Functions of lighting
Stage lighting has several functions, although to allow for artistic effect, no hard and fast rules can ever be applied. The functions of lighting include:
Illumination: The simple ability to see what is occurring on stage. Any lighting design will be ineffective if the viewers cannot see the characters, unless this is the explicit intent.
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. Harsh red light has a totally different effect than soft lavender light.
Location and time of day: Establishing or altering position in time and space. Blues can suggest night time while orange and red can suggest a sunrise or sunset. Use of gobos to project sky scene, moon etc.
Projection/stage elements: Lighting may be used to project scenery or to act as scenery onstage.
Plot(script): A lighting event may trigger or advance the action onstage.
Composition: Lighting may be used to show only the areas of the stage which the designer wants the audience to see, and to "paint a picture".
While Lighting Design is an art form, and thus no one way is the only way, there is a modern movement that simply states that the Lighting Design helps to create the environment in which the action take place while supporting the style of the piece. "Mood" is arguable while the environment is essential.
Qualities in lighting
The four main qualities or properties of lighting are intensity, colour, pattern and focus.
Measured in lux, lumens and foot-candles. For any given luminaire (lighting instrument or fixture), this depends upon the power of the lamp, the design of the instrument (and its corresponding efficiency), the presence or absence of colour gels or gobos, distance from the area to be lit and the beam or field angle of the fixture, the colour and substance to be lit, and the neuro-optics of the total scene (that is, the relative contrasts to other regions of illumination).
Colour temperature is measured in kelvin, and gel colours are organised by several different systems maintained by the colour manufacturing companies. The apparent colour of a light is determined largely by the gel colour given it, but also in part by the power level the lamp is being run at and the colour of material it is 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 or amber shift. Thus a 1000-watt instrument at 50% will appear far more orange than a 500-watt instrument at full.
LED fixtures create colour through additive colour mixing with red, green, and blue LEDs at different intensities. This type of colour mixing is also used frequently with border lights and cyclorama lights to create different colours on stage and on the cyclorama. Another form of colour mixing is CMY, or subtractive colour mixing. Cyan, magenta and yellow dichroic filters are used in different percentages to create different colours. Because it is often difficult to create true reds and greens, a green dichroic filter is often added to fixtures using this method of colour mixing.
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 lamp, reflector and lens assembly. Different mounting positions for the lamp (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 profile spotlights, 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. This is typically a thin sheet of metal with a shape cut into it. It is inserted into the instrument near its aperture. Gobos come in many shapes, but often include leaves, waves, stars and similar patterns.
Focus, position, and hanging
Many stage lights hung on a batten focused in several directions
Focus is a term usually used to describe where an instrument is pointed. The final focus should place the "hot spot" of the beam at the actor's head level when standing at the centre of the instrument's assigned "focus area" on the stage. Position refers to the location of an instrument in the theatre's fly system or on permanent pipes in front-of-house locations. Hanging is the act of placing the instrument in its assigned position.
In addition to these, certain modern instruments are automated, referring to motorised movement of either the entire fixture body or the movement of a mirror placed in front of its outermost lens. These fixtures and the more traditional follow spots add Direction and Motion to the relevant characteristics of light. Automated fixtures fall into either the moving head or moving mirror/scanner category. Scanners have a body which contains the lamp, PCBs, transformer, and effects (colour, gobo, iris etc.) devices. A mirror is panned and tilted in the desired position by pan and tilt motors, thereby causing the light beam to move. Moving head fixtures have the effects and lamp assembly inside the head with transformers and other electronics in the base or external ballast. There are advantages and disadvantages to both. Scanners are typically faster and less costly than moving head units but have a narrower range of movement. Moving head fixtures have a much larger range of movement as well as a more natural inertial movement but are typically more expensive.
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.
Stanley McCandless was perhaps the first to define controllable qualities of light used in theatre. In A Method for Lighting the Stage, McCandless discusses colour, distribution, intensity and movement as the qualities that can be manipulated by a lighting designer to achieve the desired visual, emotional and thematic look on stage. The McCandless Method, outlined in that book, is widely embraced today. The method involves lighting an object on the stage from three angles, 2 lights at 45 degrees to the left and right, and one at 90 degrees (perpendicular to the front of the object).