DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Pronounced mehta-MAIR-ik
and meh-TAM-er-ism.
For a textbook lowpass filter,
see Figure 16.23 on page 162.
To specify a color, it is not necessary to specify its spectrum
– it suffices to specify its tristimulus values. To
reproduce a color, its spectrum need not be reproduced
– it suffices to reproduce its tristimulus values.
This is known as a metameric match. Metamerism is the
property of two spectrally different stimuli having the
same tristimulus values.
The colors produced in reflective systems – such as
photography, printing, or paint – depend not only upon
the colorants and the substrate (media), but also on the
SPD of the illumination. To guarantee that two colored
materials will match under illuminants having different
SPDs, you may have to achieve a spectral match.
Scanner spectral constraints
The relationship between spectral distributions and the
three components of a color value is usually explained
starting from the famous color-matching experiment.
I will instead explain the relationship by illustrating the
practical concerns of engineering the spectral filters
required by a color scanner or camera, using Figure 21.3
opposite.
The top row shows the spectral sensitivity of three
wideband optical filters having uniform response across
each of the longwave, mediumwave, and shortwave
regions of the spectrum. Most filters, whether for electrical
signals or for optical power, are designed to have
responses as uniform as possible across the passband,
to have transition zones as narrow as possible, and to
have maximum possible attenuation in the stopbands.
At the top left of Figure 21.3, I show two monochromatic
sources, which appear saturated orange and red,
analyzed by “textbook” bandpass filters. These two
different wavelength distributions, which are seen as
different colors, report the identical RGB triple [1, 0, 0].
The two SPDs are perceived as having different colors;
however, this filter set reports identical RGB values. The
wideband filter set senses color incorrectly.
214 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES