White Paper on Projector Lamps

Tuesday, November 26th 2019. | template


White Paper on Projector Lamps


Purpose of this white paper

The aim of this white paper is to give an overview of the current market for projector lamps, including the advent of UHP lamps, compatible lamps, specialized lamp dealers and the business environment.

This paper is divided into sections. The main topics are the projector lamp technology and the economic factors. This is followed by an appendix and a glossary.

lamp technology


The lamp is the main component of the lighting system in a projector. It is usually accessible behind a door in the projector so that it can be exchanged. Sometimes a projector has two lamps. They can be used at the same time or take over one if the other one fails.

Most common projectors use metal halide lamps, ultra-high pressure lamps (UHP), UHP variants and in larger projectors xenon lamps. Although the xenon lamps are smaller than those in movie projectors, they use the same technology. Of course, xenon lamps achieve better color rendition than metal halide lamps, which have a lack of red, but are not as energy efficient and do not last as long.

What I call a lamp is actually a lamp module. It consists of an incandescent lamp and a reflector in a housing with electrical contacts for power supply.

The reflector

The reflector projects the light from the bulb onto a component in the projector called an integrator. This is done either in the form of a “fly-eye” lens (so-called because its surface consists of several lens elements in a rectangular arrangement, similar to the compound eye of an insect) or a light guide, the latter being either a rectangular glass rod or a rectangular mirror tube. Their purpose is to homogenize and shape the beam of light to ensure even illumination of each pixel with minimal loss of light.

The design of the reflector is very important as it has to collect as much light as possible from the bulb. It looks like a hemisphere, but is usually elliptical or parabolic in cross section.

Reflectors differ in sophistication. For example, a fourth power parabolic reflector is much more accurate and distributes light waves emitted from one focal point more uniformly than a second power parabolic reflector. However, parabolic reflectors of the fourth power are more difficult to manufacture accurately.
Then there are the elliptical reflectors. One of the properties of an ellipse is that it has two focal points (or foci). If you have a light source at a focus on an ellipse, the light rays that hit the ellipse are reflected to converge at the second focus (see the chart by downloading the latest white paper at tekgia.com).

This focuses the light of a lamp onto a lens, allowing you to bring as much light as possible to the screen. If your source is larger than a single point, some of the rays do not go back exactly to the first focus, missing the second focus point and getting lost. The wider the ellipse (ie, the greater the distance between the two foci), the larger the beam spot in the second focus.

Lamp size and scattered light rays

As expected, stray beams cause problems. Light that is not guided through the optics strikes other surfaces in the projector, reduces the brightness of the screen and increases the heat in the projector. You may also experience disturbing and disturbing light through the openings in the projector.

In addition, stray light can return to the optics and hit the screen where it should not. This affects the contrast of the picture. Instead of showing a continuous black, the black is lightened by the scattered light in gray.

An interesting solution to the problem of stray light rays is to reduce the size of the light source. The ideal source would be infinitely small. Any stray light would be infinitely small. There would be no noticeable loss of brightness (or luminance – the amount of light produced).

The aim was therefore to build the smallest possible projector lamp.

That brings us to the light bulb. Metal halide lamps spark over a gas-filled gap to create the light. The gaps are typically 2 mm or larger. Such sizes can cause color and luminance stability problems. They also tend to deposit materials such as tungsten on the lamp while it is on, reducing brightness at an early stage of lamp life.

In 1995, Philips introduced the ultra high pressure lamp (UHP). These lamps are not metal halide lamps. Instead, they use an arc in a pure mercury vapor under very high pressure. The pressure is typically over 200 atmospheres or 200 bar (a car tire is typically below 3 bar).

The arc gap tends to be much smaller than that of the metal halide lamps, typically 1.3 to 1.0 mm in diameter. This smaller light source is much more efficient. A 100 watt UHP lamp in a projector can bring more light to the screen than a 250 watt metal halide lamp.

Other influential factors

I will briefly discuss other factors that affect the design and manufacture of a projector lamp.

The reflector must be designed to create a uniform light field (no hotspot in the center), the glass of the lamp must be as transparent as possible, and the filament as free of impurities as possible (they affect the color temperature of) Output).

Then there is the dichroic coating of the reflector. This allows infrared light (heat) to pass while reflecting visible light. This reduces the amount of heat emitted by the LCD panel (or the DLP mirrors if your projector is a DLP projector).

Then there are the materials used. A projector lamp is made of a material that is resistant to high pressure and high temperatures.