Spectral Reflectance

Spectral reflectance is the fraction of incident radiation reflected by a non-transparent surface. Spectral reflectance must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength. However it is also commonly averaged over the reflected hemisphere to give the hemispherical spectral reflectivity:Spectral reflectance measures the fractional amplitude of the reflected electromagnetic field, while reflectance refers to the fraction of incident electromagnetic power that is reflected at an interface. The reflectance is thus the square of the magnitude of the reflectivity. The spectral reflectance can be expressed as a complex number as determined by the Fresnel Equations for a single layer, whereas the reflectance is always a positive real number.

The fraction of energy reflected at a particular wavelength varies for different features. Additionally, the reflectance of features varies at different wavelengths. Thus, two features that are indistinguishable in one spectral range may be very different in another portion of the spectrum. This is an essential property of matter that allows for different features to be identified and separated by their spectral signatures. A spectral signature is a unique reflectance value in a specific part of the spectrum.