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This article was first published in 2008.
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When you watch cars go by at night, you can see a
variety of headlight designs on display. Old car use sealed beams, often rather
yellow in appearance. Then there are the whiter designs with replaceable halogen
bulbs, while more luxury recent cars feature high intensity discharge lights,
most easily picked because of their brilliant blue/white colour.
But there’ll also be another bunch of lights that
when viewed at an angle, have red or blue beams grading to white as the car is
seen face-on. These cars have an abrupt beam cut-off and a very even spread of
light within the beam. These headlights are called ‘projector’ and use a simple
reflector teamed with a large convex glass lens.
And you want to know something? That large glass
lens can be obtained for nearly nothing from broken headlights at car wreckers.
And you want to know something else? They make excellent lenses for use in bike
lights, torches and handheld spotlights.
Projector Headlights
Here’s a typical projector car headlight. From the
front, there’s a cover plate of glass or plastic. Behind that is the convex
glass lens (normally used on only the low beam and coloured green here) followed
by the bulb and then a simple reflector. To avoid blinding oncoming drivers, the
bulb is masked so the upper part of the beam is abruptly cut-off. The single
headlight assembly also contains a high beam, which usually comprises a
conventional halogen lamp and reflector.
The headlight is near-worthless to the wrecker if
the cover glass is broken, the high beam is broken, the low beam is broken, or
the rear plastic housing is shattered.
If the low beam convex glass lens is intact but
any of the other parts of the headlight are broken, the convex lens can be
bought for near nothing.
For example, at a major wrecking yard I found and
salvaged three convex lenses, and took them to the front counter. I made the
point that I hadn’t needed to break any headlights to obtain the lenses, and
asked for a price. The counterman was puzzled: what on earth did I want these
lenses for? I told the truth – I was making a bicycle headlight – and he charged
me $10 for all three.
On another occasion, when I was buying some other
car bits, the convex lens didn’t cost me anything extra.
Many recent cars have projector headlights while
amongst older cars, the Mazda 626 and Ford Telstar are the most easily found.
Some Nissan Japanese ‘grey market’ imports also have them, including one car
that has two such lenses each side.
If you are salvaging the lens from a headlight
with a broken cover glass, be very careful. It is extremely easy to cut yourself
on the shards of glass, especially if you slip while wielding a screwdriver.
Incidentally, smaller lenses of a similar shape
can also be salvaged from old slide projectors.
Using the Lens
So you have a bunch of high quality, large, convex
glass lenses that you’ve obtained for nearly nothing. Now what? I could get all
theoretical and talk about focal lengths and beam angles and point sources, but
forget all that. The easiest way of coming up with the best design for your
particular application is to simply play around with the light source and the
different lenses.
For example, a Luxeon LED makes an excellent light
source as it is small, very bright and has high efficacy. Power-up the LED with
a suitable current-limited power supply (if it’s a 3W or 5W design, after
mounting it on a suitable heatsink) and then hold the convex lens in front of
it.
View the beam pattern on a wall or the ceiling. By
altering the distance between the lens and the LED, it’s possible to change the
beam from a broad diffuse beam to a narrow spot. In the case of the Luxeon, you
can also try matching the glass lens with the various collimators (internal
reflecting plastic lenses) available for these LEDs.
The convex glass lenses can also be used with
conventional incandescent bulbs and reflectors (and incidentally, lots of
working torches are thrown away each day; nope, I don’t know why...). Again, it’s
a case of trying different combinations and looking at the results.
If the lens is placed very close to the light
source, it’s possible to get an extremely broad beam. To put this another way,
the visibility to others of the light is greatly enhanced: it’s just the thing
for a flashing warning light or bicycle tail-light.
Building a Compact Broad Beam Light
I used a convex lens from a car headlight to make
a very bright, broad beam, flashing bike tail-light.
A stainless steel drinking cup was shortened in
length by the use of a hacksaw and file. This gave a housing with an opening
that matched the diameter of the lens. A 1W red Luxeon LED and narrow beam
collimator were installed on a small bock of aluminium (a 1W Luxeon doesn’t need
a heatsink but having one doesn’t hurt!) and then the block was mounted in the
base of the cup.
A U-PVC plastic pipe cap to suit the diameter of
the lens was obtained and its inner diameter cut out with a hole-saw, creating a
flange that fitted over the end of the cup, holding the lens in place. Silicone
was used to secure the cap in place and to weatherproof the opening. In this
application, the best results were gained by reversing the lens over its normal
car orientation - that is, the convex part of the lens faced the LED.
The 1 watt LED was powered by a 12V cigarette
lighter phone charger adaptor which was modified to act as a constant current
source (see
Luxeon Lights).
Flashing of the high powered LED was accomplished
by the use of an eLabtronics Pulser – see
The eLabtronics Pulser, Part 1. Note that the Pulser
can flash literally dozens of lights, so you’re not limited to flashing just
one.
So why go to all this bother when LED tail-light
flashers are cheaply available? Well, you’ve simply never seen a flasher
like this. It is intensely bright – from directly behind, able to be seen at
distances of 500 metres or more. The convex lens creates a broader beam that
would otherwise occur, allowing the light to be visible at a much wider angle
than would be achieved with conventional high intensity LED, and this effect is
enhanced by reflections from the internal walls of the stainless steel cup.
So next time you see some salvageable convex
lenses in broken headlights, grab them!
Careful!
These
lenses are made from high quality optical glass. So if you drop them, hit
them, or squeeze them hard enough, they’ll shatter... Ask me how I know!
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