Angular selective
Angular Selectivity is a special case of angular dependence. Angle dependant performance simply means that the relevant property, for example, the transmittance through or reflectance from an illuminated surface takes on different numerical values for different angles of incidence.
The angle of incidence, which is often designated "theta” is the angle subtended between the normal or perpendicular plane to a given surface and a ray of incoming (incident) light. A common example is that any ordinary pane of clear glass will have its highest transmittance value (and lowest reflectance value) for theta = 0° and its lowest transmittance value (namely zero) and highest reflectance value for theta = 90°.
To define Angular Selectivity, we need to introduce a second angle, namely an azimuthal angle or "phi", which describes rotation around the surface normal in a plane parallel to the surface. It is only of interest if the surface is not homogeneous but has some structure which varies according to the azimuthal angle.
Taking a vertically mounted projection and defining the upward direction by phi = 0°, we have angle selective behaviour, because the transmittance for a given fixed theta value is different for different phi values. Using a lamella or louvre array as an example, when theta = 40 ° and phi = 0°, the transmittance is quite low, whereas for theta = 10° and phi = 180°, the transmittance is higher.
If different amounts of light are transmitted, for the same angle of incidence theta, depending on whether the light comes from above or below then the sample is angle selective. By contrast, a homogeneous material like a pane of clear glass is not angle selective; it always has the same transmittance value for a given theta, independent of the value of phi.
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