=i 540 (=
A recommendation of a symbol for the "reflectance factor for
hemispherical incidence" is difficult because no elegant solution
is visible, The use of R with an appropriate index is proposed
by the CIE". But R is totally out of the range of the small Greek
letters normally used to denote material characteristics. But
because there is no other recommendation at the moment for a
symbol to characterize the conical geometry, we follow the CIE-
recommendation and propose the "reflectance factor R for hemis-
pherical incidence" as the recommended quantity to characterize
hemispherically incident radiation and conically reflected radi-
ation as the usual situation in remote sensing application.
Its definition, as given by c1E®, is as follows:
Reflectance factor (at a representative element of a surface,
for the part of the reflected radiation contained in a given cone
with apex at the representative element of the surface, and for
incident radiation of given spectral composition and geometrical
distribution): Ratio of the radiant (luminous) flux reflected
in the directions delimited by the cone to that reflected in the
same directions by a perfect reflecting diffuser identically
irradiated (illuminated).
Herewith, the defining equation of the reflectance factor is
p. LlrGhrletda, _ I car)
Lw | cos 9 da, Lu
where (#%,f- ) denote zenith angle and azimuth of reflection,
respectively. Lr(&,) is the reflected radiance averaged over the
horizontal projection of the solid angle
be, is the reflected radiance of the perfect reflecting diffuser.
(1)
The spectral reflectance factor is definied as
R) x L[ Le 5 post; da. x Lar (2r)
Law / cos Ÿ da, Law
r
(1a)
If no reflectance standard is used but the incident radiation
is measured as the irradiance E, the reflectance factor R is
obtained by substituting L_ by E/FAo , because per definition
E- T-.0,- La with 44e- 1 Yteradian.
This yields
Rs 7.29 Lc 0.)
E (2)
ya, Lar (Ar)
À su ————— a, .
RC ) Ex (2a)
Measurements using the equations (1) or (1a) require a calibrated
reflection standard which fills the field of view of the instru-
ment totally. But only the relative response characteristic of
the instrument must be known, if the field of view of the instru-
ment (not necessarily the direction of view) is kept constant for
both measurements.