Theatre Design & Technology - Spring 1979 - 26

centered at point A; the long axis of the ellipse is plotted on the
horizontal or floor plane. From point B, the short axis of the ellipse is
plotted on the horizontal or floor plane, and point C becomes an oval
area of light or the elliptical beam pattern. To establish coordinates
on the long and short axes of the oval, the field angle and beam angle
can be broken down into degrees and plotted from the centerline of
the beam. Once these points are established at point C, concentric
ellipses are drawn. In figure 1-C a circle is inscribed inside the ellipse
showing the diameter of the beam determined from the manufacturers' guide number. The diameter would be numerically or graphically shown in lighting catalogs or data sheets.
The importance of this interpretation is clearer when illumination is
related to the beam pattern. The illumination will vary with the distance of the point source to the working plane (Inverse Square Law).
The candlepower of the 12° ellipsoidal in Figure 1 is rated at 324,00Q
candles and at point b the light is calculated to be 518.4 footcandles. 1
The distance from A to a is 32' 3" and the illumination at point a is
calculated to be 311.53 footcandles' though after the light strikes the
floor plane the illumination drops to 181.51 footcandles' because
there the illumination is expressed in proportion to the cosine of the
angle of incidence.
Lighting designers normally hang lighting instruments at an angle,
however most manufacturers illustrate lighting instruments and associated photometric data in a form similar to figure 1-B/b. With respect to the beam pattern in figure 1-a, illumination figures are
overstated and the diameter of the beam is understated on data
sheets.
When the ovalness of beam patterns is not considered, a lighting
designer inadvertently designs into the plot the very hotspots and
deadspots that he spends so much time eliminating. Figure 2 shows
that unless beams of adjacent instruments overlap from center to
center, dead spots will occur. The reason for overlapping center to
center is to take advantage of the illumination in the beam angle,
putting the brightest parts of the beams next to each other. While
there are other elements which influence the brightness of a beam of
light, overlapping sufficiently allows any area(s) to receive equal
amounts of light. The purpose of achieving equal amounts of light is

to provide a framework of visibility; the framework allows an audience to overcome the strain found in artistic lighting, but permits
the lighting designer to model.
It is not necessary to go through this procedure with every instrument in every show. However, in a very short time a designer,
student or professional, can complete a reference guide developed
from data sheets allowing quick access to individual instruments,
saving time and money.
The designing of a lighting plot by feel has a long tradition in the
theatre. For the newcomer attempting to learn the craft there is only
experience to teach, and without accurate information, mistakes will
become actual practice. Though lighting design is a subjective art, it
demands a firm reference foundation. Data sheets received recently
from manufacturers show a divergence from the guidelines set by the
1958 report. It would be advantageous for the lighting design community to reevaluate for their educational and economic value the
1958 guidelines and their current operating procedures with respect
to data sheets.

Notes

Equation for zero degree positioning: E =

(E = footcandles; I = candlepower; D = distance in feet; X = angle
of incidence.)
Equation for lighting instrument hung at an angle: E =

I (Cosine X)

E = 324,000 = 518.4 footcandles
25'
, E = 3:;..~~,0 = 311.53 footcandles
, E =

324,000 (.71)
35.6'
= 181.51 footcandles

Figure 2: PLAN OF OVERLAPPING BEAM PATTERNS

Pattern of field angle

L-

Theatre Design & Technology

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lnsufficient overlap - unless
beam overlap is from center
to center deadspots occur

USITT/Spring, 1979

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