Pre-Launch Absolute Spectral Flux Calibration

A common approach to pre-launch absolute spectral flux calibration of an instrument is to break the problem into two parts [1]. In the first part, the relative spectral responsivity (RSR) is determined in the laboratory, often by tuning a quasi-monochromatic source across the bandpass of the instrument. The result from this first part is a spectrum (for each pixel and each band) normalized at some convenient wavelength. In the second part, the absolute flux calibration is measured in the laboratory, often by stimulating the instrument using a broadband calibration source of known absolute spectral flux (termed spectral irradiance, having units, for example: Wm-2nm-1). The result from this second part is a number (for each pixel and each band) having irradiance responsivity units, for example DN W-1m2 (where DN stands for Digital Number and represents the counts from the detector). These two parts are then combined to determine the absolute spectral responsivity (ASR) of the instrument, resulting in a spectrum (for each pixel and each band) having irradiance responsivity units, for example, DN W-1m2.