Spectral sensitivity describes a system's ability to detect light with respect to wavelength. As spectral sensitivity increases for a given wavelength, the probability of recording the light present at that wavelength increases (Figure 20.1). The vast majority of imaging systems are not equally sensitive to all wavelengths of light and the measure is important, as it will define the scope of use and performance of an imaging system, e.g. the ability to distinguish colour or its viability as a night vision system. Absolute radiometric spectral sensitivity is a primary objective imaging measure, from which many of the others described throughout this book may be derived.