Projector Lamp Bulb

Introduction

Purpose of this white paper

The purpose of this white paper is to provide an overview of the current projector lamp market, including the emergence of UHP lamps, compatible lamps, specialist lamp distributors and the underlying economic conditions.

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

Lamp technology

Overview

The lamp is the primary component of the illumination system in a projector. It is usually accessible behind a door in the projector so that it can be replaced. Sometimes a projector will have two lamps; they may be used at the same time or one may take over when the other fails.

Most common projectors use metal halide lamps, ultra high pressure lamps (UHP), variants of UHP and, in larger projectors, Xenon lamps. Although the Xenon lamps are smaller than those in film projectors, they use the same technology. Xenon lamps naturally achieve better colour reproduction than metal halide lamps, which are red deficient, but they aren't as energy-efficient and they don't last as long.

What I'm calling a lamp is actually a lamp module. It consists of a bulb and a reflector in a housing, with electrical contacts for receiving power.

The reflector

The reflector projects the light from the bulb onto a component in the projector called an integrator. This takes the form of either a "fly-eye" lens (so named because its surface is composed of multiple lens elements in a rectangular array, much like the compound eye of an insect) or a light pipe, the latter either a rectangular glass rod or a rectangular mirrored tube. Its purpose is to homogenise and shape the light beam to ensure uniform illumination of every pixel with minimal wasted light.

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

Reflectors vary in sophistication. For example, a fourth-power parabolic reflector is much more accurate and even in its distribution of light waves emitted from a focal point than a second-power parabolic. Fourth-power parabolic reflectors, however, are more difficult to accurately manufacture.

Then there are the elliptical reflectors. One of the properties of an ellipse is that it has two focuses (or foci). If you have a light source at one focus of an ellipse, the light beams that hit the ellipse are reflected so that they come together at the second focus (see diagram by downloading the actual whitepaper at tekgia.com).

This concentrates the light from a lamp onto a lens so that you get as much light as possible delivered to the screen. If your source is bigger than a single point, some of the beams do not originate precisely at the first focus, and thus end up missing the second focus point and straying. Also, the wider the ellipse (meaning the greater the distance between the two focal points) the larger the beam spot will be at the second focus.

Bulb size and stray beams of light

As you would expect, stray beams cause problems. Light that is not funneled through the optics will strike other surfaces inside the projector, reducing brightness on the screen, and increasing heat in the projector. You may also get annoying and distracting light leaking through vents in the projector.

Furthermore, stray light may find its way back into the optics, and end up striking the screen in places where it should not. This impacts the contrast of the image. Instead of showing solid black, the stray light will lighten the black into gray.

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

So the goal has been to make as small a projector lamp as possible.

This takes us to the bulb itself. Metal halide lamps spark across a gas-filled gap to create the light. The gaps are typically 2 mm or larger. Such sizes can cause colour and luminance stability problems. They also tend to deposit materials such as tungsten on the lamp while it is on, reducing brightness early on in the life of the lamp.

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 200bar (a car tyre is typically under 3bar).

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

Other influential factors

I will briefly cover other factors that are influential in the design and manufacture of a projector lamp.

The reflector has to be engineered to give an even field of light (no hotspot in the middle), the glass of the lamp needs to be as transparent as possible, and the filament as free as possible of impurities (they affect the colour temperature of the output).

Then there is the dichroic coating of the reflector. This allows infra-red light (heat) to pass through while reflecting visible light, thereby reducing the amount of heat shooting through the LCD element (or at the DLP mirrors if your projector is a DLP one).