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