Theatre Design & Technology - Fall 1983 - 26

FOR PERFORMANCF! LIGHTING

by J. M. Chapman

GOBO:

a beam shaping mask
in the "gate"
of a spot light.
An ellipsoidal reflector spotlight
works like a simple slide projector. The
lens is focused to project a reversed and
inverted image of the shutters onto the
stage. To produce images or textures a
gobo can be mounted at the "gate" (primary focus) of the unit. Because theatre lamps produce so much heat we normally use metal sheets cut or etched with
patterns of holes.
Projecting anything off "flat" produces distortion from the original. Image size is dependent on throw (distance) and beam angle (lens). These factors combine to make accurate prediction of gobos, onstage, a complicated
process. The slow cutting of metal gobos or the limitations of stock patterns
has meant that gobos have tended to be
restricted to three main uses:

steel patterns. The detail is very good
and they are not expensive. lesh and
glass gobos are beginning to appear and
these can produce tones and unsupported "dark." Beware-these patterns
may not use the whole clear light field of
your unit, this has to be allowed for.
Most patterns are designed flat and will
distort in angled beams.
Cusrol/1 Pa((erns. A pre-distorted
custom gobo can be made to your art
work but this may take time and will be
expensive. Use a specialist to calculate
the distortion. If you are working for a
long run get two prints for protection
against aging or damage.

Projecting Gobos.
A lot of theatre lights will take gobos
and many adjust to produce a flat field
brightness. Only a few provide a crisp,
high contrast, flair-free edge. Any unit
for "in focus" gobo work must be carefully set up. Clean the reflector and lens
to reduce internal diffusion. A new gobo may throw some light back into the
reflector, producing flare. Spray it with
mat black engine paint. By the time
I) Simple textures from punched/
this burns off the steel should be discut metal, mesh, or patterns;
colored by heat.
2) Basic light images, such as diThe lenses in theatre lights are of
rect sunlight through a window;
poor optical quality but, just as with a
and
camera, the center of the lens gives best
3) Detailed images from stock or
results. Front of lens masking, with a
pre-distorted custom gobos.
center hole, will give improved definiA designer has three basic types of tion. This reduces the overall brightness
(use a more powerful unit) but the loss
gobo to choose from:
of
flare may be essential for a high conCllr Meral. These can be made "in
trast
image. Size the mask hole by using
house" to fit a simple design or to pro\ide a rough texture or dappled light. To a large format iris in front of the lens.
Designers spend considerable time
project detail a good "edge" is required
soft
focusing beams of light to prevent
needing fine metahmrking skills. Image
hard
edges across moving actors. A godistortion can be calculated and allowed
bo
projected
on a performance floor
for. These gobos are cheap but take
may
look
fine
until the actors CUl the
time lO make.
beams.
By
using
a back angle this beam
510ck Porrel'lls. lany suppliers now
"wipe"
can
be
avoided.
carry a large range of etched stainless
2-1

Thealre Design & Technology/USITT IFall 1983

Soft Projection.
Some gobos lend themselves to
"soft" projection for textural blending,
image modification, or to generate completely new images. There are three
ways of softening a gobo: over-focus,
under-focus, and light diffusion.
Most profile/ellipsoidal spotlights
produce a different out of focus effect
depending on the direction of lens movement off-focus. Over-focus normally
provides a partially maintained image
with a soft halo around each bright area.
With under-focus the small holes in the
gobo, acting as pin-hole lenses, exaggerate the normally undesirable ring edge
affect to produce overlapping but clearly
defined layers of color fringing. These
effects will depend on the degree of offfocus, the optics of the unit and the gobo pattern. Both of these "soft" effects
can still provide a degree of hard edge
that may be undesirable.
By introducing a very light diffusion
(Roscolux USA/Supergel UK o. 114)
a true soft projection is obtained. The
size of a center hole cut in the diffusion
will determine the degree of softening.
By manipulating the off-focus and
front of lens masking, different controlled effects can be produced. As the
mask hole is expanded the center of the
image is the last area to go off-focus. By
experimenting with this technique it is
possible to provide a crisp center surrounded by a softening image.
A mask can sometimes be used to
give a hard focus to one side of an offfocus gobo by masking part of the opposite side of the lens. This technique
can be used to harden the far edge of a
gobo projected at a shallow angle where
it is impossible to obtain normal hard
focus over the whole image. This may
also be used without gobos to provide
one hard edge from a soft focused unil
-a bifocal effect from a normal unit.

Table of Contents for the Digital Edition of Theatre Design & Technology - Fall 1983