The most prevalent method of producing colored light in the theatre is by placing filters in front of the luminaires . These filters, which were originally made of gelatin, are today made of plastic (polyester and polycarbonate) or heat resistant glass (Pyrex). Filters transmit light selectively - some of the wavelengths are absorbed by the filter, i.e. subtracted from the light of the lamp, while the remaining wavelengths are transmitted. Which wavelengths will be absorbed and which transmitted depends on the color of the filter.
The light which is absorbed by the filter turns to heat. Certain filters such as deep blue #120 absorb as much as 95% of the light, and consequently become very hot. A light filter, such as light amber #102 , will transmit 75% or more of the light and absorb only about 25%.
Filters
of tints transmit over 90% of the light and barely heat up at all.
Gelatin and
plastic filters
cannot withstand high temperatures for extended periods of time and they either fade or tear at the hottest spot. Although this may be beneficial to the finances of their manufacturers, it is extremely irksome to lighting professionals, who are obliged to replace many of the filters in their luminaires every few performances.
Filters are characterized by three parameters: * The range of wavelengths which pass through the filter and produce the desired color (describing hue and saturation). * The degree of
transmission
of light through the filter (denoting brightness). * Tolerance to heat.
Filter swatchbooks contain a spectral profile for each filter, representing the wavelengths and the degree of light transmission. Some manufacturers also note the percentage of transmission. Unfortunately, data on heat resistance is usually not available to the user. The Y parameter of the graph, marked 0% to 100%, represents the percentage of total light transmitted through the filter, thus characterizing the intensity of the specific hue color, in this case 195 Zenith Blue.
The Y parameter to the right shows the percentage of light which passes through the filter. Congo Blue # 181 transmits only 0.87% of the light. Primary Green # 139 has a total
transmission
of 14.97% whereas a light color such as straw # 103 transmits 81.58% of the light.
The X parameter of the graph shows the mixture of wavelengths which pass through the filter, in nm. This
Light
Amber filter transmits almost all of the light from 580 nm to 800 nm, and very little of the light from 300 nm to 500 nm. All filters enable
transmission
of wavelengths from 700 to over 800 nm. These waves are
infrared
light which is invisible to our eyes, and thus insignificant to the color perceived. Yet if the filter were to absorb these wavelengths it would heat up even further and spoil sooner
If two identical filters are laid one on the other, they will transmit the same wavelengths but the light intensity will drop . For example: if we place a blue filter #165 in front of a luminaire, the filter will transmit light of wavelengths from approximately 400 to 500 millimicrons. In addition to the wavelengths filtered out to render the color blue, these wavelengths too will lose about 10% as they pass through the filter . If we place another blue filter on top of this filter, the light transmitted will be 90% of the light that remained after the first filtration. In other words, the blue light which is transmitted through the two filters will be about 80% of the original blue wavelengths, and will be perceived as a deeper hue than that transmitted through one filter .
The thickness of the filter effects the degree of transmission, and therefore manufacturers try to design filters which are very thin and transmit light more efficiently.
Heat resistant glass (Pyrex) filters - rondels, are available for borderlights and footlights, but their range of colors is rather limited.
When two filters of different hues are placed in front of a luminaire, subtractive mixing takes place, similar in principle to subtraction of pigments. If we place a filter of a primary color such as blue, red or green before a
white light
source, each filter will transmit only one third of the
spectrum
and absorb the other two thirds. If we use a filter of a secondary color, two thirds of the
spectrum
will be transmitted and one third will be absorbed.
