HOTOMETRICS shaft of light rather than a simple wash P going in all directions. Lenses completed the triad of mechanical parts that make up Technically, photometrics means “those the core of photometrics as used in a things pertaining to the measuring of modern lighting instrument. These are; light.” In common usage though, the light source or lamp, the reflector, and photometrics is used to describe how the lens. Most all modern fixtures contain lighting instruments, or fixtures, are all three of these elements, though some of defined as to their sizes, light output, and them place two or more within the lamp uses. One of the most exciting itself. As an anomaly, most border lights developments in lighting over the past two do not have a lens per se, although they decades has been in the area of intelligent may have glass filters that look like one. lighting; specifically that is moving fixture lights and accessories such as color changers and gobo rotators. What sets these innovations apart is their ability to be controlled digitally. The computer control of lighting has made all sorts of effects possible, and the devices that do this have revolutionized the business. They are so important, that they will be covered in a separate section. This chapter is devoted to more traditional theatre lights, which are the backbone of lighting practice. In his 1936 work, “A Syllabus for Stage In any sort of electrical lighting fixture, Lighting”, Stanley McCandless coined the the lamp is used to create light, the word instrument, which he used in reflector helps to gather the part of that referring to lights used for the stage. light that emanates in the wrong direction, Perhaps he was attempting to use a and the lens helps to either focus or scientific name to describe something that, diffuse it into a useful shape. It seems up to that point, was not given much appropriate to start this discussion with thought – lighting for the stage. Lights the lamp since that is where light begins. today are still very often called instruments, but the term fixture is also As discussed in chapter two, The Physics very popular as it fits in well with the of Electricity, most lamps generate light terminology used in commercial lighting by resisting the flow of electricity through practice. In most situations, the words the filament of the bulb. The tiny wire instrument, fixture, and light are used filament becomes so hot that it interchangeably. incandesces, which means that it has so much heat energy that some of it is The earliest electrified theatre-specific dissipated in the form of light energy. To fixtures were basically just light bulbs with keep the filament from oxidizing, or a shiny surface behind them to reflect burning up in the atmosphere, a glass bulb more of the light in one direction. Not was first used to create a vacuum around long after, lenses were used to help focus the wire filament. Modern versions have the light beam down to a more specific an inert gas like argon inside which creates 1 pressure to keep the filament from “off The arrangement of the filament in the gassing” and make it last longer. lamp is very important to the ability of the lamp to work well in concert with the A longer filament generally produces more reflector and the lens. In a theoretical light energy, so the wire is often curled sense, the source of the light should be inside the glass bulb so that it can be made from one single point for the reflector to longer without using up too much space. work at its highest efficiency. Of course Making a bulb smaller in relation to its this is not entirely possible given the output is very important for theatre lights coiled nature of a practical filament, but because they are much brighter than even so manufacturers must take great ordinary commercial fixtures. A large care to design the filament so that it lamp requires a large fixture, which is comes as close as possible to the problematic when trying to hang a theoretical model. number of instruments in a small space. One of the problems facing engineers a half century ago was that the high wattage lamps used in theatre lights got too hot for the glass bulbs of the period, and they tended to deform from overheating if the filament was too close. To accommodate the heat produced, the glass envelope surrounding a 1000 watt lamp had to be several inches in diameter, which was just too big for a practical fixture. About that time quartz glass was Another type of lamp has no filament at developed to create a lamp that could be all, and is a descendent of the carbon arc very tiny, but put out a very large amount lamps used through much of the 20th of light, and it is that type of glass that is century. In an arc lamp of that early type, used extensively today for modern lamps. two carbon rods were positioned slightly Quartz glass (which is made from a finely apart from one another and a high-voltage ground powder of that mineral) can DC current passed between them. The absorb a tremendous amount of heat gap between the two rods was carefully without being destroyed. It is important controlled so that the arc could be as large not to touch a quartz lamp with your bare as possible for the amount of voltage hand when installing one in an instrument supplied by a rectifier unit. Remember because dirt and oil on your fingers will that rectifiers change AC current into DC adhere to the glass. Upon heating, the oil current. will turn black, that part of the glass will get hotter than the rest, and it will melt in The rectifier and the rods were tuned short order. If the power to the light has together so that the space and the voltage not been switched off, the instantaneous were compatible with one another. If the heating of the lamp will burn your hand. rods were too far apart and the voltage pressure too small, the arc would not be able to jump the gap. Too close together, and the rods would fuse themselves together, and leaving no space to create 2 the arc. Since they were open to oxygen the fixture without cooling down, so in the atmosphere, the carbon rods burned extreme caution must be used when up as the light was produced, and needed changing one. be replaced frequently. Another drawback to arc lamps is that they are not dimmable in the normal sense because reducing the voltage to this type of lamp makes the arc go out all at once. Instead, a masking screen must be used. In a followspot, a dowser is used to partially (or fully) mask part of the light You can still see this sort of device today, beam, and as a result the light emitted as older “searchlight” units are used to from the fixture is reduced. Other lights send shafts of light through the night time use a system of louvers in front of the sky at store openings and the like. Arc instrument that are similar to Venetian lights in general are incredibly bright, blinds to vary the output. These which makes them useful for theatre problematic aspects make it difficult to spotlights. Modern versions of the carbon use xenon lamps for much more than arc spot can be seen in lamps such as the followspots, where an operator is standing Xenon, and the HMI. close at hand. Xenon lamps work in much the same way as carbon arcs, but the electrodes used to create the arc are sealed in a glass enclosure so that no oxygen can get in. The electrodes are made from a special alloy that, in conjunction with the gas inside the tube, helps to replace burnt parts of the electrode so that they last longer. Even so, this type of lamp is very expensive, sometimes more than the cost of the fixture itself. The lamps get very hot, the gas inside is under pressure, and thus there is a tendency for the lamp to explode, especially if it is removed from 3 Even though some aspects of arc lamps are The letters “HPL” are an example of problematic, they are so bright that they listing lamps by their ANSI code. The are the best choice for followspots and American National Standards Institute other specialty fixtures. In addition to creates standards for many different Xenon lamps, a similar type designated industries in the United States. They the HMI or Hydrargyrum Medium-arc insure that devices are compatible with Iodide is very commonly used in the one another, and that, for example, any television and film industries because of its standard screw base light bulb will fit into higher color temperature. any standard screw base socket. They have provided a three letter ANSI code for The base is the part of the lamp used to every lamp manufactured. The code is make connection with the socket, which is stamped on the shipping container, and part of the fixture itself. Everyone is should also be visible on the base of the acquainted with the screw base because lamp itself. Lamps of the same basic type, this is the type most commonly used in but different wattages, have different everyday life. Other types include the two codes. pin, the RSC or double ended, and the bipost. The HPL is a two pin lamp A reflector is used to “reflect back” the which is fitted with an aluminum heat light rays that are going toward the rear of sink to dissipate the extreme amount of the fixture.