Moving forward the light
Custom lighting assembly saves
space in hard drive
As electronic devices get smaller and more
"look-alike," and as component crowding worsens, the problems
of lighting design become tougher to solve. After some floundering on their
own, frustrated manufacturers often turn the job over to experts who can
provide complete, cost-effective solutions for indicator, display and panel
lighting.
To save valuable space near the front of the hard drive, the design team
of a major PC manufacturer separated a group of three display lights from
the light source. The source--three surface-mount LEDs attached to a circuit
board--were placed behind the hard drive. To deliver the light to the front
of the device, the team originally settled on a straight, two-section, three-channel
rigid acrylic light pipe that measured 6 in. in length. The rigid light
pipe provided excellent light transmission, but had a relatively large (three
1/8-in. dia. elements side-by-side with 0.030-in. separation) "pipe
chase" cross section for structural strength. Originally, the hard
drive's design team hoped to place the light pipe in one of the drive's
side rails but couldn't find the space. Furthermore, in that location the
light pipe didn't line up with the display lights. So a second curved light
pipe section was needed, complicating assembly. Attached to the first one,
this pipe would carry the light in a zigzag path to the display panel.
But the straight pipe couldn't be made small enough to fit in the side
rail without sacrificing optical performance. And the space for the light
pipe couldn't be increased without cutting into the hard drive's heat-sink
capacity. As an alternative, the designers discussed running the straight
pipe over the top of the hard drive and outside its envelope. But they were
reluctant to put the pipe outside the envelope of the device where it would
eat up valuable space. Moreover, it turned out that both this plan and the
first had a fatal flaw: The designers couldn't get the straight and bent
pipes to mate well enough to transmit the light.
The designers took their problem to Chicago Miniature Lamp Inc., Hackensack,
NJ. After prototyping the rigid pipe, CML recommended a different solution:
an assembly consisting of flexible fiber-optic strands with rigid polycarbonate
light pipes at both ends. The fiber-optic assembly transmits light as efficiently
as the rigid pipe, but has a much smaller cross-sectional area. Measuring
just 1/8-in. across (three 0.060-in. dia. strands encased with no gap),
the strands tucked easily into the side rail of the hard drive.
Rigid light pipes require close-tolerance assembly for precise alignment
between source and output. But the flexible strands of a fiber-optic assembly
forgive assembly misalignment. "The strands also give designers plenty
of leeway in locating display lights, regardless of the relative position
of the light source," adds CML's Norm Simms, who spearheaded the re-design.
"What's more, a fiber-optic assembly can bridge large gaps between
source and output without electric wiring, which can cause EMI problems."
Though it measured only 6 inches long, the original three-channel light
pipe design required several cross members for structural support. Unfortunately,
such cross members can create conduits for light in one of the channels
to travel into another channel. This phenomenon, known as cross-lighting,
is the optical equivalent of noise or cross-talk. But thanks to the flexible
fiber-optic strands, which don't need structural supports, cross-lighting
concerns are eliminated.
The rigid molding used at the source end snaps easily into place, ensuring
good coupling with the light source. Since the shape of the light pipe at
the display end of the assembly is independent of the shape of the source,
designers could customize the look of the output molding to be round, square,
oval, Euro, or any other style. This provides greater design flexibility
over LED lamps, which offer typically two standard shapes--the familiar
"dot of light" for through-hole versions or a square shape for
SMTs.
--RM
For more information:
Circle 472 - Chicago Miniature Lamp, or connect directly
to their website via the Online Reader Service Program at http://www.OneRS.net/107df-472
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