Electromagnetic Spectrum Transmittance, Absorptance, and Reflectance

Any beam of photons from some source passing through one medium to another will experience any one or combination of any of the phenomena, i.e., transmission, reflection, absorption, scattering. 

Because they involve ratios (to irradiance), these three parameters are dimensionless numbers (between 0 and 1), but are conunonly expressed as percentages. Following the Law of Conservation of Energy t + a + p = 1. 

A fourth situation, when the emitted radiation results from internal atomic/molec-ular excitation, usually related to the heat state of a body, is a thermal process. 

A specular surface reflects radiation according to Snell s Law which states that the angle of incidence equals the angle of reflectance. Specular reflectances within the visible wavelength range vary from as high as 0.99 for a very good mirror to as low as 0.02-0.04 for a very smooth water surface. 

In general, natural surfaces are almost always diffuse and depart significantly from specular at shorter wavelengths (into the infrared) and may still be somewhat diffuse in the microwave region. 

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