Page 43 - Fundamentals of Light Microscopy and Electronic Imaging

26 LIGHT AND COLOR

overlap significantly in the 580 nm band region and are both stimulated almost equally;
or (2) by stimulating the red and green cones separately with a mixture of distinct red and
green wavelengths, each wavelength selectively stimulating red and green cone cells in
the retina. In either case, the color yellow is defined as the simultaneous stimulation of
both red and green visual pigments. Perception of other colors requires stimulation of
one, two, or all three cone cell types to varying degrees. The mixing of different colored
paints to produce new colors, which is our common experience in producing colors, is
actually a subtractive process. Consider why mixing yellow and blue paints produces a
green color: Yellow pigment (reflects red and green, but absorbs blue) and blue pigment
(reflects blue and green, but absorbs red) gives green because green is the only wave-
length not absorbed by the mixture of yellow and blue pigments. Thus, combining blue
and yellow wavelengths of light gives white, but mixing blue and yellow pigments gives
green! Removal of a specific wavelength from the visual spectrum is also the mechanism
for producing interference colors, and is discussed in Chapter 9. A useful overview on the
perception of color when viewing natural phenomena is given by Minnaert (1954).

Exercise: Complementary colors

A complementary color is a color that gives white light when mixed with its com-
plement. Thus, yellow and cyan-blue are complementary colors as are the color-
pairs green with magenta and red with cyan. Our perception of complementary
colors is due to the red, green, and blue photovisual pigments in the cone cells of
the retina. Note that mixing wavelengths of different colors is distinct from mix-
ing colored pigments.
Combining Red, Green, and Blue Light. To experience the relationships
among complementary colors, prepare 3 slide projectors each containing a red,
blue and green color filter sandwiched together with masks containing a 1 cm
diameter hole, and project three disks of red, green and blue color on a projection
screen. Focus each projector so the edges of the circular masks are sharp. Move
the projectors to partially overlap the colors so that it is possible to see that red
plus green gives yellow, red plus blue gives magenta, and blue plus green gives
cyan. The overlap of red, green and blue gives white. Thus, when all 3 color types
of cone cells in the retina are saturated, we see white light.
Mixing Colored Pigments. As is known, mixing yellow and blue pigments
gives green. The reason for this is that blue and yellow pigments reflect green
light; all other wavelengths are absorbed by the blue and yellow dyes in the mix-
ture. To demonstrate this, prepare 2 beakers with 500 mL water and add 8 drops
of blue and yellow food coloring separately to each beaker. Display the beakers
on a light box. The generation of green by mixing the yellow and blue pigmented
solutions is different from the mixing of blue and yellow light, which gives white
light, as was demonstrated above.
Removing Colors from a White Light Source. The relationship between com-
plementary colors and subtraction colors can be demonstrated using a bright
white light source, a slit, a diffraction grating and a large diameter magnifying
glass to form the image of the slit on a projection screen. Set up the optical bench
apparatus with a bright xenon light source, a 1 mm wide slit made from razor
blades, an IR blocking filter, and a holographic diffraction grating as described in
Figure 2-10. Intercept the dispersed color spectrum with a white card and exam-