Qualities of Light

by Bill Williams

Any study of lighting design must include a thorough understanding of both the PHYSICAL and the PSYCHOLOGICAL properties of light.

Knowledge of the behavior and properties of light can help explain vision and human perception. The lighting designer is especially interested in how the properties of light affect the eye/brain process and cause feelings and emotions. An understanding of the physical properties of light can also help explain optics, lenses, color theory, lighting and projection

equipment and much more. The laws and applications of reflection, refraction and absorption are encountered and used every day by the stage lighting designer and these concepts must be thoroughly understood both in theory and in practice. These basic qualities of light are; INTENSITY, FORM, COLOR, DIRECTION and MOVEMENT. These are the lighting designer’s tools.

Almost all visual images can be described, discussed and analyzed in these terms – both physically and psychologically. There is an excellent classroom exercise that usually starts with an analysis of reproduction paintings from the ‘Old Masters’. Student learn to discuss the qualities of light, using such terms as intensity, brightness, direction, color, form, and distribution. These terms are used to discuss the painting in detail from one small area to another. In addition the painting as a whole is discussed in respect to overall lighting impact, style, mood, composition, emotional content and other qualities. (This exercise is sometimes known as the ‘postcard’ exercise as often this is the source of the reproduction paintings. The author has many in his collection.)

The experienced lighting designer also frequently relies on the qualities of light to help communicate his lighting concept to others. Example: The stage was brightly bathed in a deep blue wash. Slowly, the amber sun softly rose above the horizon gently illuminating the stage in a golden glow. Cool, textured and uncertain light slowly starts to grow and creep throughout every corner of the stage. Soon a low dominating warmth from stage right becomes evident, balanced by a diminishing and cooling of other general light.

As darkness falls, the entire stage grows shadowy and covered with sharp defined leaf projections. The blue wash unnoticeably reappears as a shaft of sharp silvery moonlight slips across the stage.

INTENSITY and BRIGHTNESS

INTENSITY typical refers to the ‘strength’ of a light source. Intensity of a source exists independent of its distance. Intensity is measured in candela (The old term was candlepower).

ILLUMINATION refers to amount of light falling on a surface. The old term for illuminance was ‘illumination’. Illuminance is measured by a light meter (corrected for the curve of the human eye) in footcandles or lux (metric). Typical stage lighting illuminance levels may range from 25 to 200 footcandles or more. The eye has an incredible power of accommodation and can comfortably adjust to illuminance levels in nature from 1 to 10,000 footcandles, or more.

BRIGHTNESS refers to the visual sensation caused by a light source when it interacts with an object and then the eye.

Brightness depends on the intensity of the source, on the distance to the object and on the reflective properties of the object. The footlambert is the unit of brightness.

Example: In theatre when we change the dimmer setting of a lighting fixture, we are changing the output INTENSITY of the source. This results in a change of ILLUMINANCE (light falling on the stage) that is perceived by the eye as a change in BRIGHTNESS.

VISIBILITY depends on many factors, not just the intensity of a source or the brightness of an object. Color, contrast, distance, movement and the conditions of the eye and visual system all play an important role towards visibility. The stage lighting designer is more concerned with the brightness of an object than the intensity of it’s light source. He soon learns that objects of higher brightness generally draw attention on stage. Light attracts! Conversely, darkness conceals – but may also put the audience to sleep. One of the prime jobs of the lighting designer is to actually keep the audience awake. This is not as funny as you may think when you consider what we do to an average audience member. Usually late, after dinner and a few drinks we seat the audience in comfortable chairs – and then turn off all the lights! The lighting designer must use the power of light to keep the audience awake and direct their attention to the stage by providing proper visibility, interest and selective focus. 

FORM and DISTRIBUTION

Light provides objects with a sense of FORM. The eye is able to recognize objects in terms of shape, size and position. Our binocular vision assists with this process by providing DEPTH.

“By means of controlling the distribution of light and creating patterns and compositions of light and shade, it is possible to produce sensations on the retina that will be interpreted as forms in space.” (A Syllabus of Stage Lighting, S. McCandless 1964).

Form as applied to light is rather complex. It is everything that intensity, color, movement and direction are not. Yet form is caused and influenced by these other qualities of light. Form has to do with the DISTRIBUTION of light or how light strikes a surface and reveals an object. We typically discuss form in terms of clarity and recognition of shapes. Form and distribution can be discussed on two levels.

First, we can discuss form as applied to the stage setting in respect to how objects appear. A stage might be evenly, softly and flatly illuminated from a low front angle.

