Spectrum A spectrum element describes a wavelength-dependent parameter. They are used for describing light sources, material properties, instrument sensivities, ... Spectra may have different units depending on their context of use : transmission spectra are unit-less, while source spectral radiance is given in W·m-2·sr-1·m-1. There are several spectrum types. For instance, tabulated spectrum uses tabulated user data. Other spectrum types follow physical laws (such as blackbody which uses Planck’s law) and are defined by a few parameters.