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HOTOMETRICS shaft of rather than a simple wash P going in all directions. 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 instrument. These are; light.” In common usage though, the light source or lamp, the reflector, and photometrics is used to describe how the . 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 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 changers and 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 lights, which are the backbone of lighting practice.

In his 1936 work, “A Syllabus for 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 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 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 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 “” 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 . 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. More than that, it is designed heat that is generated by this quartz lamp to do this in a way that works optically inside a small fixture. Earlier versions of with the rest of the instrument, creating a similar lamps were prone to failure in the specific type of light beam. The most base or socket because of the heat factor. commonly used shapes for a reflector are parabolic, ellipsoidal, and spherical. The type of reflector used will greatly affect how the light rays act after they leave the fixture itself.

4 The study of how light interacts with the equal, but the size of the angle will be natural world is known as . altered, and the light will be reflected to a is the study of the different place. and refraction of light when it interacts with lenses and . Although light is often described as either waves or particles called photons, for the purposes of a study of geometrical optics it is helpful to visualize light in terms of rays. A light is assumed to travel in a straight line unless acted upon by a lens or reflector.

The axiom of angle of incidence equals the angle of reflection also holds true for curved Reflectors are in fact mirrors that have a surfaces, but the curve affects what the curved shape, and the properties of that angle will be for each particular point on curve affect the manner in which the light the curve. Sometimes it is helpful to rays are reflected. On a , planer surface, think of a curve as a series of infinitely light rays bounce off the surface so that small flat surfaces connected together. the angle of incidence is equal to the angle Viewed in that context, a curved of reflection. The angle of incidence is the becomes a series of reflectors each with its angle at which the light ray strikes the own angle of incidence/reflection. mirror’s surface.

If the mirror is held at an angle and the It is important to remember that reflectors light source remains in the same place, the are three dimensional forms, but it is angles of incidence/reflection will still be generally easier to visualize them using

5 two dimensional shapes. Hence, a two points and allowed to hang spherical 3-D form becomes a 2-D arc downward in a curve, the shape it forms is when depicted on a flat piece of paper like a . The arc of a baseball through the one you are reading right now. the air is also a parabola. are a frequent object of mathematical study Many early lighting instruments, such as because they occur in the natural world, the fresnel, used a reflector that was yet have a scientifically predictable shape. spherical; which is to say that its shape is derived from a circle. When a parabola is used as a reflector, the light source at its natural focal point is It is the nature of a mirror curved with reflected outward with all rays pointing in this shape that a light source in the center one direction. In this theoretical will reflect itself back to the center. If a construct, the light source is viewed as a light source is placed closer to the single point. In a practical lighting reflective surface, rays of light are reflected instrument, the filament is not a single outward, all in one direction. This point, but rather a coil of tungsten wire method is not perfect however, and in the with dimensional qualities. Because of process light rays on the extreme edges of this, the light from a fixture with a the reflector are deflected in a more also tends to be random pattern. Light that falls outside somewhat diffuse, in a similar manner to the desired pattern is said to be ambient. the fresnel. If the filament is arranged As a lighting instrument, the fresnel tends laterally across the reflector, the light from to produce a beam of light which is an instrument of this type will have a brighter in the center than it is on the beam that is somewhat oval in shape. edges because imperfections in the reflection of light from a spherical reflector has a good deal of ambient light mixed in with the intended light beam.

A third type of commonly used reflector draws its shape from the ellipse. In plane The parabola is a very interesting shape geometry, an ellipse is formed by using that occurs in nature in many different two focal points, F1 and F2. The curved ways. When a string is draped between surface of the ellipse is defined by adding

6 (distance) d1=d2. This sum should be the The “string method” of forming an ellipse same for any point on the curve. makes it easier to visualize how light rays are affected by an elliptical reflector because the curved surface is such that a ray emanating from one focal point will be reflected toward the second one. In a theoretical construct, all of the light from a lamp in the position of F1 that hits the reflector will be bounced off to F2. Of course in a practical lighting instrument, the reflector cannot be an enclosed unit because there would be no way for the light to get out and reach the stage. Only part of the ellipse is used, and because of that, some light from the lamp will not be properly reflected and will become ambient light. Even so, the elliptical A string can also be used to demonstrate reflector is very efficient when compared this. Fix the ends of the string to the two to other types. focal points and stretch it taut in any direction. Use a pencil to draw a line all the way around. The resulting curve will be an ellipse. Varying the distance between the focal points produces ellipses of different proportions. If the focal points are moved close to one another, an approximate circle will be formed. If the focal points are so far apart that the string is almost tight between them, a something resembling a straight line will be formed.

