An electri-
cal field in the
depletion layer pulls
electrons out of the P-layer,
accelerating them into the N-
layer. It moves holes in the opposite
direction, toward the P-layer.6
The light passes through a layer of glass coated with
an anti-reflective film and into the P-layermade of sili-
con that has been infused, or doped, with boron. The
P-layer is no thicker than 1Rm—a micrometer, or one
millionth of a meter. That particular thickness is what
makes the photodiode responsive to light in the visible
spectrum.3
Longer wavelengths in the light make it through the
P-layer into the thicker N-layerat the opposite
side of the photodiode. Along the way, the light
also passes through the depletion layer formed
at the conjunction of the P-layer and the N-layer.4
Before light strikes the diode, electrons in the three layers of silicon
move in a fairly synchronized manner. When one electron breaks off
from a silicon atom, another electron takes its place, much like cus-
tomers at a busy but not overburdened lunch counter. When light
enters the silicon, however, it raises the energy level of the electrons so
that fewer of them settle into the vacancies left by other electrons. It’s
much as if all the customers at the lunch counter ate too much of the
same bad, overseasoned chili. Suddenly, they are rushing away from
the counter leaving empty seats, called holesin semiconductor terms.5
Eyes are not the only things that can
sense and respond to light. There are—
luckily for photographers—various chemi-
cals and minerals that, in combination with
each other, can sense when light is striking
them and respond by producing electricity.
It’s a good thing for photographers
because these inanimate light sensors
are immensely more sensitive and more accu-
rate than the human eye when it comes to measur-
ing exactly how much light they’re seeing. Before then,
exposing a photograph correctly was largely a matter of
guesswork and experience. Now, a camera does it for the
photographer with molecular precision while providing
options for using light creatively that photographers a cen-
tury ago could only imagine.
How a Camera
Measures Light
68 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES