The automotive projector unit
The reflector gathers as much light as possible so that it passes above the cutoff shield and gets evenly spread out by the curved lens:
Individual parts:
A projector unit consists of:
- a light source (bulb)
- a reflector
- a cutoff shield
- curved lens
All components are designed to work together within small tolerances. Headlight manufacturers may use the same projector unit in headlights destined for different vehicles. This is cost saving. All a manufacturer has to do is have a nice looking round hole that the curved lens can peep out of.
Most projector units look roughly the same. They all have the same proportions-- long and skinny. The differences are diameter and depth, which is always proportional. The bigger the curved lens, the deeper the projector is.
What are the most common surfaces of curved lenses:
This does not mean that one of the surface designs neccesarily are better than the other. A projector unit is designed as a complete unit and total quality and level of output depends on many other factors. The observations above are more interesting as a comparison if you want to tweak a projector unit.
What is the difference between cutoff line and cutoff band?
The cutoff line is the horizontal line where the beam turns from fully illuminated to almost darkness. If headlights are aimed correctly, the cutoff line should be within 50-100m in front of the car. The cutoff band is the area around the cutoff line that can contain various colors. These are the beautiful colors that you see when looking at projector lights from a distance.
How is the cutoff line created?
First, the reflector shapes a rough cutoff above the shield. Then, the cutoff shield defines the final shape of a horizontal line on the wall separating darkness and light. Because a razor sharp beam cutoff is needed, the reflector beam is overlapping the cutoff shield edge, so that maximum intensity is kept close to cutoff line. The light that hits the gray cutoff shield will partially be converted to heat and partially deflected into stray light.
Why do projector units create such a colorful cutoff band?
We have all seen it. Projector units sometimes stick out like a sore thumb in the big yellow halogen reflector crowd. The cutoff band can contain the entire range orange-red-blue in bands. The phenomenon is called diffraction. Simply stated, diffraction is waves of light bending around the razor sharp edge cutoff shield. The longer the wavelength, the more it will bend. The longest wavelength is red, and the shortest is blue. Refraction also causes light waves to interfere and vary in intensity. The intensity has a few peaks - the first being around 60% of beam intensity--and the next is about 15%.
There have been speculations as to whether or not car manufacturers have intentionally equipped their cars with a distinct blue colored cutoff band. This way they will look different from the crowd. And that is just what the trademark of Audi and BMW is. Current DOT and ECE regulations do not allow colored beams in headlamps. Since it is only a few degrees of the beam, it has slipped through...
What creates the colorful cutoff band?
It is the edge of the cutoff shield that creates the colors in the cutoff band, not the bulb. It does not matter if bulb is 3500K or 7000K, halogen or HID. The curved lens only amplifies the colors. In standard configurations, most projector units has a yellow/orange cutoff when distance to screen (=wall) is close. At longer distances, the blue color band gets appearant and sometimes even deeper in color. This would be the view that most people whatch when meeting cars with OEM HID.
The colorful cutoff band is probably what started the demand for blue bulbs in late 1990's.
Why does moving the cutoff shield change cutoff band color?
It all has to do with making the right color hit both the 60% peak and as much angle on the curved lens as possible. The further away from the center of the curved lens the beam hits, the more will the cutoff band be enlarged.
How can the blue color in the cutoff band be increased?
Moving the cutoff shield closer to the reflector will accomplish this task. This is done by pressing the shield with a finger towards the reflector. Note that cutoff will be even sharper. Moving curved lens away from shield has same effect, but will decrease output hight and width.
Can the color of the projected beam below cutoff band be altered by the cutoff shield modification?
No.
How do I optimize bulb or shield placement in a projector?
This is mostly applicable to halogen projector units upgraded with HID bulb. The idea here
is to make 30-50% of hotspot rise below cutoff line. Pictured beam would be a typical E-code
or harmonized pattern. The dotted circle represent the hotspot. Both bulb and vertical
shield hight will affect outcome. Moving bulb or shield too much away from default position
will have other negative beam consquences.
To be able to see the circle better, use one or two pairs of sunglasses. This will increase contrast between hostpot and
surrounding light.
What is a top shield?
This has been found on some projector units. At this location in the path of light where the shield is mounted, the image is upside down (some projector units are mirrored as well, but that's another story). Which means that the top shield you are referring to actully cuts off light right in front of the car. This does make a lot of sense. A driver want to have the best vision down the road. This means that bright light 2-5 meters in front of the car is actually glaring to the driver, especially on light pavement and will REDUCE down the road vison. On some vehicles, this light will automatically be cut off by front bumper, other vehicles has a less forward design on bumper, so this light has to be cut off inside headlamp. Hella really bake in a lot of sense in their design and makes minor variations to the same projector unit depending on vehicle its going into. The idea to remove any extra shielding might actually hurt more than do any good.
Can low beam projector modules be used as high beam?
Yes, it can. There is not really a good reason for using low beams as high beams. A high quality high beam will give good results out of the box - a well known "Free Form Reflector" design does the trick. It cost less. An all metal design can take almost unlimited amount of overwatt bulb. And the most important part: there are alot more of them available on the marked. Probably the best complimentary high beam units would be a spot type beam. An HID projector low beam alread has a lot of width and foreground lighting.
Anyways, if you still want to, this would be the way to do it:
A low beam projector comes with a bottom shield. Simply remove this.
This will make it send out light further down the road with no cuttoff line.
Then a
top shield should be made. For background info - see above section above.
Just use some aluminum foil. It does not need to have same accuracy as
a low beam shield. It should typically cut off all light hitting the road
closer than 30 meters (100ft). This will require some trial and error to
get it right.
What path does light rays take from reflector to the road?