HP 83487A User Manual page 49

Figure 3-3. Offset Zero Calibration
Dark Calibration
The dark calibration is for optical measurements, or electrical measurements
if an external O/E is being used. This calibration measures the optical channel
offset signal when there isn't any light present and then uses this information
in performing extinction ratio measurements. Dark calibrations should be
done for the following conditions:
• Before any critical extinction ratio measurements are made
• After a plug-in vertical calibration
• If a module has been removed
• If the mainframe power has been cycled
• If extinction ratio measurements are being made after the vertical scale or the
offset has changed.
If the line power has been cycled, the dark calibration invokes either the offset
zero calibration or plug-in vertical calibration as needed. This increases the
time required for the dark calibration to complete. The Dark cal softkey is
located within the Extinction ratio menu.
Calibration Overview
User Calibrations—Optical and Electrical
3-13