3. GENERAL DEFINITIONS AND MEASUREMENT METHODS 
The Spectral reflectance factor R, with a defined conical 
FOV Cg, for hemispherical irradiation, is usefull for 
the analysis of spacecraft and satellite data. This de- 
finition considers the sky radiation which cannot be 
neglected compared to the sun's irradiation in an open 
field as recommended by the ISP Working Group VII-9^4. 
The measured spectral signatures are obtained by using 
Slightly different equations and experimental techniques 
in order to fit the previous definition of the reflec- 
tance factor R. This depends on the instruments used for 
the measurements. 
3.1 Radiometers 
    
   
     
  
    
    
   
      
    
  
Instruments as EXOTECH 100 have been designed with a 
fixed number of spectral bands and allow to measure both 
hemispherical irradiance and reflected radiance by chang- 
ing the input optics of the radiometer in absolute units. 
Measured data are integrated over the bandwidth. Con- 
sequently, it is appropriate to define a mean spectral 
reflectance factor for each band which is given by the 
formula: 
2. = 7 i 
| f, (3) E, dà 
As € 
  
LÀ = 14 2, eo... n of bands_/ 
where: 
E, is the spectral irradiance (w/cm2) 
L4 ( 99) is the spectral reflected radiance (w/cm? sr) 
averaged over the horizontal projection of 
the solid angle ( 2p) 
£5 (2) is the filter function of band i 
is the bandwidth of band i.