=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.