Illumination: The simple ability to see what is occurring onstage.
Revelation of Form: Altering the perception of shapes onstage, particularly three-dimensional stage elements.
Focus: Directing the audience's attention to an area of the stage or distracting them from another.
Mood: Setting the tone of a scene.
Location and Time of Day: Establishing or altering position in time and space.
Projection/Stage Elements: Lighting may be used to project scenery or to act as scenery onstage.
Plot: A lighting event may trigger or advance the action onstage.
In the pursuit of the Principles of Lighting, the three main Qualities or Properties of interest are:
Intensity: Measured in both lux and lumens. For any given luminaire (lighting instrument or fixture), this depends upon the wattage of the bulb, the design of the instrument (and its corresponding efficency), the presence or absence of color gels or gobos, distance from the area to be lit, the color and subtance to be lit, and the neuro-optics of the total scene (that is, the relative contrasts to other regions of illumination).
Color: Color temperature is measured in kelvin, and gel[?] colors are organized by several different systems maintained by the color manufacturing companies. The apparent color of a light is determined largely by the gel color given it, but also in part by the power level the lamp is being run at and the color of material is it to light. As the percentage of full power a lamp is being run at drops, the tungsten filament in the bulb glows orange instead of more nearly white. This is known as Amber Drift. Thus a 1000-watt instrument at 50% will appear far more orange than a 500-watt instrument at full.
Pattern: Pattern refers to the shape, quality and evenness of a lamp's output. The pattern of light an instrument makes is largely determined by three factors. The first are the specifics of the bulb, reflector and lens assembly. Different mounting positions for the bulb (axial, base up, base down), different sizes and shapes of reflector and the nature of the lens (or lenses) being used can all affect the pattern of light. Secondly, the specifics of how the lamp is focused affect its pattern. In Ellipsoidal Reflector Spotlights (ERS) and their derivatives, there are two beams of light emitted from the lamp. When the cones of both intersect at the throw distance (the distance to the stage) the lamp has a sharply defined 'hard' edge. When the two cones do not intersect at that distance, the edge is fuzzy and 'soft'. Depending on which beam (direct or reflected) is outside the other, the pattern may be 'thin and soft' or 'fat and soft'. Lastly, a gobo[?] or break up pattern may be applied to ERS's and similar lamps. This is typically a strip of metal with a shape cut into it. It is inserted into the lamp near its aperture. Gobos come in many shapes, but often include leaves, waves, stars and similar patterns.
In addition to these, certain modern instruments are 'movable', refering to motorized movement of either the entire lamp or a mirror placed in front of its outermost lens. These lamps and the more traditional follow spots add Direction to the relevant characterists of light.
It is important to note that the above characteristics are not always static, and it is frequently the variation in these characteristics that is used in achieving the goals of lighting.
The above elements of lighting are primarily the domain of the Lighting Designer (LD). In consultation with the director and the scenic or stage designer and after watching sufficient late rehearsals, the LD is responsible for providing an Instrument Schedule and a Lighting Plot. The Schedule is a list of all required materials, including gel colors, gobos, color wheels, barndoors and other accessories. The lighting plot is typically a plan view of the theatre in which the performance will take place, with every luminaire marked, including its 'rough' focus (the direction it should be pointing), its instrument number, any color/gobo/accessories required, and the specifics of its connection to the lighting control systems (channel number). These form the basis of the work the Lighting Crew is to undertake, under the supervision of the Head Electrician and the direction of the Crew Chief.
There are a variety of instruments frequently used on the stage. They may be characterized as follows:
It is important to note that virtually all theatrical bulbs are incandescent. Fluorescent lights are rarely used outside of work lights because, although they are far more efficient, they cannot be 'dimmed' (run at less than full power), they do not produce light from a 'point' or easily concentrated area, and have a warm-up period, during which they emit no light or do so intermittently. Carbon arc lamps do exist, though they are rare.
All lights are either Floodlights[?] or Spotlights[?]. The distinction has nothing to do with the area covered, but is based upon the presence or absence of a focusing lense[?] or lenses. Lamps that lack a focusing lens or are in a permanent fixed relationship to their lens are Floodlights. These include:
PAR lights (Parabolic Anodized Reflectors) These resemble car headlights. They possess a lens, but it is an integral part of the lamp housing, and its position relative to the filament cannot be altered.
