Introduction

The first question that begs to be answered is what is an Ellipsoidal Reflector Spotlight (ERS)? An ERS is a theatrical lighting instrument which provides a well-defined spot of light on stage. The ERS is also called a "profile spot" because the light beam will create a shadow for any soild object placed at it's gate. Unlike the Fresnel spotlight or the Parcan, the ERS's reflector and lamp do not move within the body of the instrument. Instead the lens assembly moves closer or farther away from the beam to sharpen or soften the focus of the instrument's beam. Furthermore, the Ellipsoidal Reflector Spotlight's beam is more controllable by far than that of other instruments. For these reasons the ERS is one of the most important lighting fixture in a theatre's inventory.

The Lamp (or, the all important light source)

Let us begin our study of the physics of the Ellipsoidal Reflector Spotlight at a very appropriate place, the lamp. All modern ERS fixtures use a Tungsten-Halogen lamp, rather than the older and less efficient Tungsten filament lamp. Lamp technology is always dealing with the problem of brightness versus life. The brighter the lamp is, the less life the lamp will have, and vice versa. The Tungsten-Halogen lamp still faces the same brightness vs. life trade off, however the Halogen lamp is brighter and lasts longer than the simple Tungsten filament lamp. The problem with the Tungsten filament lamp is sublimation. Sublimation is a process by which a solid changes to a gaseous state without first becoming a liquid. This process is greatly speed up under high-temperature, low-pressure conditions, such as those present within the envelope of a lamp. When the filament of a Tungsten lamp is heated by the flow of electricity through it, the Tungsten slowly turns to a gas, floats through the atmosphere of the lamp. Eventually, the free Tungsten condenses on the relatively cooler (600* C) interior of the glass surface in the form of a black, sooty Tungsten deposit. Sublimation of the Tungsten filament is the cause of death for many a lamp: the Tungsten finally wears so thin that the filament breaks and the lamp is "burn out."

The Tungsten sublimation problem has been solved by the discovery of the "Halogen cycle." The Halogen cycle occurs when a heated Tungsten filament is surrounded by an atmosphere of a Halogen gas: Fluorine (F), Chlorine (Cl), Bromine (Br), Astatine (Al), and Iodine (I). When the filament of a Tungsten-Halogen lamp is heated, the normal sublimation of the Tungsten filament occurs, and the Tungsten condenses on the cooler interior surface of the lamp's envelope. Almost immediately after the Tungsten deposits itself on the envelope's surface, the molecules of the Halogen gas combine with the Tungsten solid forming the gaseous Tungsten Halide. The molecules of Tungsten Halide diffuse into the lamp's atmosphere, and when they come into contact with the hot filament, the Tungsten Halide is split into Tungsten and Halogen and the Tungsten is deposited upon the filament. The Halogen again is a free gas with is left to begin the cycle again.

Conceivably, the Halogen cycle could continue ad infinitum, however the certain problems do occur in a lamp employing the Halogen cycle. The first of these problems stems from the Tungsten's tendency to form long, spindly crystals when deposited on the filament. These crystals may become so long that they reach the other filament and cause the lamp to short circuit. A second problem is referred to as filament erosion. The filament may develop a spot which is not hot enough to cause the Tungsten Halide to deposit the Tungsten on it. The filament eventually breaks at this spot and the lamp's circuit is broken. The final and most common problem is caused by a seal failure in the lamp's envelope near the base. The excessive heat of the lamp may cause the seal to compromise, flooding the lamp's atmosphere with oxygen. The Halogen cycle breaks down, and the envelope fills with a yellow oxide gas. Newer Tungsten-Halogen lamps (such as the HPL series used on ETC's Source Four) has a built in heat sink, a metal device with a large surface area that serves to dissipate heat.

