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Frequently Asked Question

What are the calibration factors for the GUV radiometers?

Overview: In our GUV-based instruments, incident light strikes the photodetector surface, resulting in a current (IM) flow. Thus, IM is the primary measurement. Note that stray electrons from a variety of sources, often called collectively the “dark current” (ID) are also included in IMID has both a stable component (measured in the lab, IDL) and a variable component (measured in the field, IDF). By subtracting the dark current, we determine the signal current (IS) due only to the incident light: ISIM – (IDLIDF). For engineering reasons, voltage is easier to deal with than currents, and the currents are converted to voltages during data acquisition.

GUV (and PRR) Calibration. Skipping the engineering details, the head is responsible for acquiring the measurement and dealing with the programmable gain amplification in the acquisition circuitry. There are three gain settings used by the instrument, each controlled by a resistor (Rx) of category small, medium, or large (SML). The head transmits a digital representation of the signal voltage (VRx) to the software accompanied by the resistor setting that was used during the measurement. The calibration table that is used by our software contains the necessary coefficients to convert from VRx to the desired end unit. The values of the individual feedback resistors are obtained from the calibration table. Each channel has a scale and offset value for each of the three resistor settings:

ScaleS, ScaleM, and ScaleL as well as OffsetS, OffsetM, and OffsetL

The offsets (SML) in volts are measured in the darkroom as part of the calibration. The scale values (SML) are computed as the product of the feedback resistor values (in ohms) and the responsivity (in amps per end unit). The responsivities for each channel are also listed in the table. Thus, each scale factor (SML) has units of volts per end unit (e.g., V/(μW/cm2 nm)).

In applying the calibration, the software decodes the digital information to recover the voltage VRx.
The software then uses the resistor-specific scale (SRx) and the resistor specific laboratory dark
voltage (VdarkRx) to determine a calibrated measurement M in the end units. Analogous to the
discussion of measurement current, M is composed of a signal measurement (Ms) and a field
measurement of the contribution in the dark (MFD):

M = Ms + MFD = (VRx - VdarkRx)/SRx

Note that MFD depends on the field conditions and cannot be accounted for with the application ofVdarkRx. That is why we introduced the “field offset” term into the calibration table.

Data recorded under field conditions with the instruments capped/covered results in a measurement  of the field offset, MFD. This variable component of the dark current is subsequently removed:

MFD = M (when IS = 0)

MS = M – MFD

where MS is the measurement of signal only, i.e., the final calibrated data.

From GUV calibration overview.pdf (jhm)