Theatre Design & Technology - Dec 1969 - 25

Glass Properties Important to
Stage and Studio Lighting
JOHN P. HOXIE, Corning Glass Works
Corning, New York
Mr. Hoxie's paper is based on a presentation before
the Illuminating Engineering Society's annual Symposium on Theatre, Television and Film Lighting,
held May 26-27, 1969. Editor

Glass is the envelope material common to
all lights used in the studio, stage or outdoor photography. This article discusses
briefly the properties of glass which are vital
to various classes of usage_
Starting with the most rudimentary source,
the carbon arc which modern technology
has been able to maintain as our most
powerful lighting device, we locate a fresnel lens about 8" from a 200 to 300 ampere arc crater. This lens accomplishes
(1) a closure for the light producing cavity,
(2) efficiently controls the direction of light
emanation, and (3) protects skin and eyes
from burning by the ultraviolet emission.
These glass functions are inherent in all
high energy lighting devices for this field
of application whether they are incorporated
into a single piece of glass or distributed
among several components.

It is important to understand the importance
of low coefficients of thermal expansion to
lamp envelope manufacture. We employ a
scale of numbers common to the industry.
In brief, we express the coefficient of
linear expansion in parts per million per
degree centigrade which gives us numbers
between 1 and 100. That is, numbers we
multiply by 10'7 power. Ordinary nonheat resisting bulb glass has an expansion
of 92, borosilicate glass as used in Pyrex
brand lenses and "hard glass" projection
bulbs is 36. Quartz is 6 x 10- 7 per degree
C., or 6 on the same scale.
"Pyrex" and "Vycor" are registered trade names.

Laboratory rig for determining coefficient of
glass expansion

With the hermetically sealed source enclosure (which may house either an incandescent filament or a gaseous discharge),
there is not only the tendency to destroy
that enclosing bulb by termal stresses due
to uneven heating, but as well the corrosionlike attack by hot interior gases which
becomes more severe as we approach the
softening point of the glass. High intensity
light-producing reactions are among the
most destructive processes known to man
and each advance in light source technology has called for more durable lamp
envelopes.
As a measure of heat resistance we rate
glasses according to how Iowa coefficient
of expansion they have. A lens made of
very low expansion material will experience
less stress between its hottest and cool er
zones, therefore it runs less danger of
breakage.
Expansion coefficient is determined in the
laboratory on glass rod samples. A rod is
successively held in a cold chamber, then
in a hot chamber, and the amount of elongation noted as the rod goes from a temperature such as O°C to 300°C. (See photo.)

THEATER DESIGN AND TECHNOLOGY

Quartz is a pure sil ica glass derived either
from selected natural deposits or refined
industrially by chemical procedures. By virtue of its purity, it transmits the ultraviolet to a spectral degree about equal to
the air around us. This means that light
coming to us from the sun and passing
through a thick layer of atmosphere, including ozone in the upper regions, has
more of its UV removed than lamp light
through quartz. Depending upon the amount
of UV generated, a quartz lamp without
shielding may result in sun burn or eye
damage.

DECEMBER,1969

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