How Digital Photography Works

How Digital Cameras Create a

Whiter Shade of White

Eyes do only half the work of seeing. They generate a lot of nerve impulses in response to stimulations from light striking the retina. But you
ain’t seen nothin’ until the brain interprets the impulses. All the information that seems to come so naturally through the eyes—shape, size,
color, dimension, and distance—is the result of the brain organizing sensory information according to a set of rules the brain starts compil-
ing soon after birth.

The truth of this is found in how we can play visual tricks on the brain with optical illusions, 3D movies, and trompe l’oeilpaintings. The
brain plays one trick on itself: It convinces itself that white is white even though under different lighting—daylight, incandescent, fluorescent,
and so on—white might actually be gray, brown, or blue. Photographs, though, are callously truthful. They capture light that the brain
insists is white and reveal its true color. This is why digital cameras have a feature called white balance.

White light is the combination of all the colors of the visible spectrum, something easily demonstrated by shining

a white light through a prism. Different light sources, however, produce what the brain is willing to accept as

white by combining the colors of the spectrum in different proportions from those found in daylight. Some

sources use colors from a continuous spectrum. Others use light from only a few narrow bands in the spectrum.

1

The colors these light

sources produce are corre-

lated to the temperature of

the light measured in Kelvin

(K), which starts with 0 at

absolute zero (–273°

centigrade [C]). The use of

Kelvin is more convention

than science. The Kelvin

temperatures assigned to

colors do not translate to the

heat generated to produce

the colors. In other words,

light that has a warmer

appearance has a lower

temperature and light that

has a cooler appearance

has a higher temperature.

2

Artificial Light Sources Kelvins

Match flame 1700

Candle flame 1850

40-watt incandescent tungsten lamp 2650

100-watt incandescent tungsten lamp 2865

Photoflood or reflector flood lamp 3400

Common fluorescent tube 4000

Xenon arc lamp 6450

Daylight Sources

Sunrise or sunset 2000

Early morning or late afternoon 4300

Noon 5400

Overcast sky 6500

Average summer shade 8000

All Kelvin measurements are approximate.

78 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES