Visual Cryptography for Color Images - Visual Cryptography and Secret Image Sharing - page 32

FIGURE 2.1
(See color insert.) Pixels superposition: black and white (left) and colored
(right).
today we have a good understanding of light and colors; the topic is quite
complex and a rigorous and detailed explanation goes beyond the scope of
this chapter. However, we will try to explain the basic properties of light and
colors because they are crucial for any visual cryptography scheme that deals
with color images.
Roughly speaking, light consists of electromagnetic energy with wave-
lengths in the approximate range of 350{750 nm, as shown in Figure 2.2.
The visible range represents only a small fraction of the full electromagnetic
spectrum.
FIGURE 2.2
(See color insert.) Electromagnetic spectrum.
When a particular wavelength in such a range hits the retina in our eyes, it
is perceived as a color. In the visible range, shorter wavelengths are perceived
as bluish colors, middle wavelengths as greenish colors, and higher wavelengths
as reddish colors. When our eyes are hit by several wavelengths we perceive a
color that is a sort of "sum" of the wavelengths. If the eyes are hit by all the
visible wavelengths, the perceived color is white. That is, a (pure) white light
consists of all the visible wavelengths.
The expressions "red light," "yellow light," etc., are technically incorrect,