al 
Jo 
x 
>. 
sr) 
The spectral signature of a surface determined by this 
method is a series of i values of mean Spectral reflec- 
tance factors Rj over the i spectral bands. In the EXOTECH 
100 radiometer these correspond to the four LANDSAT bandse 
Generally two radiometers are employed for field measure- 
ment campaigns of the J.R.C. One has the 2x diffuser 
optics and is directed towards zenith to measure the 
integral of the spectral irradiance in the four bandse 
The other one has 1° or 15° field of view optics to 
measure the reflected radiance from the target in the 
same bands. From equation (1), the four numbers which 
define the spectral signature are obtained. 
3.2 Spectroradiometers 
Spectroradiometers employed at J.R.C. are not equipped 
with an integrating sphere or a 27 diffuser. Therefore, 
hemispherical spectral irradiation cannot be measured 
in absolute units. The spectral reflectance factor of a 
surface element may however be obtained from the formula: 
D$ 
La 
(2) 
  
R (à) = 
w 
where: 
Ly, is the reflected radiance of a perfectly white 
reflecting diffuser irradiated exactly as the target. 
This means that for the experimental technique which use 
equation (2) a calibrated reflection standard, which 
fills the total field of view of the instrument, is re- 
quired. For practical field use it is not possible to 
prepare a perfectly white reflecting diffuser moreover 
it is not the best solution for the gain setting of 
spectroradiometers. At the J.R.C., it was preferred to 
have a grey reflection standard. This was prepared with 
a mixture of fixed quantities of black and white paints. 
The standard was calibrated in the laboratory on a CARY 
14 spectrophotometer equipped with the cell space total 
diffuse reflectance attachment. The laboratory measure- 
ment provides the spectral reflectance factor of the 
reference grey standard Rg CM so that the equation (2) 
becomes 
  
ba (fg) (3) 
  
R(A) = Rg(A) 
Ing