Optical Calibration

The output from most detectors is usually converted from the native units of the detector into the desired measurement. 

Stated simply, when exposed to a flux, E_l(λ), a photodetector produces a sensor output voltage, V:

where R(λ) is the spectral responsive function of the sensor. 

A calibration scale factor, C_n, results when the source is known (and hopefully traceable to NIST) and the appropriate protocols are observed:

where V_n is the output voltage of from the sensor when exposed to the calibration lamp irradiance, E_l(λ_n), at the nominal center wavelength of the device, λ_n.

The value of an unknown but spectrally similar source may be calculated from the scale factor, taking into account a measurement of the dark offset voltage:

Note that in systems having multiple amplifier stages, such as the PRR-800, PRR-2800, or PUV-2500, each amplifier requires a separate calibration for the appropriate scale factor.

But what is a dark offset?