The relative amount of electromagnetic radiation reflected (absorbed, transmitted, emitted) by the matter at different wavelengths depends on its specific chemical composition and physical properties. The plots of corresponding physical quantities (reflectance, absorbance, transmittance, emissivity) against wavelength, are termed spectral signatures of the specific matter under study. In principle the analysis of spectral signatures obtained by multispectral EO sensors could allow us to identify/discriminate different cover types.
The interpretation of spectral signatures requires to well understand the e.m. radiation-matter interaction process. In very simple term we expect that incident radiation I(λ)can be reflected, absorbed or transmitted by the matter so that for the energy conservation should be:
I(λ)=I(λ,R)+I(λ,A), I(λ,T)
being I(λ,R), I(λ,A) and I(λ,T) the reflected, absorbed and transmitted fraction of I(λ). From the previous relation descends (dividing both members for I) that:
1=R(λ)+A(λ)+T(λ)
being:
R(λ)=I(λ,R)/I(λ) named Reflectance
A(λ)=I(λ,A)/I(λ) named Absorbance
T(λ)=I(λ,T)/I(λ) named Transmittance
They are all specific properties of the considered matter and are not independent each others.
In particular for an opaque medium with T(λ)=0 it is:
R(λ)=1-A(λ)