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21.1. Colorimetry 533
offeran explanationas to why certain wavelengths of light are absorbed and others
are reflected. We can therefore not use reflectance functions to explain why the
light reflected off a banana has a spectral composition that appears to us as yellow.
For that, we would have to study molecular orbital theory, a topic beyond the
scope of this book.
Finally, when light reaches the retina, it is transcoded into electrical signals
that are propagated to the brain. A large part of the brain is devoted to processing
visual signals, part of which gives rise to the sensation of color. Thus, even if
we know the spectrum of light that is reflected off a banana, we do not know yet
why humans associate the term “yellow” with it. Moreover, as we will find out in
the remainder of this chapter, our perception of color is vastly more complicated
than it would seem at first glance. It changes with illumination, varies between
observers, and varies within an observer over time.
In other words, the spectrum of light coming off a banana is perceived in the
context of an environment. To predict how an observer perceives a “banana spec-
trum” requires knowledge of the environment that contains the banana as well as
the observer’s environment. In many instances, these two environments are the
same. However, when we are displaying a photograph of a banana on a moni-
tor, then these two environments will be different. As human visual perception
depends on the environment the observer is in, it may perceive the banana in the
photograph differently from how an observer directly looking at the banana would
perceive it. This has a significant impact on how we should deal with color and
illustrates the complexities associated with color.
To emphasize the crucial role that human vision plays, we only have to look
at the definition of color: “Color is the aspect of visual perception by which an
observer may distinguish differences between two structure-free fields of view of
the same size and shape, such as may be caused by differences in the spectral
composition of the radiant energy concerned in the observation” (Wyszecki &
Stiles, 2000). In essence, without a human observer there is no color.
Luckily, much of what we know about color can be quantified, so that we
can carry out computations to correct for the idiosyncrasies of human vision and
thereby display images that will appear to observers the way the designer of those
images intended. This chapter contains the theory and mathematics required to
do so.
21.1 Colorimetry
Colorimetry is the science of color measurement and description. Since color
is ultimately a human response, color measurement should begin with human