The FASCAL has been in continuous operations since 1970s for the calibrations of spectral radiance of sources. The measurements are performed by the use of a prism-grating double monochromator whose spectral radiance responsivity is determined using a variable-temperature blackbody. The temperature of this blackbody is determined using the gold-point blackbody and spectral radiance sources are calibrated using the knowledge of the temperature of the blackbody and the Planck radiation law. The area of the calibration source for the measurement is limited to a rectangular area of 0.6 mm by 0.8 mm.
Prism-grating double monochromator with photomultiplier tube, Si diode and extended-InGaAs diode detectors are used to cover a range of wavelengths from 220 nm to 2500 nm. A variable-temperature blackbody with emissivity of > 0.999 with an 2 mm diameter opening is used as the calibration source. Tungsten-ribbon filament lamps and integrating sphere sources are calibrated. Dry-air purge of the integrating sphere sources are possible to remove the effects of moisture in the path of the radiation in the integrating sphere sources.