The lamp produces an intense oval pool of light with soft edges. The only adjustment is a knob that allows the lamp unit to be rotated within its casing, thus changing the orientation of the oval.
These types of instruments come in varying diameters, the most common being designated PAR56 and PAR64 – the number indicates the diameter of the housing in eighths of an inch (eg a PAR64 is eight inches in diameter).
Because of the lack of control over beam diameter, shape and sharpness, PARs are seldom used in theatre (except for special effects) but are used extensively at rock concerts, especially in combination with smoke/haze machines which make the path of the beam visible.
Strip or Cyclorama (Cyc) lights These are long housings typically containing multiple bulbs arranged along the length of the instrument and emitting light perpendicular to its length.
Most 'practical' fixtures onstage are also floodlights.
Fresnels: Fresnels typically are 6 or 4 inch, refering to the size of the lens. The lens is the distinctive 'Fresnel lens' type, with a 'stepped' appearance instead of the 'full' or 'smooth' appearance of other lenses. The stepped nature of the lens causes a corresponding pattern of circles of light, so Fresnel lenses are usually 'stippled' on the flat side. This pattern of small bumps helps to break up the light passing into the lens to smooth out its eventual pattern.
Fresnels use a parabolic reflector, with the filament of the bulb at the focus. Due to this, the bulb and reflector cannot move independently of one another, and remain a fixed unit inside the housing. It is this unit that is moved back and forth inside the lamp to focus the fresnel. This is done by a slider on the bottom of the light, or by a worm track.
Fresnels are not very efficient. The reflector cannot be larger than then lens apature, and thus all the radiated light that is neither reflected by the parabola behind the bulb or emitted directly through the lens is absorbed by the casing as waste heat. Additionally, the degree to which the lamp may be focused is limited by the length of the housing. The tighter the focus ('spotted in') the less light is able to escape. Thus fresnels are not good for tight focus on small areas. Fresnels also lack internal shutters, and must rely on barndoors, large metal flaps that may be mounted just beyond the color slot at the front of the light. Due to these restrictions, Fresnels are most often used at middling distances for area lighting. Fresnel bulbs are almost always 'base down': mounted with the bulb up.
ERS: The Ellipsoidal Reflector Spotlight is also sometimes known as a 'Leko'. It comes in very many forms, and is the most numerous and important instrument type in use. The flexability of the ERS allows them to fulfill the bulk of the lighting roles in the theatre, from area lighting to close specials, from long throws from the back of the house to shin kickers on the stage.
ERSs may have more than one focusing lens, while all posses internal shutters for cropping the emitted light, and all accept a color gel in front of the lens and a gobo in front of the shutters. ERS's may have bulbs mounted axially, or with the base either up or down (it is important to hang a lamp in the proper orientation). The lenses are smooth and full, not stepped, and it is the lens or lenses that move in the ERS, not the bulb and reflector assembly as in the Fresnel.
The ERS improves over the efficiency of the Fresnel by surrounding the bulb in an ellipsoidal reflector[?], with the filament of the bulb at one focus and the aperture to the lens housing at the other. The shutters and gobo are ideally in focal point of this apature.
The Paralipsphere is a further improvement of the ERS. It replaces the portion of the Ellipsoid behind the bulb with a parabolic reflector focused on the filament, and additionally replaces the portion of the Ellipsoid immediately surrounding the aperture with a spherical reflector focused on the bulb. Para(bola)+(El)lip(soidal)+Sphere = Paralipsphere. From a lighting design point of view, Paralipsphere's are merely a more efficient ERS, but from a technical point of view they are an interesting and significant variation.
It is important to note that gobos and color gels both suffer from 'burn in'. The power (> 1000 W) and efficiency of modern lights, particularly Paralipspheres, is such that they are able to seriously degrade metal gobos and plastic color after relatively short periods of time. Currently, the limit on improvements in the efficiency of lighting instruments is the heat- and light-tolerance of color materials.