-'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