-'590-- 
The basic physical quantity which is used for the charac- 
terization of the spectral reflection properties of the 
various species is the spectral reflectance factor R(A)/1/: 
{ Lar (Vr) cos X da, 
J Lar, COS da, 
  
R (A) 
It relates the radiation flux reflected from a surface 
element of the target into the solid angle N and the 
direction of Ÿ. to that flux which is reflected into the 
same cone and the same direction by an ideal reflection 
standard (white Lambertian reflector wL) with unchanged 
irradiation conditions. As an averaged value L for the 
radiance of a natural target will be measured Dy an in- 
strument with a certain field of view N. this formula 
reduces to 
Lacy, n) dositdo, — Dey, n.) 
R(A) = - 
Lar wL / ws da, Làr, wL 
2 
This function calculated from field measurements of radi- 
ance values of the target and the reflection standard will 
still depend on the field of view of the instrument and the 
measuring distance. As the observed target area is generally 
formed by several components of different reflectance the 
enlargement (or reduction) of the observed surface area can 
cause a change of the relative distribution of these compo- 
nents leading to values for the averaged radiance signal. 
Furthermore, vegetated surfaces do generally not show a 
Lambertian response so that the angular dependence of the 
reflected radiation can influence the result if different 
instrument apertures are used. A list of parameters affect- 
ing the in situ determination of the spectral reflectance 
factor is given below: 
a) Parameters being independent of the momentaneous irradi- 
ation of the scene: 
- nature of the object 
- relative composition of the dominant components of 
the objects(s) 
- age and sanitary status of the object 
- season 
local conditions and environmental influences