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Types of Lamps and Their Design Considerations

Parabolic Lamp
This design has been around since before the development of electricity. These lamps have a parabolic reflector and a lens with fluting a.k.a a Fresnel lens. The reflector collects the light and the fluting in the lens does the beam control. They put about 27% of the light created by the light source (bulb) on the road where needed. The balance of the light is lost to refraction and stray light.

Free Form lamp
With the advent of the computer came the first real improvement in lamp construction in decades. The reflector is not a parabola but rather is a computer designed complex shape. Hella uses over 50,000 calculated points to define this shape. The reflector collects the light and shapes the beam while the clear lens simply keeps the dirt out. The lack of fluting on the lens reduces defraction of the light and increases output. Free form lamps typically collect about 45% of the light available.

Projector or DE Lamps
The next generation of lamps was developed not only to improve the output of headlamps, but also to reduce the diameter of the package for styling reasons. They have a free form reflector that is very deep. This results in a large surface area that collects a lot of light while having a small overall diameter. Unfortunately, the reflector is so convoluted that it takes a special projector lens to collect the light and get it going in the right direction. These lamps put as much as 52% of the light the bulb makes where it belongs. While this package is small in diameter it is by its nature, very deep.
Bi-Xenon and Bi-Halogen
A projector lamp can not produce both a high and low beam using a twin filament bulb as Parabolic and Free Form lamps can. The only system that seems to work well is known as a Bi-Xenon or Bi-Halogen. This lamp uses a shield that produces the low beam cutoff and then, when signaled, lifts to allow the full high beam. This system allows the use of Xenon (HID) high beams as well as low beams in one lamp without the loss of "Flash to Pass" and the detrimental effect of turning an HID ballast on and off quickly. A number of new cars now incorporate these lamps including Audi, VW, BMW, Mercedes, Ford (Mustang) and more to come.

Lamp Size
As in most things, bigger is better. A larger reflector will collect more of the available light from a given bulb. In addition, because of the larger size, you can increase the wattage of the bulb and better dissipate the heat generated.

Lamp Shape
Light bulbs produce luminous flux in a sphirical pattern. Round lamps are more efficient in handling the light than rectangular or oval lamps. To compare the performance of non-round lamps, the better performer will have the larger inscribed circle.

If you haven't heard of the manufacturer, there is probably a reason. There are some truly horrible projector lamps out there. Recently, a supplier was fined over a million dollars for selling a projector lamp as meeting the DOT standard, which it didn't. The DOT standard is easy to meet, sealed beams meet it.
City Light or Position Light
A European thing. In the big cities of Europe the street lighting is excellent and cars are not permitted to use even their low beams, in order to control light pollution. They are simply a 5 watt bulb that lights up the reflector to make you more visible to other traffic and prdestrians. They creates no appreciable illumination. Several new types of city lamps have been introduced including Celis "Angel Eye" rings and LEDs.