Theatre Design & Technology - Summer 1980 - 18

Distribution Control Through Field Pattern Adjustment
(Idealized)

1()()O.Walts

2 S·x 12# lenses

150,000

100.000

-+_

-+-\-

50,000

held--Gale

lull-open

Degrees from Center

Peak. Gate reduced
Cosine. Gate full
Smooth. Gate full

Distance
(12'
Circle)

Efficiency

Central
Footcandles

Spread

36'
20'
20'

25CC
37"<
30r ,

131
202
140

19°
33°
33°

6. The range of spreads and intensities required for throws
of 10 to 100 feet and central footcandle intensities of 25 to
100 footcandles were tabulated in "Choosing a Spotlight." It
has been customary to list each unit with a single spread, that
of the field or the angle between those points where the intensity has dropped to '/,0 the central intensity; excepting
zoom equipped units, for which the extremes of spread and
intensity are usually given. The addition of the field pattern adjustment may result in the publication of limits for both spread
and intensity since, coupled with a variable gate, this adjustment introduces a range often greater than that obtainable
with a zoom system while retaining a desirable cosine distribution characteristic.
A theoretical unit with two 6" x 12" lenses has a range of
spread (to cut-off) of 23 ° to 31 0, with central intensities of
210,000 candela to 97,000 candela, respectively, using a
1 ,OOOW lamp.
Other focal length lens systems may be used with the same
lamp house, reflector and gate resulting in the following:

Lens System
2-412"x6~2"

2 - 6" x 9"
2-6"X12"
2-6"x16"
1 - 8" x 13"
1 - 10" x 23"

Spread
(to cut-off)
42° - 56°
34 ° - 41 °
23° - 31 °
l r - 23°
12° - 15°
7120 _ 8°

Central Candela
in thousands
61
104
210
255
523
1000

51
60
97
159
358
840

The figures are partly theoretical (idealized cosine distributions), while the efficiencies and spreads have been verified in
actual tests.
16

Theatre Design & Technology

Note that the range nearly fulfills the spread and intensity
requirements in the tabulation referred to earlier. Where
lesser intensities are needed, wattages of 750 or 500 may be
used. At shorter throws and wide angles, these smaller lamps
should suffice. A 250W lamp of matching description could be
obtained if there were an adequate demand, which would provide a lumen rating of approximately 5,000 lumens. Life ratings of these lamps are from 300 to 2,000 hours with lumen
au puts of 10,000 to 28,000. Coupled with extended use is the
potential for economy: Selection of proper units can reduce
wattage requirements which, in turn, can reduce the costs of
lamps, wiring and control.
7. Projections with ellipsoidal spotlights vary from the
simple shaping of the beam to the projection of complex images. This property makes ellipsoidal spotlights unique among
theatrical equipment. Instead of concentrating the light with a
condensing lens as was done with early spotlights, ellipsoidals employ a variant of an ellipsoidal reflector to capture almost 90% of the light from a small source and redirect it to fill
a gate. A lens system projects an image of the gate onto the
field in the same way a slide projector puts an image of the
slide onto a screen. Slide projector lenses are more refined
than those of theatre spotlights so the image from the
spotlight may suffer chromatic fringes, linear distortion, and
other aberrations.
Because the lighted area is an image of the gate, whatever
shaping takes place at the gate is reproduced in the field. The
design of shaping shutters, iris, templates, or projection slides
must anticipate the temperature at the gate frequently exceeding 500°C (932°F). Problems of warping, difficulty in
achieving smooth movement of the blades in and out, and in
angling blades or having them pull out or spill light is all too
common. Much of the success of the design of these units lies
in an adequate solution to these problems.
Simple patterns made of perforated aluminum enjoy widespread use. A more sophisticated procedure of photoelectrically etching stainless steel to produce more refined patterns
is available if the design requires more intricate image projection.
The ellipsoidal spotlight is a highly refined instrument but
its final effectiveness lies in the user's hands. It is hoped that
these notes will assist in obtaining the best performance from
the equipment.D

THENEW

A
N

575

METAL HALIDE

HOTELS
ARENAS
COLLEGES
THEA TRES
COLISEUMS
RINKS

MODEL 41000
FOLLOW SPOTLIGHT

M
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R
I
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- .. ;. NEW','

HALIDE::

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RESTAURANTS
MUSEUMS
FAIRGROUNDS
UNVERSITIES
AUDITORIUMS
NIGHTCLUBS

u-'

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-~.,

'C.

;.~

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S. ',.- .... ' :

$3875.00

STRONG 575

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METAL

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PtlolOIT'{;!r,_ Dan Small sc ',ra:,ns,,', Gt 225100' aroCllt:s ill a 100
lOOIH", ...
lull ~11( IOtl'Clr<_ oala please tall Of MIte us IOf S~~I.
I Cal_"n data shed

AMERICAN STAGE LIGHTING CO. INC
1331 C. NORTH AVE.
NEW ROCHELLE NY 10804
TEL. 914··636·5538

USITT /Summer, 1980

Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1980