EXFO FPM-600 Power Meter - Manual (Page 25)

Power Meter/Light Source/Optical Loss Test Set 19
4 Measuring Power or Loss
(FPM-600 and FOT-600)
Nulling Electrical Offsets
Temperature and humidity variations affect the performance of electronic
circuits and optical detectors. Nulling the electrical offsets eliminates these
effects. Your unit has been 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: Factory-defined values will be reinstated when you turn off the unit.
To perform an offset nulling:
Hold down
a few seconds. The unit
displays NULL while nulling the offsets, then
returns to normal mode.
Note: Keypad is disabled during the operation.
I
MPORTANT
If light reaches the detector when nulling offsets, LIGH appears on
the display and the nulling is not performed. You will need to press
a key to return to the previous display.