Building Super Lights

This article was first published in 2008.

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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