1401 - Illustrate basic radiation-matter interactions and related concepts of spectral reflectance, absorbance and transmittance as specific properties of the matter

Concepts

• [PP1-2-5] Concepts of Transmittance, Absorbance, Reflectance, Scattering.
  Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the Earth's surface. There are three (3) forms of interaction that can take place when e.m. radiation strikes, or is incident (I) upon a surface. These are: absorption, transmission, and reflection. The total incident radiation will interact with the surface in one or more of these three ways. The proportions of each will depend on the wavelength of the incident radiation and the specific chemical/physical properties of the surface material. Absorption occurs when incident radiation is absorbed into the target, while transmission occurs when radiation passes through a target. Reflection occurs when radiation "bounces" off the target and is redirected. The spectral reflectance  is defined by the ratio of reflected radiance to incident radiance  at a prefixed wavelegth . The spectral transmittance of a medium is defined by the ratio of the transmitted radiance  to the incident one  at a prefixed wavelegth . The absorbance of a medium or target is defined by the ratio of the absorbed radiance to the incident one   at a prefixed wavelegth . Conservation of energy require that, at a certain wavelenght: R+T+A=1. To express the circumstance that the reflection can occurre in different direction as the surface deviates from a specular one, becoming rough, the concept of surface scattering has been introduced (ref. [PP1-2-10] The Rayleigh roughness criterion).
• [PP1-3-3] Spectral Signatures of the matter
  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)