One interesting aspect of a fixture with an elliptical reflector is that the light rays actually cross over each other as they pass through the second focal point. If this type of fixture is used to project an image by using a metal pattern placed between the two focal points, the image will be upside down and reversed. Projecting patterns is a very useful quality of an ellipsoidal instrument.

The exact shape of the ellipse is important in determining the width of the beam of light that will be projected by an ellipsoidal. A narrower ellipse will produce a narrower beam, a fuller ellipse a

7 wider beam. Different beam angles are You might actually observe several important to lighting designers so that different effects on the straw, because not they can efficiently shape the light that only is light bent by the water, it is also will be projected onto the stage. affected by the transparent glass container. Manufacturing different reflector shapes is The density of the material makes a difficult, and rather than do that lenses are difference in how much refraction takes typically used to create different beam place. A denser material creates more angle widths. refraction than a less dense material, so the glass has more of an effect than the water. Glass lenses were first invented at the end The angle at which the light ray intersects of the 13th century, and use the principle with the refractive medium is also of refraction to change the angle at which important. A light ray that strikes the light rays travel. In the case of a refractor at a steeper angle is bent more magnifying glass, light rays are gathered than one striking at a shallower angle. If by the lens as they pass into the human the intersection is at a 90 degree angle, eye. In the case of a lighting instrument, there is no refraction. light rays are affected as they pass out of the fixture, but the optical principles involved are the same.

Dutch mathematician Willebrord Snell determined that light rays passing through a denser medium were slowed, and that this process bent, or refracted the rays so that they moved in a different direction. Most everyone is familiar with the “broken straw” example, where a straw in a transparent glass appears to bend where it enters the water. The straw is not really bent, but appears to be because light passing through the water and glass is slowed, and according to Snell that refracts the light into a new direction. Useful lenses for the theatre are generally of a type known as Plano-convex, meaning that they are flat on one side, and curved on the other. The curvature of the lens affects the amount of bending that occurs, and by carefully calibrating the curve it is possible to focus light rays from an instrument into a cohesive unit. The thickness of the lens is directly proportional to the amount of bending a lens will create, because a thicker lens has a more pronounced curve. In the typical ellipsoidal instrument, two Plano-convex lenses are used in conjunction in order to reduce the thickness that would be required if only one were used.

8 instrument, which creates a soft pool of light where precision is not required, unlike an ellipsoidal instrument. Fresnel lenses have a slightly pebbled surface on the plano side in order to diffuse the pool of light they create. This helps to hide light and dark spots resulting from the imprecision of the stepped lens.

Diffusion is the main objective of some

lenses, especially those used on fixtures with a parabolic reflector such as PAR Augustin Fresnel was a 19th century lamps, or the ETC Par. In French scientist working with the either of those two types, the lens serves to properties of light who developed a spread the beam outward a specific specific type of lens often used in amount. The lenses have a similar lighthouses, and also in theatrical lights. appearance, and can be identified by the One of the problems of the day was that texture of the surface. the very thick lenses used in lighthouse lights tended to crack. This was because impurities in the glass would cause it to TERMS USED IN THIS CHAPTER heat up when the light was turned on, and the heat caused the glass to expand. Photometrics When turned off, the glass would contract, and this movement often meant that the somewhat brittle glass would crack, Instrument ruining the lens. Fresnel solved this Fixture problem by creating a stepped lens that maintained its convex shape on the curved Lamp side, but did so in steps, so that the Reflector resulting lens was much thinner. Lens Filament Quartz glass Carbon arc Xenon HMI

Dowser

Although this works perfectly well in Followspot theory, the formation of the glass is not completely exact, and a certain amount of Color Temperature ambient light is created. Fresnel lenses are Base used theatrically in the fresnel lighting

9 Socket Two pin RSC/double ended Bipost ANSI American National Standards Institute Parabolic Ellipsoidal Spherical Reflection Refraction Geometrical optics Rays Incidence Fresnel Ambient Focal Points Pattern Plano-convex Diffusion

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