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6 Measurement Considerations
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Photodiode sensor: It can follow short pulses in the ns range. It is important that the pulse
peak power is within the maximum power range of the sensor. It is also important that the
power range is set that accordingly the peak power is within this range, otherwise the reading
will clip at the range end and lead to a wrong average value. Further it is important to use a
power range in manual mode that can measure the peak value, therefore the Min-Max
function is very helpful. Depending on the pulse length and repetition rate the bandwidth
setting will influence the power reading. It is recommended to use the low bandwidth setting
for a stable display; when the pulse should be monitored via the analog output, the bandwidth
should be set to ‘HI’.
6.4 Line Width of Light Sources
The line width of light sources can be neglected only when using a broadband thermal or pyro-
electric sensor. Photodiode sensors show a strong dependency of the operating wavelength so
if the line width of light sources is greater than 10nm (e.g. LED) there may be an influence on
the displayed power. To achieve the best result for broadband light sources with a photodiode
sensor it is necessary that the response curve is nearly linear over the line width. When
entering the center wavelength of the light source as operation wavelength the PM320E will
nearly show the right optical power for a symmetrical spectral response shape.
6.5 Temperature Effects on Thermal Sensors
Thermal sensors react on any temperature differences that occur between thermal disc and
heat sink. The measurement results can be influenced by airflow disturbances or by heating up
the heat sink as a result of a long exposure of the thermal disc by the laser beam.
To avoid disturbances it is recommended to shield the sensor as good as possible from airflow
and to zero it properly in the operating condition. That means for short term measurements
zero the cold sensor, for long term measurements zero the sensor when it is in a state of
thermal stability (e.g. after 10 minutes light exposure).
6.6 Ambient and Stray Light
Ambient or stray light can strongly affect the measurement accuracy in free-space applications.
A permanent background light level can be subtracted by conducting a zero adjustment. More
complicated is varying ambient light like daylight or turning on/off room light. In such cases the
only solution is a proper light shielding of the sensor.
6.7 Back Reflection
Photodiodes, ND filters and even black coatings of thermal sensors show a certain kind of back
reflection of the incoming light. So back reflections to a HeNe or diode laser may interfere the
power stability of the laser, therefore it is recommended to slightly tilt the power meter sensor in
the laser beam.
In order to totally avoid back reflections it is recommended to use an integrating sphere based
S14xC series sensor where the incoming light gets nearly completely absorbed in the sensor.
