Calibration Of Photodiode Type Sensors - OPHIR vega User Manual

9.2 Calibration of Photodiode type Sensors

Photodiode detectors are inherently very linear but also
have a large variation in sensitivity with wavelength. In
addition, most sensors from the Ophir PD300 series are
equipped with both a built in filter and removable filter to
allow measurement of higher powers without detector
saturation. These filters also have a transmission that
depends on wavelength. Therefore, when the PD300 is
being used with Vega, the Vega has a built in calibration
adjustment for wavelength which is described in the next
paragraph.
9.2.1. Method of Factory Calibration
The sensitivity of various Ophir photodiode sensors can
vary from one to another as well as with wavelengths.
Therefore, Ophir photodiode detectors are individually
calibrated against NIST traceable standards over the
entire operating range of wavelengths for both filter out
and filter in. The calibration curve is normalized to the
correct absolute calibration at 632.8 nm using a HeNe
laser against a reference meter traceable to NIST.
The spectral sensitivity curve of the detector as well as
the spectral transmission curve of the filters is fed into
the sensor EEROM and this information is used to set the
gain to the proper value at wavelengths other than the
wavelength the instrument was calibrated. When the
user selects his wavelength on the Vega, the correction
factor for that wavelength is applied.
9.2.2. Accuracy of Calibration
Since the instruments are calibrated against NIST
standards, the accuracy is generally ±2% at the
wavelength the calibration has been performed. The
maximum error in measurement will be less than the sum
of the calibration accuracy, linearity, inaccuracy due to
errors in the wavelength curve and variations in gain with
temperature. The linearity of the photodiode detector is
extremely high and errors due to this factor can be
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