The
optical system
of the profile-spot consists of a light source, an
ellipsoidal reflector
(or sometimes a spherical reflector), an
aperture
called a gate, and a lens, or a compound of lenses, mounted on a moveable sleeve. The gate, which can be shaped by shutters, iris or a gobo, is projected onto the stage.
The
reflector
gathers the light rays and reflects them through the gate to the second focal point of the ellipse. This second focal point, where all the light passing through the gate is concentrated, is located quite near the lens, enabling it to collect and project .
nearly all the available light .
The efficiency of profile-spots is further enhanced by the addition of a spherical
reflector
which gathers light from the
lamp
and projects it onto the ellipsoidal reflector.
Note the light concentrated by the the
reflector
to the second focal point of the ellipse, and the wide collecting angle of the lens.
In profile-spots the gate consists of four plates of thin metal called as shutters, which are used to
cut
off part of the light rays and shape the light
beam
in various geometric shapes. At the gate there is also usually a slot in which a gobo or an iris can be placed. The iris can be closed or opened, changing the size of the light
beam
by masking the light into a narrower
beam
angle.
Typical Gobos by Rosco.
Typica irises by Altman.
Some types of profile-spots employ a spherical
reflector
system and condenser lenses to achieve the same light concentration near the
lens
as the
ellipsoidal reflector
provides. Modern profile-spots often employ
condenser lens
to improve light collection at the gate. The
beam
angle of a profile-spot depends on the focal length of the lens, and it can not be changed unless the
lens
(a single
lens
or a compound of lenses) is replaced. A more complex system of lenses, compounding two or more lenses, is the variable-lens system, popularly known as a
zoom
system. The variable
lens system enables variations of the
beam
angle (or
image
size). The change in
beam
angle is achieved by adjusting the distance between the two lenses, thus changing the focal length of the
lens
compound.
