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.