# Theatre Design & Technology - May 1968 - 19

```can be made in terms of a beam angle and a field angle.
The beam angle is defined as the angle between those
points of the intensity curve where the intensity falls to 50
per cent of the maximum intensity.3 The field angle is defined as the angle between those points of the intensity
curve where the intensity is 10 per cent of the maximum
intensity. These angles have several uses. One application
is to give a simplified indication of the beam shape. When
instruments are used individually, the intense part of the
beam usually is of greatest interest. Then the beam angle
may be the best single estimate of the beam size.
The relation between beam and field angles is an indication
of how rapidly the intensity changes. If the beam and field
angles are close, then the beam is relatively uniform over
the central region (variation of less than about 2: 1), and the
edge is fairly sharp. If the beam angle is small and the field
angle is large, the beam tends to be peaked in the center
but has a gradual drop-off. If the beam angle is large and
the field angle is considerably greater, the central intensity
is reasonably uniform, and the drop-off is gradual. These
statements are general trends and are illustrated in Fig. 1.

CP

ANGLE-------l
ANGLE-----

CP

l,
A-·J
~

--.I
BEAM I-~
~ ANGLE

FIELD ANGLE

CP

.1-1---- _ _....30_ _

~BEAM ANGLE~

-----FIELD ANGLE - - - - - - - I

Figure 1. Illustrations of beam angle and field angle for various
beam shapes.

The efficiency of a lighting instrument is defined as the
ratio of the instrument's lumen output to the lumens generated by the lamp. The beam efficiency expresses the
lumens within the beam angle, and the field efficiency expresses the lumens within the field angle. When only the
central part of the beam is important, the beam efficiency
is the better measure of the instrument's effectiveness in
utilizing the generated light. If instruments are used in

lusm

May 1008

groups over an area, the lumens within the field angle will
be a better measure of the instrument's effectiveness. This
distinction can be very important since the field efficiency of
some instruments can be more than twice the beam efficiency.
The beam and field efficiencies are important specifications
for many instrument types such as fresnel lens spotlights
or scoops. In other cases, such as the ellipsoidal reflector
spotlight, these efficiencies are of less import in comparing
instruments. H ere, the edge of the beam is sharp, and it is
determined by masks, or shutters, imaged by a lens. The
light in the beam of a well-designed instrument is sufficiently uniform that the instrument is useable up to the maximum
beam size permitted by the masks. The total efficiency using
the total lumens of the maximum beam size normally is the
best measure of the instrument's effectiveness. Since the
size of the utilized beam is variable and determined by
masks, the beam and field angles are of limited significance.
The most useful measure of beam size is the maximum
Several different criteria may be used when two instruments
of the same type are to be compared. The variation of intensity versus angle, the beam angle, the field angle, and
the uniformity of the beam from point to point are some
possible evaluations. If the instruments are not adjustable,
a direct contrast is easy. When the beams are variable,
some common basis must be used. For example, the beam
angles might be made equal and the beam efficiencies and
the maximum intensities compared. In some cases, a single
comparison cannot lead to a definitive conclusion. This is
typical if the beam shapes vary. Then the beam angles and
the field angles cannot be made equal simultaneously. At
such times, several comparisons may be required, and the
ways the unit is to be used must be taken into consideration.
Photometric comparisons are not always sufficient, and a
visual evaluation may be necessary. It is best to visually
judge the uniformity of a beam for small localized variations.
Again, the use of the luminaire will enter into any decision.
From the nature of its applications, a mottled pattern is commonly acceptable for beam projectors, but in some other
instrument types, visual uniformity is necessary. Also, the
sharpness of bright or dark zones affects visual interpretation of uniformity.4 When an instrument is directed perpendicular to a surface and the illumination pattern is
viewed, the same magnitude of change in illumination between two points becomes more apparent as the line of demarcation between the two points becomes sharper. For
example, a 2: 1 smooth, gradual variation between the center and edge of a pattern may not be obvious, but a sharp
10 per cent change at some point within the pattern will be
obvious.
One is not always able to obtain a unique set of data for
evaluation even with a specific instrument and a fixed focus
employed for most instrument types, there are fixed adjustments of the optical components. As an example, a lamp
must be positioned within the reflector of an ell ipsoidal reflector spotlight. The "correct" adjustment of this lamp is
not unique. The testing standard. 3 states that the lamp is
to be adjusted to produce a beam of greatest uniformity.
This involves a value judgment. If the ratio of maximum to
minimum illumination on a surface is used, the results will
differ from those obtained by using a maximum to minimum
intensity ratio. More important, if the minimum, or a low

19

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