some
typical characteristics
which
must be considered in use:
1. The
substitution effect
is one source of measurement
uncertainty. During calibration of the sphere photometer
some of the light irradiated into the sphere will exit the
Picture 6.3: Integrating Sphere Photometer
Picture 6.4: Substitution Effect
Picture 6.2: Light Measurement Quantities
Luminous Flux
Luminous Intensity
Φ
[ lm ]
I [ cd ]
Light Detector
Baffle
Calibration:
Dark Room
without Re-reflected Light
Measurement:
DUT Reflec-
ted Light back into Sphere
