Theatre Design & Technology - Spring 1978 - 13

Choosing A Spotlight
by Louis Erhardt
Theatre spotlights come in a variety of types and sizes.
With accessories, they are designed to fulfill the many requirements of the theatre, television, photography, and the
multitude of other situations in which people or objects are to
be brought to the attention of the viewer or camera. At the
moment our concern is directed toward the selection of an
ellipsoidal, framing spotlight to be mounted in the auditorium
and intended to light overlapping areas on the stage.
The Illuminating Engineering Society defines the field of
the lighting unit as: Those points of the candlepower distribution curve where the candlepower is 10 percent of the maximum candlepower. The included angle is the field angle. The
beam angle is similarly defined by those points of the candlepower distribution curve where the candlepower is 50 percent
of maximum candlepower, the beam being the included
angle.
To select a spotlight of the ellipsoidal type there are a
number of sequential steps in the analysis and decision:
1. Determine the spread in feet to cover the object or area to
be lighted. The usual stage area varies between 10 feet and
12 feet, although there are instances where larger or smaller
areas may be desirable.
2. Establish the distance of throw-the distance from the
mounting position of the spotlight to the area or objectagain, in feet.
3. From the chart below convert this information into the
minimum spread needed to meet those conditions. Should
some allowance in size be desired, a wider spread may be
selected. Smaller areas can always be accommodated
through the use of the built-in shutters, or templates. The
"spread" obtained from the table will be the field angle listed
in manufacturers' catalogs. The actual area covered will usually be about a foot larger since the light continues outward a
bit beyond the 10 percent point to the edge shadow.

Degrees in the Field Angle of Spotlights
Distance
of throw
Feet

10
20
30
40
50

33.4
17.1
11.4
8.6
6.9

43.6
22.6
15.2
11.4
9.1

53.1
28.1
18.9
14.3
11.4

61.9
33.4
22.6
17.1
13.7

73.7
41.1
28.1
21.2
17.1

90.0
53.1
36.9
28.1
22.6

60
70
80
90
100

5.7
4.9

7.6
6.5
5.7
5.1

9.5
8.2
7.2
6.4
5.7

11.4
9.8
8.6
7.6
6.9

14.3
12.2
10.7
9.5
8.6

18.9
16.3
14.3
12.7
11.4

Circle Diameter-Feet

4. Select from the various manufacturers' lists those units
having the desired spread. Remember that the diameter of
the area lighted will vary directly with the distance of throw. If
one selects a spotlight for a 10 foot throw and then mounts it
fifteen feet from the area, the diameter will be one-and-a half

times as great as planned, and the intensity will be less than
half that received at 10 feet.
Spread
Field
Angle

50°
40°
40°
33°
30°

Manufacturer
Berkey
Colortran

Electro
Controls

Kliegl

Strand
Century

2211
213/052
2321
7367A
213/062
zoom

28°
27°
24°
23°
20°
18°
15°
13°
12°
11°
10°
9°
5°

2331
1355W
7367A
2337
213/072
1355
2457
1357/8
213/092
1355/8
213/102
1357/10
213/112

To achieve different degrees of spread from one lamp
housing it is necessary to employ lens systems of longer or
shorter focal length. This may be accomplished by interchanging lens heads with varying focal lengths, by inserting
other lenses in a lens head-sometimes changing their spacing as well-or by employing a zoom system of lenses. Interchanging heads or repositioning lenses is more economical
when the problem is specific and known. If continuous variability from a single unit is essential the added costs of a zoom
system may be justified.
Note that the length of the total spotlight is determined by
its longest throw. If a zoom system is employed the unit will be
longer for shorter throws than one designed specifically for
the lesser distance. With zoom systems two lenses must be
positioned individually for spread and sharpness. Since the
adjustments interact such settings are more difficult than
those of fixed focal length units. (This problem should not be
confused with the use of sophisticated zoom systems in follow spots wherein only an initial distance setting must be
made, after which the lenses are moved differentially with but
a single manual change.)
5. Decide upon an intensity level for the illumination on the
stage. A generalized figure of 50 footcandles is often used
as the criterion in stating the "throw" of a spotlight. This is a
convention that mayor may not satisfy your needs. For the
small stage it is an adequate level if not too heavily filtered,
and it is usually augmented by the light of other spots needed
for modeling. Experience in providing the emphasis needed
for design purposes dictates the ultimate intensity level.

Louis Erhardt, Consultant to Strand Century Inc., is author of Radiation, Light, and Illumination.

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 1978