EXFO FTB-500 (Window XP) - Manual (Page 122)

Using the Optional Built-In Power Meter and VFL
112 FTB-500
Setting Thresholds and Correction Factors
You may apply a correction factor (CF) to measured power to compensate
for inaccuracies or drifts. You should change the CF after performing an
offset nulling.
where:
Power
corrected
= the corrected power value
Power
measured
= the measured power value
CF = the correction factor
For each favorite wavelength, the CF is set to 1.00 at the factory (even if
the unit indicates “----”), but allowed values range between 0.85 and
1.15.
Note: Some other products express the CF in dB, so the CF would be added to
measured power.
I
MPORTANT
If you revert to factory settings, you will erase all your thresholds
and correction factors. The pass/fail status feature will be disabled
automatically.
Power
corrected
Power
measured
CF  =