Alternately, the same stage might be unevenly dappled with tight circular pools from a high overhead angle. We can also discuss form as applied to the light produced by a stage lighting fixture. (Example: “The fixture produced a sharply defined square shaped beam with a very wide dispersion angle”.)  Form becomes much more complex when you consider that an image projector can be used as a stage lighting fixtures. As a result of this technology the light produced from the ‘fixture’ can take on absolutely any shape, form or distribution. As in nature, stage light sources may produce either soft diffused shadowless light or hard shadow producing light, – or anything in between. The edge of a lighting beam may also range from a soft almost invisible edge to a hard, sharply defined edge. A beam of light may also have a broken, uneven distribution, as in the case of a gobo or template pattern projection.

COLOR

All light is colored. White light is simply a mixture of all visible wavelengths (colors). The human eye is most sensitive to light in the yellow-green portion of the visual spectrum (about 550 nanometers), than it is to red or blue at the ends of the spectrum.

Color is usually discussed in terms of HUE, VALUE and CHROMA.

HUE is the classification of a color that the eye sees as red, green amber, etc. VALUE indicates lightness or darkness of a color.

CHROMA indicates the purity or saturation of the color.

The PRIMARY colors of light are RED, GREEN and BLUE. These three colors can mix together to produce any other color, including white. (The primary colors of pigments are RED, YELLOW and BLUE.)  The SECONDARY colors of light are formed when any 2 primary colors are combined. The 3 secondary colors are MAGENTA (red & blue), YELLOW (red & green) and CYAN (blue and green).

THE COMPLEMENTARY colors are any combination of a primary and a secondary color that, mixed together make white light. Examples of complementary colors: MAGENTA & GREEN, YELLOW & BLUE, and CYAN & RED).

When white light is passed through a color filter only the wavelengths corresponding to the color are transmitted. All other wavelength are absorbed. This is referred to as SUBTRACTIVE filtering.

When 2 or more colored beams of light combine to illuminate a surface, they mix together through ADDITIVE mixing. Stage lighting fixtures produce colored light using high temperature plastic filters. There are more than 100 different colors available from several manufacturers. These filters ‘pass’ or TRANSMIT their own color and ‘block’ or ABSORB all others.

Sometimes glass filters are also used. Conventional glass filters generally come in a limited range of colors however they are useful for high temperature applications or where prolonged life of the filter is required. A new generation of ‘dichroic’ glass filters are also sometimes used for entertainment lighting applications where ‘vibrant’ colors are needed that will not fade over time. Dichroic filters are made with thin film technology, tuned to specific wavelengths. These filters transmit a specific color and REFLECT all others. (Unlike conventional filters that absorb not reflect unwanted wavelengths.)

DIRECTION and MOVEMENT

Direction

The direction of light is one of the most important attributes in stage lighting design. All light has direction. A bare candle radiates light in all directions. A spotlight radiates light in a very specific direction. In nature most light comes from the sky, from above. In theatre lighting this is also generally true as most lighting positions are above the stage or audience.

Low front lighting is often considered to be ‘flat’. Very high lighting angles may cause shadows on the actor’s faces. Lighting from more than one direction can add ‘plasticity’ and dimension to an actor. Lighting from the ‘balcony rail’ can fill in shadows on the actor’s face however this position can also cause shadows on upstage backdrops or scenery. Very low lighting angles

have always been associated with rather unnatural lighting and are usually used for effect lighting only. Footlights, once common in many theatres are seldom used today. Clearly the lighting designer must chose the direction of light very carefully. In theatre, like in nature the ‘floor’ reflects some light from below, usually filling in shadows. The color and reflective qualities of a stage floor are very important and for this reason should always be selected with assistance from the lighting designer.

Interestingly enough, the property of DIRECTION was not really considered by McCandless as one of the ‘qualities of light’ in his ‘Syllabus of Stage Lighting, 1964’ He did however discuss (briefly) the importance of direction in respect to plasticity of objects and the actual ‘position’ of the light source.

MOVEMENT

Movement in light is generally taken to mean any change in INTENSITY, COLOR, FORM or DIRECTION. Dynamic changes in all of these qualities take place in nature on a regular basis. Movement may also include the physical movement of a source, such as; a search light, police beacon, color wheel, special optical effect, moving projections, mirror ball, etc.

Movement may be rapid or very subtle, slow and imperceivable. Such may be the case of a designer that provides a slow shift in sunlight from one side of the stage to the other throughout the duration of a play. The audience may not notice the shift, however they often may ‘feel’ the result of the change emotionally. A sunrise or sunset might also change so slowly that the movement in light is imperceivable and the audience may only feel the result and not actually see it.

Up until recently movement was probably the least utilized quality of light, by the stage lighting designer. This all changed in the 1980’s when the automated lighting fixture was born. The modern automated fixture can now move physically – directing it’s beam from one part of the stage to another. In addition the automated fixture can ‘move’ from one color or effect wheel to another, at any speed. The changes and combinations of intensity, form, distribution, color and movement are endless.