Even the most elaborate exposure system begins with a tiny device called a photodiode
orphotovoltaic cell. It’s a specialized form of a transistor, or semiconductor, so called
because in one situation it conducts electricity and in another situation it acts as an insula-
tor, blocking the flow of current. A lens on one side of the semiconductor concentrates light
toward the photodiode.1
The light passes through a thin layer of glass and through a
metallic grid that makes up the negative of two electrical con-
tacts that connect the diode to the rest of the light-measuring cir-
cuit. The grid must have enough open space for the light to hit
the diode beneath it, but the grid must be substantial enough to
easily carry the current the diode will produce. A metal layer
covering the entire surface of the opposite side of the diode is
the positive contact.2
The activity creates an imbalance in positive and electrical
charges. If the negative and positive contacts on opposite sides
of the diode are connected with an electrical circuit, the elec-
trons corralled in the N-layer rush through the circuit to join
with the holes, creating a flow of electricity. This is the
photovoltaic effect.7
In old camera light meters, the electrical
current passed through an electromagnet,
which in turn moved a metal needle on a
gauge a distance proportional to the
amount of current, which was propor-
tional to the strength of the light creating
the current. Photographers read the light
level from the gauge and set their expo-
sure accordingly.8
Today, the current passes through an analog-to-digital converter (ADC), which translates the
strength of the current into a numerical value that a camera’s exposure system and microproces-
sor uses to set the exposure automatically.9
CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 69