Guidelines / Manual
BTS256-LED Tester / Page 8
About LED Measurement ( Continued )
Version 01.2009-01
Picture 8.1: Pulse Width Modulated Signal
in both signal and dark signal. To improve the signal to
noise ratio offset compensation becomes an important rule
for diode array sensors. Best offset compensation is done
with a dark signal measurement using the same integration
time as the signal measurement. A remote controlled shut-
ter supports synchronized integration time on-line offset
compensation. Low light detection with CCD photodiode
array technology can be achieved by employing offset com-
pensation in combination with very long integration times.
Due to
thermal drift
LEDs are often operated in pulse
width controlled mode (picture 8.1). The reduced average
power under this mode produces a higher peak intensity in
light output. For accurate flux measurements at lower fre-
quency the light meter should be synchronized to the op-
eration frequency of the LED. The synchronization to the
light output of the LED can be done with photodiodes be-
cause of their fast response time.
Time
In
te
n
s
ity
Peak Intensity
Average Intensity
Frequency Synchronized
Measurement Sequences
