Getting the colour of light just right isn’t the only thing that a projector lamp has to achieve, says Stephen Dawson.

On the face of it the lamp would seem to be the most mundane part of a projector. All it has to do is produce light. Surely all the sexy stuff is in the flipping mirrors of a DLP projector, the extraordinary lenses that provide sharp focus over a wide zoom range, the processing power that adjusts and allocates the disposition of more than two million pixels up to sixty times a second.

For sure, but none of those things will produce a good picture without the right light. This is where the lamp comes in.

What it must it do
Colour temperature is not something that was generally in the public consciousness, until recent years with the advent of compact fluorescent lamp (CFL) home lighting. Then we became all too aware, with complaints that these produced a harsh, yet dim, light. Funnily enough, their light was actually far closer to daylight in colour than incandescent bulbs, which are heavily biased towards the red end of the spectrum. So what people were really complaining about was that CFLs weren’t like incandescent light bulbs.

The market responded, of course, with CFLs using phosphors that produce ‘warm’ light, albeit with a likely reduction in efficiency since the choice of phosphors is no longer optimised for a maximum light-output to power-input ratio.

But room lighting is arbitrary. Warm lighting dominates because that was what was produced by the first viable electric lighting system.

The colour of light that must be produced by the lamps in projectors is anything but arbitrary. It must be well defined in order to deliver accurate colours when projecting images.

But that’s not the only consideration for a projector lamp.

It also has to be bright. Most projectors work by pushing the light through filters, which means that much of the light is discarded. Two thirds or more, in fact. And while home theatre projectors tend to be used, as they should, in darkened rooms, they often have to light up rather large screens. Business projectors often have to compete with room lighting.

And it has to operate a long time. Sure, the 2,000 to 4,000 hour rating of the bulk of lamps might not seem long, but that is enough for more than a thousand movies, or two years of use for three hours every night.

Furthermore, it ought to avoid changing its light temperature, or losing too much brightness, over that operating life. Indeed, it has to deliver a similar light temperature when running in low output mode.

That’s why projector lamps typically cost two or three hundred dollars rather than twenty dollars.

How it’s done
About the only things in common between projector lamps and incandescent light bulbs is that you pour electricity in and light comes out. The latter works, of course, by running the electricity through a fine filament which glows hotly. Projector lamps are basically arc lamps, although far better than the exposed ones of the early days of cinema, where the electrodes had to be constantly tweaked to maintain the spark jumping over the gap.

With projector lamps, the electrodes are held in a small glass or quartz bulb that is contained within the larger bulb. In that small globe there are high pressure gases or materials that will become high pressure gasses. (A lot of lamps have names like ‘UHP’, which stands for Ultra High Pressure, and they’re not kidding.)

When switched on a high voltage is first applied to get the spark to jump the gap. This is some thousands of volts. Later, when the lamp gets up to operating temperature, that’s reduced to tens of volts. The various metal halide materials heat up and then vaporise, creating the high pressure gas through which the electrical arc is maintained.

The composition of the gas determines the colour of the light. Different manufacturers use different compositions in order to best realise high performance in their particular projectors.

As the lamp ages the electrodes gradually deteriorate and the interior of the inner bulb is marked with deposits. This reduces light output (and changes its colour characteristics) and increases the temperature of operation. Using far beyond the designed lifespan can result in the lamp exploding, which is why most projectors will not operate once the lamp timer has reached its specified operating life.

One major reason for this technology is efficiency. Most of the energy fed into an incandescent lamp is converted to heat. Less than 5% – usually, well less – becomes usable light. High pressure metal halide lamps can be as much as 25% efficient.

The modern projector lamp
A modern projector lamp is sometimes just the lamp bulb itself, but more often it comes in an assembly. That’s in part for protection of the lamp, and in part because of the fine tolerances required in managing the light within a projector.

Any light globe, including one in a projector, sends its light in all directions. But in a projector it needs to be sent in one carefully selected direction (where it will be sent to projection panels, first being split if it’s a multiple panel projector) if any kind of reasonable efficiency is to be achieved. This is where a lot of the engineering takes place: carefully placing reflective surfaces close to the lamp to ensure all the light goes in the right direction.

Epson’s E-TORL lamp technology, for example, is claimed to increase lamp efficiency by 20% not in the production of light, but in managing it by the careful placement of mirrors so that little is wasted.

Electronic adjustments
Projector lamps are just part of an integrated system. They produced light of specific characteristics which are well known to the manufacturers. Likewise their performance gradients over time are well known.

So when producing a projector manufacturers don’t just feed a naïve raw signal to the Digital Micro-mirror Devices or the 3LCD or the LCoS panels which actually control the image production. It is tailored for the characteristics of the lamp.

Let’s say the engineers discover that they can achieve a bit more efficiency, or a longer lamp life, if the lamp has a very slight blue bias. It might be worth doing since it can be largely offset by the image processing in the projector reducing the blue levels slightly in the signal. Likewise, by referring to the lamp timer the projector could, theoretically at least, offset expected colour changes over the life of the lamp.

And all that means, replacement with the genuine manufacturer’s lamps is pretty important for home theatre applications.

Going in a different direction
Some lamps, though, will likely never have to be replaced. These are high powered LED lamps.

There has much work been done in recent years to achieve white LEDs. But that is not what is wanted in an LED projector. Instead it is better if these produce red, green and blue light individually.

LEDs are efficient, of course, but with the three separate colours they are even more so. Normal projectors throw away some two thirds of their light through filtering. A LED lamp only produces each colour as required, and so does not need to be filtered.

These work with single chip DLP projectors, so instead of white light being filtered by a colour wheel linked to the signal processor, the LED bank itself produces only the correct colour of light required from instant to instant.

This is a very exciting development of course and has another advantage over standard lamp technology: a very long life. These LED’s tend to be rated at 20,000 or more hours of life, which means you’re likely to be in the market for a new projector years before the lamp needs to be replaced.

Their downside, though, is limited brightness. At their best they tend to be capable of only one third the output of conventional projectors.

But give it a few years and that may well change.