The Reflector

All this talk of the ERS's lamp is well and good, but the power of the ERS lies in its ability to control the light of its lamp. First, the omni-directional light emanating from the lamp must be collected and redirected by the instrument's reflector. A reflector, in this case, is a curved piece of glass, coated on one side with a thin, silvery layer which redirects light at an angle equal to the angle of incidence at a tangent to the curved surface of the reflector. All theatrical lighting instruments employ a reflector in some form or another. Instruments such as the Fresnel, the Parcan, and the old beam projectors use a parabolic reflector, in which the mirrored surface curves around the light source in a parabola. The light beams from such an arrangement are projected parallel to the central axis of the parabola (see figure 2, Appendix A). The shape of the ERS's reflector is an ellipsoidal, which unlike the parabolic reflector, focuses the light emanating from the lamp into a focal point, dictated by the shape of the ellipsoidal (see figure 3, Appendix A). (Fuchs 81) This type of reflector creates a gate, a point at which the beam's shape and size may be manipulated by shutters, irises, or patterns cut into heat resistant metal called "gobos." When placed at the gate, gobos create a pattern in the light projected by the instrument by blocking some light, and allowing some light to pass.

It is interesting to note that more advanced ERS fixtures such as ETC's Source Four feature reflectors that are coated with dichroic layer which reduces the color temperature of the light emitted by the lamp. Infrared radiation (heat) is passed through the reflector and out the back of the instrument, rather than reflected out the instrument's barrel. This serves to reduce the wear, warping, and damage to gobos, color gels, shutters, and lenses that normally occur because of the high temperatures produced by the instrument's lamp.

And on we go to the Lenses...

Once the light has been collected by the reflector and manipulated at gate, the light is molded into a coherent beam by a pair of lenses. In modern ERS fixtures, the lenses used are in the double plano-convex arrangement. These lenses (like all others) employ refraction to control the direction of the shape of the light. Without lenses, light could not be focused into the spot that an ERS provides, rather the light would splay in random directions and flood the area being lit. A Parcan has no lens arrangement, and is most often used as an area-flood light.

Each of the lenses in an ERS has a set focal length: a point on the optical axis at which the rays of light are made to meet. The focal point of the first lens is the same focal point as the ellipsoidal reflector. The light passing through the first lens is refracted, leaving the lens parallel to its flat surface. Once past the first lens, the light enters a second lens (convex side first), and is refracted again. Once passed through the second convex lens, the light is projected at a set angle from the lens' optical axis, which is known as the beam angle.

The beam angle for a given instrument varies; it depends upon the distance the two plano-convex lenses are apart. In older designs, such as those still employed by the Altman Stage Lighting*, the beam angle for each instrument is permanent and cannot be changed much. For example, Altman's three principle configurations of ERS's are the 6x9, the 6x12, and the 6x16. The first number (in this case '6' for each) denotes the diameter of each plano-convex lens. The second number is the distance between the center of each lens in relation to the other. The closer the two lenses are together, the larger the beam angle. The distance between the two lenses can not be changed considerably, usually never more than an inch or two, which makes the focus on the beam's outer edges either hard or soft. Newer designs, such as ETC's Source Four, have improved upon Altman's design. By swapping out lens configurations, the distance between the lenses may easily be changed, making it much easier for the electrician to change the beam angle. Likewise, by changing the distance between the reflector's focal point and the first lens slightly, the hardness and softness of the beam edge on the Source Four is set. Both of these innovations make this newer design more flexible than its predecessor.

The beam angle and the distance of the instrument from a given point (in most instances the stage floor or a piece of scenery) can be used to determine the size of the pool of light the ERS will produce. For example, the Source Four (arguably the best ERS available) with a beam angle of 50* will at 10 ft produce a lighting field with a 9.5 ft diameter. As the distance from the instrument increases, the field diameter increases, in this case, by a factor of .95. In the case of a Source Four with a 26* beam angle, the factor is .45. Therefore, a 50* Source Four produces a field with the same diameter at 10 ft as a 26* Source Four does at 21.1 ft*. Practically, the beam angle of the instrument must be decreased as the instrument's distance from the stage increases.

To Conclude...

One can see that something as seemingly simple as a stage lighting instrument can be thoroughly complex. By only limiting the discussion of the light producing and emitting qualities of the ERS, one can see that if the topic were expanded to include other topics of interest concerning ERS fixtures, such as heat dissipation, color saturation, and so forth, the possibilities are endless. And technology in the field of stage lighting is expanding every day by leaps and bounds. New short arc lamps, moving light fixtures, PARnels*, control advances, and color projection are just a few of the new and exciting additions brought to the lighting world thanks to advances in physics and technology.