Image Processing Reference
In-Depth Information
2
Color
Color
does not exist as a label inherent to a surface, but rather it is a result of our
cerebral activity, which constructs it from further processing of the photoreceptor
signals. However perceptional, the total system also relies on a sensing mechanism,
which must follow the strict laws of physics regulating the behavior of light in its
interaction with matter. These laws apply from the moment it is reflected from the
surface of an object, until the light photons excite the human photoreceptors after
having passed through the eye's lens system. The light stimulates the photorecep-
tors, and after some signal processing both in the retina and in other parts of the
brain, the signals result in a code representing the color of the object surface. At the
intersection of physics, biology, psychology, and even philosophy, color has attracted
many brilliant minds of humanity: Newton, Young, Maxwell, and Goethe to name
but a few. Here we discuss the color sensation and generation along with the involved
physics, [166,168], and give a brief account of the signal processing involved in color
pathways, evidenced by studies in physiology and psychology [146, 235].
2.1 Lens and Color
The role of the lens is to focus the light coming from a plane (a surface of an ob-
ject) at a fixed distance on the retina which contains light-sensitive sensors. Without
a lens the retina would obtain reflected rays coming from different planes, at all
depths, thereby blurring the retinal image. For a given lens curvature, however, the
focal length varies slightly, depending on the wavelength of the light. The longer
wavelengths have longer focal lengths. A light ray having a wavelength interpreted
by humans as red has the longest focal length, whereas bluelight has the shortest
focal length. Humans and numerous other species have dynamically controlled lens
curvatures. If human eyes are exposed to a mixture of light having both red and blue
wavelengths, e.g., in a graph, the eyes are exposed to fatigue due to the frequent lens
shape changes. The lens absorbs light differently as a function of the wavelength.
It absorbs roughly twice as much the blue light as it does the red light. With aging
this absorption discrepancy with wavelengths is even more accentuated. As a result,
