Ralf Reulke 
  
4 SELECTION OF THE TRUE COLOUR CHANNELS 
4.1 Human Colour Perception 
The colour values F = { R, G, B }" describe human colour perception with regard to a colour synthesis system and are 
ultimately defined by the comparison method. Historically, as the first colorimetric interface, the RGB-CIE coordinates 
are based on three narrow-band stimuli (10 nm) at 700 nm (R), 500 nm (B), and 400 nm (B). Because technical colour 
synthesis systems (i.e. in colour televisions and monitors) are based on the spectral emissions of phosphors, modified 
colorimetric interfaces are relevant. In Europe, this interface corresponds to the EBU norm (European Broadcasting 
Union) for so called EBU phosphors. For every colour stimulus @(À), a synthesised emission must be generated by 
weighting of the normalised spectral densities from the EBU phosphor pr(A). This spectral density is according to the 
sensitivity of the cones in the human eye, 
z={z, 22.23) =[P,d,4)' 2) 
and leads to the identical colour sensation F as in the case of the stimulus @(A). 
780nm 780nm 
Jarmo = [arz) Rp) +6 poh) +R ppb G) 
380nm 380nm 
A true-colour camera gives these RGB weights. 
F -(R,G, BY. -k [4A 900 50) (4) 
À 
s={sy, 5,53} is the overall sensitivity of the three camera channels. Substituting (4) into (3), and considering 
narrow-band emissions (À) 2 9(A-Ag) 
780nm 
k [dr 6(A-20) shy =k sho) and [dA 2A) SCA- Ag) =; (Ao) (5) 
À 380nm 
leads to the system of linear equations to determine the necessary sensitivity of the three sensor channels at Ao 
780nm 
z00)=| Jah 4% p" @ ks) (6) 
380nm 
The variation of Ao (Ao € [380nm, 780nm] ) gives the spectral values curve of the EBU phosphor, 
SA) 7 (5,4) $5) 5300 Y" - (0), 800, b)" (7) 
which, as the overall sensor sensitivity, must be obtained for true-colour cameras (including negative portions). With 
the uniform sensitivity s( A ) of the sensor material and the Luther condition 
s(A) 2 5 (A): s(À) (8) 
the required filter curves s(À) (along with their negative portions) can be separated. In colour cameras, these 
technologically achievable transmission functions are approximated by implementable filter curves, with the help of 
linear transformations: 
s (A) 2T: sc (Q) (9) 
Requirements for this are three wide-band filter curves with different band centres, and sufficient overlap, so that the 
entire visual spectrum is covered. 
4.2 Optimisation of True Colour Representation 
This paper starts with the foundation of multispectral and true colour channels and explains the scientific derivation of 
these channels. With this set of spectral channels and an additional panchromatic channel, three different approaches for 
colour transformations were investigated for an optimal true colour representation: 
e Optimisation based on an overall sensor sensitivity 
e Optimisation of colour values of the expected target 
e Non-linear adjustment. 
Calibration tests were run using the results of all optimisation approaches. The calibration tests used 17 DIN test colours 
and 65 NASA reflectances under D; lighting conditions. The results of the first optimisation approach showed mean 
square error AE with values of 6.09 for the DIN test colours and 3.89 for the NASA reflectances. The perceptible 
threshold level is 3.0. 
  
248 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 
  
ENG Gly un wl