White Paper on Projector Lamps


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

Lamp technology


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).

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

To see the sections on Economics (Market Conditions, Investment, The Supply
Chain, Compatible Lamps) and the Appendix (Lamp Life, The Difference
Between a Bulb, a Lamp, a Housing and a Module), download the actual White
Paper on Projector Lamps from tekgia.com.

Bob Wilkins, Tekgia (http://www.tekgia.com

Download a White Paper on Projector Lamps: Projector Lamps

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