Reflection is the process by which electromagnetic radiation is returned either at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection), whereas transmission is the passage of electromagnetic radiation through a medium. Both processes can be accompanied by diffusion (also called scattering), which is the process of deflecting a unidirectional beam into many directions. In this case, we speak about diffuse reflection and diffuse transmission (Fig. II.14). When no diffusion occurs, reflection or transmission of an unidirectional beam results in an unidirectional beam according to the laws of geometrical optics (Fig. II.15). In this case, we speak about regular reflection (or specular reflection) and regular transmission (or direct transmission). Reflection, transmission and scattering leave the frequency of the radiation unchanged. Exception: The Doppler effect causes a change in frequency when the reflecting material or surface is in motion. Show Absorption is the transformation of radiant power to another type of energy, usually heat, by interaction with matter.
Fig. II.14 - a-c: Direct, mixed and diffuse reflection d-f: direct, mixed and diffuse transmission Fig. II.15 - When directly reflected or directly transmitted, an unidirectional beam follows the laws of geometrical optics: direct reflection (left): ain = aout, direct transmission (right): n1 · sin(ain) = n2 · sin(aout) with
n1 and n2 denoting the respective medium's index of refraction a d Fe,incident = Ee dA and the (differential) reflected radiant power is given by the exitance Me, multiplied with the size of the surface element: d Fe,reflected = Me dA Thus,
or r EeTotal reflectance is further subdivided in regular reflectance rr and diffuse reflectance rd, which are given by the ratios of regularly (or specularly) reflected radiant power and diffusely reflected radiant power to incident radiant power. From this definition, it is obvious that r = rr + rd The transmittance t of a medium is defined by the ratio of transmitted radiant power to incident radiant power. Total transmittance is further subdivided in regular transmittance tr and diffuse transmittance td, which are given by the ratios of regularly (or directly) transmitted radiant power and diffusely transmitted radiant power to incident radiant power. Again, t = tr + td The absorptance a of a medium is defined by the ratio of absorbed radiant power to incident radiant power. Being defined as ratios of radiant power values, reflectance, transmittance and absorptance are dimensionless. Quantities such as reflectance and transmittance are used to describe the optical properties of materials. The quantities can apply to either complex radiation or to monochromatic radiation. The optical properties of materials are not a constant since they are dependent on many parameters such as: •
thickness of the sample The measurement of optical properties of materials using integrating spheres is described in DIN 5036-3 and CIE 130-1998. Descriptions of the principle measurements are presented in paragraph III.1.f below. II.8.b. Radiance coefficient
qe, Bidirectional reflectance distribution function (BRDF) In the USA, the concept of Bidirectional reflectance distribution function BRDF is similar to the radiance coefficient. The only difference is that the BRDF is a function of the directions of the incident and the reflected beam (Fig. ). In detail, the (differential) irradiance dEe impinging from a certain direction causes the reflected radiance dLe in another direction, which is given by dLe = BRDF · dEe This BRDF depends on more arguments than the radiance coefficient. However, its advantage is the simultaneous description of the material's reflection properties for all possible directional distributions of incident radiation, whereas the radiance coefficient generally is valid for just one specific directional distribution of incident radiation. The unit of radiance coefficient and BRDF is 1/steradian. The BRDF is often abbreviated by the Greek letter ρ, which bears the danger of mixing the BRDF up with reflectance (see foregoing paragraph).
Fig. II.16 - Geometry used for the definition of the bidirectional reflectance distribution function (BRDF). The BRDF depends on the directions of incident and reflected radiation, which are given by the angles Ji and Jr, which are measured relative to the reflecting surface's normal, and the azimuth angles ji and jr, which are measured in the plane of the reflecting surface. What is reflection Refraction absorption and transmission?Reflection Refraction Absorption Transmission
When any light passes from rarer medium to the denser medium, one of the three things happen: Reflection: The light may get reflected from the surface. Absorption: This happens when the light gets converted to another form of energy.
What is the difference between reflection Refraction and transmission of light?This phenomenon usually occurs in lenses. Reflection can simply be defined as the bouncing back of light when it strikes the medium on a plane. Refraction can be defined as the process of the shift of light when it passes through a medium leading to the bending of light.
What is reflection and absorption of light?If light is not absorbed by a surface, it is mostly reflected. Reflection occurs when incoming solar radiation bounces back from an object or surface that it strikes in the atmosphere, on land, or water, and is not transformed into heat.
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