1. INTRODUCTION
Colour Infrared (CIR) aerial photographs are widely used
in many kinds of rural operations and planning. The high
reflectance of vegetation in the infrared band as well as
the great distinctiveness between different plants and
conditions of vitality make it an indispensable means for
most applications especially in those areas where inter-
pretation is needed.
Large scale orthophotos - especially digital orthophotos -
are of increasing importance as a base of maps: The fast
production, the many details visible, the high grade of
actuality, etc., are just some advantages over traditional
maps. These benefits, combined with the nowadays
available huge amount of computing power and storage
capacity, will certainly show a triumphant march of the
digital orthophoto in most large scale applications in the
future.
The problem now arises that the non-specialist often
feels confused and irritated by the false colour infrared
images. People, like politicans, officials, citizens, want to
see the vegetation in realistic colours rather than in bright
red. One solution could be to have two photo flights (or
one photo flight with two cameras aboard): One to take
photographs with infrared film material in order to
produce orthophotos for interpretation purposes; the
other one to take photographs of the terrain on true
colour based film material in order to produce
orthophotos for the final visualization and presentation - a
way which is normally not possible because of the high
costs.
Therefore the only practical solution of this dilemma is to
take photographs with that film material that has better
spectral and radiometric characteristics for the
interpretation of vegetation - this is the colour infrared
film material - and to redisign it to a true colour photo.
2. THEORETICAL ASPECTS
The information transfer from objects to remote sensing
sensor systems is done by electromagnetic radiation.
Photographic systems record the object reflected sun
radiation onto the emulsion of a photo. The
photographically produced picture describes on the one
hand the geometric characteristic of an object, on the
other hand there is also a pysical description of the object
in form of the colour of the object. The colour on a photo
is the result of intensity and spectral compounds of the
reflected sun radiation.
Using colour-infrared-film the visible and near infrared
part (400nm - 900nm) of the electromagnetic spectrum is
used for the information transfer. In case of a true-colour
photo the emulsion of the film is sensitized only for the
visible part of the spectrum (400nm - 700nm). As the
spectrum of true colour photograph is a matching part of
the spectrum of colour-infrared photograph it seems to
be realistic to find a direct relationship between the
colours of objects recorded to both of the different film
types. However this is only on the first sight: By a detail-
led consideration of the problem a lot of colour desturbing
influences - as shown below - can be found:
470
daily and seasonal change of the angle between
sunposition and surface element: due to this fact the
intensity of the reflected sun radiation is reduced ty
sin8 (0: sun height) related to a vertical angle of inc.
dence (090^).
Interaction mechanisms of sun radiation and reflected
sun radiation with the atmosphere: penetrating the
atmosphere the sun radiation will be scattered, ap.
sorbed and reradiated by particels of clouds, haze
vapour, fog, smog etc. (irradiance of sunlight and
skylight). Furthermore the reflected part of the sun
radiation will be desturbed by similiar effects, The
amount of these effects is correlated with the
condition of the atmosphere. All effects concerning
the interaction mechanisms of sun radiation and
reflected sun radiation with the atmosphere are
wavelength dependent.
cameraspecific distortion of radiation: Within the ob-
jective of a photogrammetric camera the radiation de-
creases again: besides a position invariant reduction
factor there is a component that depends on the field
angle 4:
AE = prop. cos*t (1)
(for modern photogrammetric cameras) and on the
wavelength A of the incoming radiation.
filter specific distortion of radiation: Dependent from
the type of filter used the intensity and/or the spectral
structure of the incoming radiance. Colour infrared
photographs usually are taken with a yellow filter to
avoid the influence of skylight. So the effective
electromagnetic spectrum that is recorded wih
colour-infrared film material lies between 500nm and
900nm.
film specific recording of radiation and photographic
developing process: Dependent on the intensity and
the spectral structure of radiation, on the
characteristic and spectral sensitivity curves of the
film, and last but not least on the time during which
the irradiance is incident on the emulsion surface, the
three dye layers (cyan, magenta and yellow in the
case of a subtractive photographic process) of the
film - after the developing process - will be coloured in
relation to the specific parts of the spectrum.
object specific reflectance of radiation: Besides the
above mentioned influences the main part of radiome
tric differences (colour and intensity) on the photo
graph is caused by the characteristic of reflectance o
the specific object. The variation in reflectance
dependent on the geometry (form, structure of sur
face), and the chemical and physical characteristic of
the objects. A complete and definite description of the
reflectance characteristic of an object can be given bj
the reflectance function that characterizes the relatio
between the incoming radiation (L, within a differenti
solid angle dO, with the direction 0, ;$, ) and the re
flected radiation (dL, with the direction 0,9)
dL.(8., D, ©, dy
Pe iSt) E
Le( ©. o.) cos .- dQ.
f(8. CON e. ®,) =
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
By
be
er
3.1
Wit
hei
nec
ger
(ap
usi
apr
nat
eac
par
ad
For
me:
den
duc
ject
orth
can
colc
the
feel
the
sim
The
Wer
all t
Aus
and
