Absolute radiometric characterization of the transfer radiometer unit of RASTA in the UV, VIS and NIR spectral range

The German Aerospace Centre (DLR) is operating the Calibration Home Base (CHB) as a facility for the radiometric calibration of airborne hyperspectral sensors and field spectrometers, the calibration concept relying on the application of both absolutely calibrated source- and detector-based radiometric transfer standards. Following this concept, DLR has designed a new and dedicated radiance standard (RASTA).To provide a source-based SI traceability to the CHB facility, RASTA has been recently calibrated at the Physikalisch-Technische Bundesanstalt (PTB). To provide redundancy to the source-based calibration and to take advantage of the superior stability of detector-based radiometric transfer standards, the RASTA design includes a multiple detector unit, the so-called transfer radiometer. This transfer radiometer is built up of five individual filter radiometers (FRs), each consisting of a detector, a dedicated amplifier and an associated temperature controller. Three of these FRs use silicon photodiodes as detectors and coloured optical glass filters as wavelength selecting elements with centre wavelengths at 400 nm, 550 nm and 850 nm. The remaining two FRs are of broad-band design without any filters, applying a standard-type and a long-wavelength-type InGaAs photodiode for covering the wavelength range from 850 nm to 1.7 µm and 850 nm to 2.5 µm, respectively. The measurement of the spectral irradiance responsivity of each of these FRs was performed at the spectral comparator facility of PTB. In the wavelength range from 250 nm to 1.7 µm, this was accomplished by comparison with absolutely calibrated transfer detectors, traceable to the primary detector standard, the cryogenic radiometer. In the wavelength range from 1.7 µm to 2.5 µm a relative calibration was done by applying a thermopile-detector with a previously characterized, spectrally flat absorbing layer. The instrumentation, the comparison procedure and the calibration results including a detailed uncertainty analysis will be presented.