Secondary Optics
This section contains practical design tools for
secondary optic design. More accurate and
sophisticated techniques exist which are beyond
the scope of this application note. The design
methods discussed here are proven, but no
analytical technique can completely replace
empirical testing. Designs should always be
prototyped and tested as early in the design
process as possible.
Secondary optics are used to modify the output
beam of the LED such that the output beam of
the finished signal lamp will efficiently meet the
desired photometric specification. In addition,
secondary optics serve an aesthetic purpose by
determining the lit and unlit appearance of the
signal lamp. The primary optic is included in the
LED package, and the secondary optics are part
of the finished signal lamp. There are two
primary categories of secondary optics used,
those that spread the incoming light (diverging
Figure 5.5 Point source model of a SuperFlux
LED.
optics), and those that gather the incoming light
into a collimated beam (collimating optics).
The most common type of diverging optic used
in automotive signal lamp applications is the
pillow lens. The pillow lens spreads the incoming
light into a more divergent beam pattern, and it
breaks up the appearance of the source
resulting in a more uniform appearance. A cross
section of an LED signal lamp with a pillow lens
is shown in Figure 5.6.
Figure 5.6 Cross section of an LED signal
lamp with a pillow lens.
Figure 5.7 Cross section of an LED
signal lamp with a reflector cavity and
pillow lens.
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