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In addition to multispectral applications, true colour images become more and more important for photogrammetric 
applications and are to be derived from colour processed multispectral images. Spectral position and spectral bandwidth 
of both multispectral and true colour imagery are different. Multispectral applications need non-overlapping narrow 
bands and an infrared channel close to the vegetation red edge, whereas true colour channels are closely related to the 
eye's sensitivity and therefore require broad band overlapping channels. The main focus of the spectral optimisation of 
the colour channels lies therefore on the multispectral applications. True colour images are derived from multispectral 
channels afterwards. Table 1 lists spectral channels of spaceborne and airborne multispectral instruments. 
The optimisation of the multispectral channels can only be achieved with dedicated applications. The channels are 
selected by focussing on classification and information content for different vegetation types, bare soils and sealed 
areas. Simulations also show that the wavelength range of the panchromatic channel must be related to the multispectral 
channels. Therefore only small changes in the spectral position and bandwidth are possible for the optimisation of the 
true colour channels. To generate true colour from multispectral channels an optimal transformation is used with three 
colours and, if necessary, an additional panchromatic channel. The transformation parameters must be related to future 
applications of the airborne scanner. This restricts colour space. 
2  ADS40 
The LH Systems ADS40 airborne digital sensor is the next generation digital sensor for airborne photogrammetry and 
remote sensing. The ADS40 has a modular customised concept with three panchromatic and four multispectral CCD 
lines. The three panchromatic sensor lines produce the forward, nadir and backward views along the strip. Each 
panchromatic line consists of two linear arrays, each with 12000 pixels, staggered by 0.5 pixels. Each sensor line is 
measured during camera calibration so that the precise position of each pixel on the focal plane is known. 
21 Design Criteria for Multispectral Systems 
The number of channels desirable in a given spectral region depends on their half-width, which limits on the one hand 
the signal-to-noise ratio and the energy reaching the detector. But on the other hand the larger the number of channels 
used the better defined is the information measured. This describes the situation of a typical optical spectrometer. 
Because of the limited energy, spectrometers with high spectral resolution cannot reach fine spatial resolution. The 
ADS40, however, is tailored above all to photogrammetric tasks and needs a very fine spatial resolution. Hence, the 
channel width should not be smaller than 50 nm. With this assumption four or a maximum of five channels provide the 
maximum information. Some of the tasks given below require corrections to the data sets (atmospheric correction, 
correction with respect to the viewing angles, BRDF effects) or a suitable geoinformation system. 
22 Design Criteria for True Colour Systems 
As mentioned above it is impossible to derive an overall approach to transform multispectral to true colour images. 
Therefore a restriction on the future tasks of this sensor system is necessary. The optimisation criterion during the 
derivation of the transformation parameters is the colour impression of imaged objects. For that purpose 65 spectral 
surface reflectances where selected from the NASA-Reference (1985), hereafter referred to as “the NASA 
reflectances". The influence of the atmosphere on the colour impression was simulated with a standard atmospheric 
model (MODTRAN 3.5). During simulations several parameters - flight altitude, illumination, extinction and aerosol 
type - were changed. To fill the whole colour space an additional standard colour test set (DIN 6169 part 7) was used. 
This approach allows the fine tuning of the spectral channels and the derivation of transformation parameters for true 
colour representation. 
3. SELECTION OF THE SPECTRAL CHANNELS 
The combination of the channels depends on the object that is going to be investigated and the task to be performed. 
The following tasks are identified: 
l. Presentation in true colours is preferred in order to 
* observe and document the surface 
e map out land use 
* eliminate cloudy areas 
* locate ice sheets in water areas. 
It is also desirable to use the blue channel, because in this wavelength range the scattering by air particles is very 
high, which allows the sensor to receive information from the shadow areas. Mapping of land use and 
documentation of the development of landscape is required by government ministries and authorities (for example 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 245