EXFO FTB-500 (Window XP) - Manual (Page 120)

Using the Optional Built-In Power Meter and VFL
110 FTB-500
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 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.
To perform an offset nulling:
1. Tighten the protective cap on the power meter port.
2. From the Instrument View tab, tap Offset Nulling.
The nulling process takes approximately 5 seconds.
I
MPORTANT
Light must not reach the detector when nulling offsets. Always use
an EUI or protective screw cap. Do not use a soft rubber cover.