NOTE: This definition was changed in RP-16-17 Addendum 4. Deletions have a strike-through. New text is bold.
ρ = Φr /Φi
The ratio of the reflected flux to the incident flux. Reflectance is a function of:
(a) of the incident flux
(b) of collection of the reflected flux
(2) Spectral distribution:
(a) characteristic of the incident flux
(b) weighting function for the collected flux
(a) of the incident flux
(b) component defined for the collected flux
(i) Unless the state of polarization for the incident flux and the polarized component of the reflected flux are stated, it shall be considered that the incident flux is unpolarized and that the total reflected flux (including all polarizations) is evaluated.
(ii) Spectral reflectance depends only on the beam geometry and the character of the reflecting surface (and on polarization). Luminous reflectance also is a function of the spectral distribution of the incident flux.
(iii) If no qualifying geometric adjective is used, reflectance for hemispherical collection is meant (see the various definitions for hemispherical, conical, and directional reflectance for other modifying adjectives).
(iv) Certain of the reflectance terms are theoretically imperfect and are recognized only as practical concepts to be used when applicable. Physical measurements of the incident and reflected flux are always biconical in nature. Directional reflectances (see directional-conical reflectance, conical-directional reflectance, directional-hemispherical reflectance, hemispherical-directional reflectance, and bidirectional reflectance) cannot exist since one component is finite while the other is infinitesimal; here the bidirectional reflectance-distribution function is required. However, the concepts directional reflectance and hemispherical reflectance have practical application in instrumentation, measurements, and calculations when including the aspect of the nearly zero or nearly 2π conical angle would increase complexity without appreciably affecting the immediate results.
(v) In each case of conical incidence or collection, the solid angle is not restricted to a right cone but may be of any cross-section, including rectangular, a ring, or a combination of two or more solid angles.
(vi) For the various related geometrical reflectance properties (hemispherical, directional, and conical) it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of incident radiance (luminance) radiant (luminous) flux as well as the beam geometry and the character of the reflecting surface.