EXFO IQS-1700 High Performance Power Meter for IQS-500/600 - Manual (Page 47)

Preparing Your High Performance Power Meter for a Test
High Performance Power Meter 39
Nulling Offsets
Nulling Offsets
Temperature and humidity variations affect the performance of electronic
circuits and optical detectors, which can offset measurement results. To
compensate for this offset, the unit is equipped with an offset nulling
function.
Your unit is designed not to require offset nulling under normal operation,
but you should perform it whenever environmental conditions change
significantly or when measuring very low power values.
Note: The manual nulling offset is valid for the current test session only; it no
longer applies when you turn off your power meter.
I
MPORTANT
Light must not reach the detector when performing an offset
nulling operation. Always use a protective screw cap. Do not use a
soft rubber cover