Page 42 - Fundamentals of Light Microscopy and Electronic Imaging

POSITIVE AND NEGATIVE COLORS 25

Absorbance

B Rh G R

400 500 600 700
Wavelength (nm)
Figure 2-8
Absorption difference spectra of the four human visual pigments. The four pigment proteins
were cloned, purified, and characterized with respect to their absorption spectra in vitro.
Photobleaching difference spectra were obtained by subtracting an absorption spectrum
measured after light exposure from one measured prior to light exposure. The pigments
show maxima in the expected red, green, and blue regions of the visual spectrum. The
values are close to those measured for rod and cone cells in vivo and confirm Maxwell’s
theory for RGB-based color vision over a century ago. (Courtesy of Jeremy Nathans, Johns
Hopkins University.)

1 cm diameter hole in the center. Three color disks can be projected on a screen and
made to overlap as shown in Figure 2-9. Try it and experience why mixing magenta and
green light gives white. Negative colors, in contrast, are generated by the subtraction
(absorption) of light of a specific wavelength from light composed of a mixture of wave-
lengths. A pigment that looks red, for example, absorbs blue and green wavelengths, but
reflects red, so it is red by default. To appreciate this point, it is informative to examine
colored objects (paints and pigments) with a handheld spectroscope under bright white
light illumination. It is fascinating that yellow, cyan-blue, and magenta pigments are
composed, respectively, of equal mixtures of red and green, green and blue, and blue
and red wavelengths.
Thus, perception of the color yellow can arise in two ways: (1) by simultaneous
stimulation of the red and green cone cells by a monochromatic yellow (580 nm) light
source—the red and green photovisual pigments exhibit broad excitation spectra that

Y = yellow
Green Y Red
M = magenta
W C = cyan (blue-green)
C M W = white

Blue

Figure 2-9
Addition colors of the red-green-blue tricolor system. This color display can be produced by
projecting the colors of RGB color filters with three separate slide projectors on a screen as
described in the text.