0 Color Fidelity versus Spatial Resolution

computational photography

 The family of CFA patterns selected in this chapter is motivated by recalling contrast
sensitivity research showing human sensitivity to luminance contrast is very different from
human sensitivity to chrominance contrast.      Reference [70] examines the dependence of
chrominance contrast sensitivity on spatial frequency and on illuminance level; it was found
that contrast sensitivity degrades at lower luminance levels, for both chrominance and lumi-
nance. Despite limited comparison with luminance contrast sensitivity, the results suggest
that chrominance sensitivity degrades past one cycle/degree, while luminance sensitivity
peaks  near  two  cycles/degree.  Reference  [71]  provides  a  more  in  depth  comparison  of
chrominance and luminance contrast sensitivity.


 This work finds that red-green and blue-
yellow contrast sensitivity functions have similar spatial bandwidth, which is roughly 13
of the bandwidth of luminance contrast sensitivity.  It was also found that luminance con-
trast sensitivity degrades below about 1 to 2 cycles/degree, while chrominance sensitivity is
constant below about 13 to 23 cycles/degree. The similarity of red-green and blue-yellow
contrast sensitivities and their substantial different from the luminance contrast sensitivity
suggests the decoupling of spatial detail and luminance sensitivity from color sensitivity.
The clearest way to accomplish this is to provide a highly sensitive luminance channel in
addition to three channels for chrominance data.  The spectral response of the panchro-
matic channel is colorimetrically inaccurate for luminance,but it provides the best possible
signal-to-noise for a given sensor.


Introducing panchromatic pixels into a three-channel CFA pattern and allowing the color
sampling to drop off provide color resolution that is roughly 13 to 14 the panchromatic
sampling.  Assuming that color artifacts and chromatic aliasing are successfully limited in
demosaicking, this approach, fusing a panchromatic image with a lower resolution color
image, provides a capture system most closely mimicking the capabilities of the human
visual system under most imaging conditions.