|Definitions & qualifiers|
The emittance (ε) of a surface is the ratio of the energy radiated from it to that from a blackbody at the same temperature, the same wavelength and under the same viewing conditions. The emissivity is the emittance of a specimen that has an optically smooth surface and is thick enough to be opaque
Reflectance (ρ) is the fraction of radiant energy that is reflected from a surface.
Reflectivity is the reflectance at the surface of a material so thick that the reflectance does not change with increasing thickness
It is not sufficient just to say "emissivity" or "reflectivity" because the terms have a lot of qualifiers, depending on wavelength and direction of emission or detection. A list of qualifiers is summarised below.
Spectral: at one wavelength, designed by (λ) following the symbol for the property, e.g. ελ.
Total: over all wavelengths, designed by the symbol (τ) following the property symbol, e.g. ετ
Directional: in a given direction. The direction is completely specified by two angles, θ and φ; θ is the angle between the specified direction and the normal to the surface and φ is the azimuth of the specified direction from the specimen. The symbols θ and φ are enclosed in parentheses following the symbol of the property as , θ, φ, θ'', φ'; those indicating the incident radiation first, followed by those indicating the direction of the reflected or transmitted rays.
Normal: a special case of directional where θ=0°. In practice, for θ<15°, a normal condition is assumed.
Angular: a more general case of directional where θ>15°, that is, for cases other than normal.
Hemispherical: over a complete hemisphere, designated by the symbol 2π replacing θ and φ.
Specular: in the direction of mirror or specular reflection. In this case, θ=θ'' and φ'=φ+180°
Diffuse: applied to a surface it means reflecting or transmitting in all directions over a hemisphere centred at the point of interest on the surface. Applied to incident radiation: incident at a point from all angles over a hemisphere centred at the point.
Perfectly diffuse: with the same radiance in all directions from a surface.