November 1961 Popular Electronics
Table of Contents
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In 2012, Webster's dictionary defines
luminescence as, "the low-temperature emission of light
produced especially by physiological processes (as in the firefly),
by chemical action, by friction, or by electrical action." Per
this 1961 article on electroluminescence in the November 1961
edition of Popular Electronics, Webster defined it back then
as, "any emission of light not ascribable directly to incandescence,
and therefore occurring at low temperatures." Interesting that
the contrast to incandescence is no long used. Maybe in 1960
the phenomenon of electroluminescence was still new and novel
enough to emphasize the distinction. Barely a decade had passed
since it had moved out of the laboratory and into the marketplace.
The ability to economically produce large panels for lighting
was deemed groundbreaking, and indeed it was. Today, our large
screen TVs rely heavily on the technology. Now, as in 1961,
a relatively short useful lifetime plagues electroluminescent
At one time only a dream of science, heatless illumination
is now a reality. Electroluminescence is being put to work to
make paper-thin panels which someday may light your home.
By James E. Pugh, Jr.
In early times, the glowing tree-trunk seen by a lone traveler
in the woods or the strange "burning" of the sea noticed by
a wide-eyed sailor seemed to portend evil and disaster. Today
we know that both are caused by chemical reactions in the cells
of tiny living things, and our superstition has changed to active
interest. Scientists have been fascinated by the heatless light
produced by certain organisms for decades but, until recently,
they couldn't even come close to duplicating it on a practical
The introduction of the fluorescent lamp (1938) was a step
in the right direction. Here, less power was wasted in the production
of useless heat than with an incandescent bulb, though the loss
was still substantial. In the late 1940's, however, the phenomenon
of electroluminescence, a direct conversion of electricity to
a "cold" light much like that found in nature, moved out of
the laboratory. Today you can buy an electroluminescent lamp
for a few cents in any hardware store.
Sandwich-like construction of an electroluminescent
lamp can be seen in this simplified drawing. The front plate
has a transparent conductive coating which acts as an electrode,
yet passes light.
But just what is electroluminescence? Well, Webster defines
luminescence as "any emission of light not ascribable directly
to incandescence, and therefore occurring at low temperatures."
Generally speaking, luminescence is caused by excitation of
the electrons in a chemical compound called a "phosphor." In
the case of living things which luminesce, the excitation is
brought about chemically and the phenomenon is usually referred
to as chemiluminescence. Therefore, electroluminescence, as
you might guess, is luminescence produced by a phosphor which
has been excited electrically.
The manufacturing equipment, in theory, imposes
the only limitation on the size of the lamps. Here, a phosphor
layer is being baked in an infrared oven.
Electrical "Sandwich." Though electroluminescent lamps are
not yet efficient enough to provide general lighting (being
most commonly used as night-lights), they have some unique physical
and electrical characteristics which suggest many uses. The
lamps are made by sandwiching a layer of phosphor (often zinc
sulfide) between two plates of electrically conducting material.
The front plate usually consists of glass having a special transparent
coating which allows it to act as an electrode, and the. complete
unit is not much thicker than that plate.
When an alternating current is connected across the two plates,
the electrons in the phosphor are excited to a higher energy
level - causing it to emit a soft, glareless light with almost
no accompanying heat. The color of the light depends on the
selection of the phosphor and the impurities which are deliberately
introduced into it. Green, blue, orange, yellow, gold, red,
and white phosphors are available, but at present green yields
the most light.
the way in which light is produced by an electroluminescent
lamp with the operation of standard fluorescent or incandescent
units. Though a phosphor is also used in the fluorescent lamp,
it is not excited directly by an electric current - but rather
by ultra-violet rays. These rays are produced (along with wasted
radiations at other frequencies) by an electrical discharge
through mercury vapor gas. In the case of the incandescent lamp,
of course, the light is only a byproduct of the heat generated
in a fine wire by the passage of an electric current.
Graphs show brightness versus frequency (left)
and voltage (right) for lamps of different ratings.
Advantages and Drawbacks. The new light source is usually
produced as a flat sheet, but it can be cut into almost any
conceivable shape. And, since no vacuum is required for its
operation, the only limits to its physical size are those imposed
by the manufacturer's equipment. One unit recently made by Sylvania
measures eight by two feet - a whole wall could be made to glow
with a soft, pleasant light by installing a number of these
panels side by side.
Typical Characteristics of Electroluminescent
Freedom from "burn out" is one feature of the new light source. Light output
simply diminishes with time as illustrated in this graph of
a typical lamp operating at 120 volts, 60 cycles. One manufacturer's
conservative estimate of the maximum usable life of his product
under these operating conditions is 5000 hours. Another maker
claims up to 20,000 hours of useful operation.
The electroluminescent lamp's main drawback is its present
inability to produce enough light for high-intensity applications.
The light output of a given phosphor, however, can be increased
by raising its supply voltage and/or frequency. Though 120-volt,
60-cycle operation is common, special oscillators have been
used to provide higher frequencies (up to 20,000 cycles), and
lamps rated at voltages up to 600 are readily available. An
interesting idiosyncrasy of these lamps is that, with some phosphors,
a color shift occurs as the frequency is raised. In the case
of one green phosphor, the color changes to blue at frequencies
over 400 cycles.
Characteristics of Electroluminescent Lamps
||Available in standard voltages from 120 to 600 volts
||Most useful range: 25-1000 cycles
|Resistance (of a square foot)
||24,000-55,000 ohms, depending on voltage rating
||0.35-0.109 uf./sq. ft., depending on voltage rating
(@ 120 v., 60 cps)
|16-36 ma./sq. ft.
(@ 120 v., 60 cps)
(@ 120 v., 60 cps)
It will be a while before you can light your house with glowing
ceiling or wall panels but, in the meantime, electroluminescent
lighting will find plenty of applications. In addition to their
other advantages, these "miracle" lights have a usable life
expectancy of up to 20,000 hours. During this time a burn-out
never occurs - the light intensity simply decreases slowly until
it reaches a level so low that the unit must be replaced. Then,
too, power consumption is quite low. A Westinghouse "Rayescent"
lamp, for instance, draws only 0.6 watt per square foot when
operating at 120 volts, 60 cycles.
What may be the world's largest electroluminescent
lamp measures eight by two feet. It has a brilliance of five
foot-lamberts, an estimated life of five years.
Present and Future. Long life, freedom from "catastrophic"
burn-out, and glareless light make these lamps particularly
well suited for use as indicators of all kinds. Among the applications
in this category which have been suggested or already developed
are lighting for aircraft, auto, and boat instrument panels,
highway signs, and "read-out" numbers.
>Interior decorators, too, are excited by the possibilities
of this flexible light source. Glowing walls and ceilings (primarily
for decorative effect rather than illumination) are already
a reality, and electroluminescent panels have been incorporated
into many types of furniture. Perhaps in the not-too-distant
future this revolutionary lighting material will be improved
to the point where it can be used for general lighting as well
as decoration - and you may have to look in an antique store
to find one of today's "old-fashioned" light bulbs.
Electroluminescent lamps are well suited for use as indicators
of all kinds.
Electroluminescent street sign.
Night-and-day photos of a tractor dashboard
incorporating the new light source
Table top emits a soft "candlelight" glow
flattering to diners
Posted September 2, 2012