where 
N = collected radiation 
£ s = surface emissivity 
N cn ~ black body radiance of the real surface at the 
temperature T and wavelength 
= radiance of the source 
In the bands of the visible light and the near infrared 
the contribution of the term 1 is small in comparison to that 
of the term 2; at 10 micron wavelength viceversa. 
The first term represents the emitted variation, while the se 
cond one represents the reflected radiance. 
The general criteria of the multispectral investigation (that 
is to analize an object under different wavelengths ) is to 
achieve additive information beyond the single spectral images 
by the combinations of them. 
a) False colour composition. 
The information collected in different spectral bands is 
recorded on negative film, one for each band. 
For instance we can obtain 4 transparencies describing in black 
and white the reflection of a surface concerning blue, green, 
red and infrared portions of the light. 
Normally we combined everytime 3 bands, each band having a par 
ticular colour: the vectorial addition is realized by projec 
tion, following the relationship. 
S, , . = A.a + B.b + C.c 
(a,b,c) 
where S is the resulting information, a,b,c, the versors of 
the utilized colours, and A,B,C the modulation of the respec 
tive colours. In that way we get a false-colour image, where, 
point by point we can understand the relative contribution of 
each spectral band. 
Moreover we .can enhance the contrast of the image tone depen 
ding on the argument of our investigation. A particular kind 
of elaboration is the simulation ( by means of the images re 
lative to green, red and infrared portions of the light) of 
the false colour infrared film : in this way we get an image 
similar from the point of view of the chromatic interpretation 
but with a sharper spectral resolution. 
b) Density slicing. 
Each negative frame corresponding to a single band can 
be subdivided in series of density levels, that is, in iso- 
reflective areas. In a photographic way this result is achie 
ved by the AGFA Contour system. The position of the level 
chosen within the grey scale depends on the exposure time whi 
le the width of the level itself depends on the density of 
the yellow filter used.