Advantages of Projector Headlights

In the world of technology, the projector is not a new invention. Modern projector units have existed since the early 1920′s. They have existed as long as slides (still photos) and movies have been shown. The technology actually belongs to the AV (Audio/Visual) world, but arrived in the automotive industry in the early 1990′s.

What is a curved lens?
The term “curved lens” is used for simplicity. The optical name for it is “Plano convex”. One side is plano, the other is convex. Another commonly used name is ellipsoidal lens. It is also called a collector lens, because it collects light rays to a concentrated beam. They may seem very propitiatory, but if look around, you will find single, generic curved lenses in various sizes available. For example, sells 3″ Ellipsoidal lenses.

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 (HID Conversion Kit)
• 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 lens 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:

• Clear – Gives the best down the road illumination – typically found in ECE units.
• Ribbed (Fresnel like) – Gives less down the road illumination, bleeds slightly more light above cutoff and generally spreads light slightly better.
• Soft – Same effect as above, but will reflect some light back into reflector which means less efficiency.

This does not mean that one of the surface designs necessarily 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.

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.

How do I optimize bulb or shield placement in a projector?
This is mostly applicable to halogen projector units upgraded with HID Conversion Kits. 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 consequences.
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 path does light rays take from reflector to the road?

• foreground (2-25meters) passes high above shield edge from upper part of reflector
• down-the-road (25-90m) passes right above the shield edge from area closest to bulb
• Bi-Xenon high beam (90m and upwards) passes right above and right below shield edge from area closest to the bulb

 

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