Figure 5.8 Cross section of an LED signal
lamp with Fresnel and pillow lenses.
As the spacing between the pillow and the
LEDs is increased, each LED will illuminate a
larger area of the pillow lens. As the spot
illuminated by each LED grows and as the
adjacent spots begin to overlap, the lens will
appear more evenly illuminated. The trade off
between lamp depth and lit uniformity is a
common consideration in LED design, where
both unique appearance and space saving
packages are desired.
Collimating optics come in two main varieties:
reflecting and refracting. Reflecting elements are
typically metalized cavities with a straight or
parabolic profile. A cross section of an LED
signal lamp with a reflector cavity and a pillow
lens is shown in Figure 5.7.
Refracting, collimating optics typically used in
LED signal lamp applications include plano
convex, dualconvex, and collapsed plano
convex (Fresnel) lenses. A cross section of an
LED signal lamp with a Fresnel lens and a pillow
lens is shown in Figure 5.8.
In general, designs that use collimating
secondary optics are more efficient, and produce
a more uniform lit appearance than designs
utilizing only pillow or other non collimating
optics. Fresnel lenses are a good choice for thin
lamp designs and produce a very uniform lit
appearance. Reflectors are a good choice for
thicker lamp designs and are more efficient than
Fresnel lenses at illuminating non circular areas.
This is because reflectors gather all of the light,
which is emitted as a circular pattern for most
SuperFlux LEDs, and redirect it into the desired
shape. In addition, reflectors can be used to
create a unique, “jeweled” appearance in both
the on and off states.
The dependency of reflector height on reflector
efficiency will be covered later in this section.
Figure 5.9 Half-angle subtended by an individual
pillow (A) for convex (upper) and concave (lower)
pillow lenses.
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