AQ-00073-000, Rev. 7 31
Appendix D Reflectance Glossary
Diffuse Reflection.
Reflection in which flux is scattered in many directions by diffusion at or below
the surface.
Diffuse Transmittance.
Ratio of the flux transmitted by a specimen to the incident flux, the trans-
mitted flux being measured for all forward angles except the regular transmittance angle.
Diffusion.
Change of the angular distribution of a beam of radiant flux by a transmitting material or
a reflecting surface such that flux incident in one direction is continuously distributed in many
directions, the process not conforming (on a macroscopic scale) to the laws of Fresnel (regular)
reflection and refraction and there being no change in frequency (wavelength) of the monochro-
matic components of the flux.
Lambertian Diffuser.
Ideal surface that reflects or transmits radiation completely in accord with
Lambert's cosine law. When illuminated from any direction, its radiance is the same for every
direction of view.
Lambert's law
. The intensity (flux per unit solid angle) emitted in any direction from a surface var-
ies as the cosine of the angle between the normal to the surface and the direction of the emitted
flux (also called Lambert's cosine law).
Mixed Reflection.
Partly specular and partly diffuse reflection.
Mixed Transmission.
A combination of diffuse and regular transmission.
Perfect Reflecting Diffuser.
Ideal reflecting surface that neither absorbs nor transmits light, but
reflects diffusely, with the radiance of the reflecting surface being the same for all reflecting
angles, regardless of the angular distribution of the incident light.
Perfect Transmitting Diffuser.
Ideal transmitting specimen that neither absorbs nor reflects light,
but transmits diffusely, with the radiance of the specimen being the same for all transmitting
angles, regardless of the angular distribution of the incident light.
Reflectance.
Ratio of the reflected radiant or luminous flux to the incident flux in the given condi-
tions.
Reflectance factor.
Ratio of the flux reflected from the specimen to the flux reflected from the per-
fect reflecting diffuser under the same geometric and spectral conditions of measurement.
Regular Transmittance.
Ratio of undiffused transmitted flux to incident flux.
Regular Transmittance factor.
The ratio of the flux transmitted by a specimen and evaluated by a
receiver to the flux passing through the same optical system and evaluated by the receiver when
the specimen is removed from the system.
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