American Cinematographer - April 2018 - 16

SHOT CRAFT

How LEDs Work
By Jay Holben

LED is an acronym for light-emitting diode. The modern LED
was invented in the 1960s, and initially was only capable of emitting
a red light at very low intensity; further advancements paved the way
to brighter red, then orange and green. These early LEDs were often
relegated to serve as simple indicator lights in electronic devices -
like the power indicator on your long-gone VHS player. They required
very little energy and were long-lasting.
LEDs create light from an electroluminescence effect in a
semiconductor material. To achieve this, the semiconductor is
"doped" by adding additional elements that change its properties,
so that it can have both a positive region and a negative region. A
junction is formed between these two regions, aptly called the P-N
junction (positive-negative). In the doping process, the electrons from
each atom on the positive side are removed, creating an "electron
hole"; this polarity imbalance makes each atom positively charged
and eager to find its missing electron in order to bring balance to the
Force - er, semiconductor.
When a current is applied to the semiconductor, electrons
pass from the negative side, across the P-N junction and into the
positive side, where they fall into these "holes," causing the electrons to release energy in the form of a photon, which in turn emits
light. So light is thereby produced without heat and without requiring gas to energize the photons, as happens in fluorescent and HMI
fixtures. This process is known as electroluminescence or solid-state
lighting because the creation of photons happens entirely within the
semiconductor.
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April 2018

This also means that the LED is a remarkably simple piece of
hardware. There is a cathode and an anode, forming a diode - the
semiconductor substrate itself - and an epoxy dome that acts as a
lens to concentrate the emission of light.
In 2014 Shuji Nakamura, Isamu Akasaki and Hiroshi Amano
were awarded the Nobel Prize in Physics "for the invention of efficient blue light-emitting diodes which has enabled bright and
energy-saving white light sources," as the official website for the
Nobel Prize explains. The blue LED has paved the way to making
brighter and more effective fixtures that have replaced utility fixtures
such as sodium- and mercury-vapor streetlights, traffic lights and
architectural lights. The use of LED technology for these applications
requires significantly less energy. It's estimated that 20 percent of
global energy is dedicated to lighting; the sufficient use of LED technology can reduce that number to 4 percent.
Through specific phosphor mixtures, we can alter and control
the color of the diode, resulting in LED fixtures that are suitable for
photographic use. An "applied phosphor" is one that's applied
directly to the surface of the diode, while a "remote phosphor" is
placed a distance away from the diode. For our purposes as cinematographers, the important colors are red, green, blue, tungsten
(with a CCT of 3,200K) and daylight (with a CCT of 5,600K or
6,500K). Each diode is only capable of emitting a single color; many
fixtures today incorporate LEDs of different colors - for example,
red, green and blue - enabling the cinematographer to fade
between the different diodes in order to create nearly any color of
the rainbow from that single fixture.
Phosphors that are placed on the surface of the diode can
shift in color output as the diode heats up. Yes, LEDs do create some

American Cinematographer

Rogue One photo courtesy of Lucasfilm Ltd.

A massive array
of WinVision Air
9mm LED panels
provide a view
of hyperspace
for a scene in
Rogue One: A
Star Wars Story,
shot by Greig
Fraser, ASC,
ACS.

Table of Contents for the Digital Edition of American Cinematographer - April 2018