In the world of theater, it takes many things to make a good show. Scenery, costumes, makeup, and, increasingly, lighting. Until about fifty years ago it wasn't uncommon for stage lighting to consist of only a few high-powered, wide-beamed lights that did nothing more than illuminate the stage.

Today, though, a good show almost always requires good lighting. In fact many touring productions are little more (in technical terms) than a few set pieces, some props, and a lot of lights. This transformation is due, in no small part, to substantial advances in stage lighting technology.

As late as the 1980's stage lighting instruments (units, in the parlance of the business) had a major problem: they gave off far too much heat. The heat, as you might imagine, caused no end of troubles. For one thing, it made the units difficult to handle. For another, the heat was incredibly focused -- in the same general direction as that the light traveled. Why a problem? Because in between the lamp (literally the light bulb inside the unit) and the object onto which the light was projected (an actor, a set piece, ect) there were a couple of important things.

First were the shutters. These flimsy pieces of aluminum allowed the lighting designer to shape the beam of light. Meaning they could, say, keep the light on the performers, but keep it off the ugly speaker hanging on the proscenium down-stage left. Unfortunately, due to the need to keep a lighting instrument relatively light (as in, not heavy) the shutters had to be rather thin and after repeated heating and cooling they tended to warp, or in extreme case, burn through.

Next was the lens, which focused the light, providing a narrow or a wide beam. And immediately after that came a paper-thin sheet of plastic called gels.

Many people have a least a passing familiarity with gels. If you've ever been to a stage performance -- play, rock concert, ballet, what have you -- and looked up you probably saw a multitude of colors floating in space over the audience. These are the gels. Illuminated by the lights behind them, they allow the lighting designer to paint the stage with light -- in less grandiose terms, to set the mood.

To do this work gels depend on absorption. A dark blue gel, R85, for example, will allow all light with a wave length of about 450 nanometers (nm) to pass through; the rest of the visible spectrum -- from about 460nm to 700nm -- is absorbed by the gel. Since the lamp inside the lighting instrument gives off white light, and since white light is nothing more than all colors (a.k.a. wavelengths) combined, the gel absorbs a heck of a lot of energy. In addition the gel has to absorb nearly all the infrared energy (heat) the lamp gives off.

All this energy absorption would cause, and can still cause today, saturated (deeply colored) gels, such as R85, to burn. A burnt gel can be a slightly curious or a stupefyingly embarrassing thing to watch, depending on which side of the stage you're on. From the audience all you see is a beautifully lit scene start to look kind of . . . strange. The deep blue hues which had heretofore indicated night slowly start to lighten. "Dawn?" you ask. Then small circle of bright, white light appears on the stage. "What an odd cue," you think. Meanwhile, the lighting designer, if she or he is present, is making a mad dash to the control booth, cursing under their breath.

To solve this problem of dashing and cursing modern stage lighting equipment employs the dichroic filter.



In previous incarnations, the lamp inside the lighting unit was supported by a reflector. Situated behind the lamp the reflector (as you might have guessed) reflected the light forward to the lens. While the same idea is in use in today's units, the reflector has been highly modified.

Introduced in 1992 the ETC Source Four fast became the industry standard. In addition to a number of other innovations the Source Four uses a dichroic reflector composed of borosilicate glass. You probably have some borosilicate glass in your kitchen, or at least used it in chemistry class. In the US it's sold under the brand name Pyrex.

The advantage of the dichroic reflector is that it focuses all the visible light toward the front of the unit, while allowing the heat to escape out the back. This happens because of the principle of interference.

As the light strikes the reflector it comes in contact with successive layers of optical coating -- not unlike the coating on some glasses which reduces glare. This coating allows the manufacturer to control which wavelengths pass through the filter -- the passband -- and which are reflected back -- the stopband. In the case of a Source Four reflector the passband is all infrared energy, or electromagnetic (EM) radiation with a wavelength of 750nm and above. The stopband, then, is all visible light, or EM radiation with a wavelength below 700nm.

When two waves -- whether from the ocean, the vibration of a string, or an EM source -- are superimposed the shape of the resulting wave depends on whether or not the two original waves are in or out of phase. In the case a Source Four reflector, the two original waves are the EM radiation from the lamp and the optical coating on the glass. If the waves are in phase, that is, if the troughs and peaks line up, there is constructive interference and the radiation passes through the reflector and out the back of the unit. If the waves are out of phase, then there is destructive interference and the radiation is reflected toward the lens.

Which means, of course, that in today's modern world, there are far fewer burnt gels and far more happy lighting designers.

There is, however, another stage application for dichroic filters -- they can replace the gels themselves.

Operating on the same principle of interference, filters can be made with a wide variety of passbands. Say, for example, a designer wanted to use a very saturated blue, say, R85 for an entire show. A natural concern, even with Source Fours, would be the previously mentioned, insanely awkward burnt gel. To avoid heartburn the designer might decide that all units with R85 should have their gels removed at intermission and replaced with fresh ones. A fairly simple task, but what if the units are in hard to reach spots, like over the stage?

The solution is a dichroic filter. In a manner similar to the way the dichroic reflector allows the heat to pass through it, a dichroic filter allows certain wavelengths to pass through, while reflecting the rest away.

In the case of R85, the filter allows light with a wavelength of 450 nanometers to pass through while reflecting nearly all the rest of the visible spectrum. In fact, because of constructive inference, the filter actually increases the amplitude (the distance from the top of a peak to the bottom of a trough divided by two) of the light in the passband, making for a deeper, more vibrant hue.

Despite all their advantages dichroic filters still haven't caught on in any meaningful way within the theater industry. The hang-up for most companies is the cost. One filter can cost as much as one sheet of gel. But, whereas a sheet of gel can color four to eight units, a filter colors only one.

Then again, maybe some designers relish the nostalgia of hot-swapping color during intermission.

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