|
Plano-Convex Lens:
|
A lens curved on one side and flat on the other. The more
pronounced the curvature of the convex side, the closer to the
lens will be the point at which light rays entering the lens from
the convex side will converge. The distance from the lens to
this point is called the focal length.
A plano-convex
lens is described by its diameter and focal length. For example,
a 6"x9" lens will have a diameter of 6" and a 9" focal length.
The shorter the focal length, relative to the diameter of the
lens, the wider the beam of light; thus, a 6"x12" lens
will emit a beam of light 3/4 the width of the 6"x9" lens. When
two plano-convex lenses are used "belly-to-belly", their
effective combined focal length is halved. For example, two
6"x9" lenses belly-to-belly will have an effective focal length
of 4½".
Fixtures using plano-convex lenses typically project sharp-edged images.
|
|
Step Lens:
|
Plano-convex lenses with the flat side cut away in steps. Step
lenses are optically similar to plano-convex lenses, but lighter and
less prone to cracking from the heat. The light from a step lens is
usually not as even as that from a plano-convex lens.
|
|
Fresnel Lens:
|
Fresnel lenses, as opposed to step lenses, are cut away from the
front. They are extremely thin and therefore efficient and less
likely to crack from heat
Unlike step lenses, each of which has a single focal length, each
concentric ring of a Fresnel lens has a different diameter and a
slightly different focal length.
Fixtures using Fresnel lenses project soft-edged images.
|