Calibration equations to convert unitless values from an optical Satellite imager in Digitals Numbers (DNs) to engineering or scientific units such as at-satellite radiance or spectral radiance. Radiance is power (energy per unit time) per unit area per unit solid angle, in units such as Watts per steradian per square meter (W sr-1 m-2). Spectral radiance is radiance per unit wavelength, in units such as watts per steradian per square meter per Micron (W sr-1 m-2 mm-1) for a specific spectral band-pass. The coefficients for such calibration equations are normally developed by laboratory tests of the instrument prior to launch with reference to sources that are traceable to National Institute of Science and Technology (NIST) radiance standards. Also see NIST traceability. (Source: Dr. John Barker).