Elements of OCR calibration and characterisation Sample Clauses

Elements of OCR calibration and characterisation. The Ocean Colour Radiometers (OCR) to be deployed in the field measure radiances and irradiances with an associated uncertainty. This uncertainty is broadly separated into “calibration” and “characterisation”. Calibration (radiometric) refers to the conversion of Digital Numbers from each photodetector to a radiance or irradiance in SI units at a certain wavelength, generally by use of linear slope (“responsivity”) and offset (“dark”) coefficients, which may vary in time and may require special attention to immersion effects for in-water measurements [24,25]. A calibration result shall always include a statement on uncertainty. Characterisation refers to all other processes which modify the assumed linear calibration model, including [3,20,26]: • Non-linearity • Thermal response • Angular response (especially imperfect cosine response for planar irradiance sensors) • Spectral response (including straylight for hyperspectral spectrometers, out of band response for filter radiometers) • Polarisation sensitivity • Temporal response • Pressure effects These instrument properties have been well-known for decades and corresponding characterisations are considered necessary already by the 2003 NASA Ocean Optics Protocols [26], with reinforcement of the need for such characterisations and updated review of the state of the art in the 2019 IOCCG Protocols [20]. In the FRM context, estimation of uncertainty related to all such instrument properties is needed to complete the total measurement uncertainty estimate ... but is often lacking. The need for regular8, e.g. annual, calibration of radiometers is well-understood and recognised by measurement scientists, who make appropriate preparations for the financial, down-time and transport/customs/logistical implications of such calibrations9. The need for full characterisation of instruments is less well-established, presumably because of the high cost of such characterisations and the low availability of laboratories equipped to perform such characterisations. The FRM4SOC-1 review of radiometer instruments [AD-4] revealed a high diversity in the availability of information on and understanding of such characterisation from the instrument manufacturers. The possibility of performing such characterisations is beyond the capabilities of some instrument manufacturers and, if not required by their customers, is difficult to justify financially because of the high cost of both infrastructure and expertise. 8 Even the require...