What Is A Color Filter?
A color filter blocks passage of some portion of the visible spectrum from any light source by absorbing and transmitting selected spectral ranges. For example, a primary red filter will allow red-associated light frequencies to pass and block all others. Of the radiant energy that is blocked, by far the largest part is absorbed by the filter as heat.
The filter contains light refracting elements, normally dyes, which are suspended in or coated on a transparent base. The dyes are organic, primarily aniline colorants that must be sufficiently heat stable to survive the high temperatures of entertainment lighting fixtures.
There are three ways to mate color with the filter base: (1) it can be coated on top of the base; (2) it can be diffused into the surface of the base material; or (3) it can be dispersed completely through the base. Coating one surface of the base is easier technically. A coated filter can be recognized by scratching the surface coating with a sharp object. The color scrapes off, leaving a faulty filter.
In any color filter, the dye eventually migrates from the hottest area. The rate at which the filter fades is a function of the dye employed and the depth of penetration by the dye into the base material. When simply coated on the surface, a dye will sublimate from the base into the air as a gas more easily than a dye which is uniformly locked by chemistry into the surface of the material. The greatest stability is achieved when the dye is dispersed uniformly through an extruded plastic base.
Effect Of Dyes On A Filter’s Melting Point
The base substrate of color filters was, at one time, gelatin derived from animal by-products (hence the nickname – gels). Plastic color filter substrates in current use include acetates, vinyls, polyesters, and polycarbonates. When it comes to heat stability; acetates and vinyls have the lowest melting temperatures, and polycarbonates have the highest.
In any color filter, the dye eventually migrates from the hottest area. The rate at which a filter fades is a function of the dye employed and the depth of penetration by the dye into the base material. When simply coated on the surface, a dye will sublimate from the base into the air as a gas more easily than a dye that is uniformly locked into the substrate. The greatest stability is achieved when the dye is dispersed uniformly through an extruded plastic base.
The transmission of the particular dye used has a dramatic influence on the degree of heat a filter can withstand. Dark greens, for instance, absorb the rich output of reds in incandescent light and are thus subjected to heat damage much faster than dark reds, independent of the plastic base used.
Color Filter Bases Polyesters And Polycarbonates
Of the resins available today, polycarbonate plastic offers the best combination of properties for a color filter base. It is extremely heat-stable and does not deform until temperatures in the 300° to 325°F (149° to 163°C) range are reached. These filters are the most heat-resistant type manufactured today. Rosco's Polycarbonate filters are inherently self-extinguishing.
Rosco exclusively uses polycarbonate filters in its Supergel range of filters, and approximately 65% of their popular Roscolux line is made from polycarbonate plastic, with the remaining 35% are made from polyester. Polyester filters can be expected to resist deformation when raised to temperatures as high as 260° to 290° Fahrenheit (126° to 143°C).
If you have any specific questions regarding filter applications, please email us at info@rosco.com.
Notes On Handling And Using Plastic Color Filters Comparisons
Keep in mind that an audience never looks at the filter itself. Instead, it sees only the filter’s effect on the transmitted light. Side-by-side comparisons of filters themselves are convenient, but the best indicator of a filter’s effect is the appearance of surfaces lit by the filtered light. When you have no surface colors in particular to light, use your arm or hand to check the effect on skin tones.
All plastic filters will ultimately be degraded by heat. As thermoplastics, they change dimensionally as the temperature approaches the melting point of the plastic. With extreme heat, the distortion sometimes brings the filter closer to the heat source, thus accelerating the melting process and shortening its effective lifespan. By holding the filter firmly at four or more points around the frame opening, the plastic will retain its shape through slightly higher temperatures. If your fixture operates with excessive temperatures at the filter frame (most easily recognized a when filters sag in the frame), try fastening the filter down all around with double-faced polyester tape.
Two or more layers of lighter (higher transmission value) filters may be used to produce a darker or more intensely colored light beam. However, the combination will be significantly less efficient than a single filter. The inside surfaces between the two layers will form an insulating barrier, and become hotter than the outside surfaces. The net result may not be desirable when the filter is used in a hotter fixture.
All color manufacturing is subject to subtle variations in colorant chemistry from one product to the next. Whether it be printed wallpaper, house paint, artist’s oils or color filters - the finished product does change over considerable time spans. In filter production, new color dyes are added, basic dyes are changed or discontinued, and more heat-resistant dye formulations are adopted. Therefore, it is recommended to replace your Rosco sample swatchbook every 3-5 years.
If a gel fails prematurely in a fixture, first check that the fixture has the voltage and wattage lamp for which it is designed, then check the general alignment and assembly of the fixture. Often a filter may be placed at a beam crossover point, causing a hotspot at the color frame, and a small focusing adjustment or movement of the filter will solve the problem.
Using A Heat Shield Between Lamp And Filter
Special formulations of clear glass and plastics to absorb, reflect, or deflect heat before it reaches the color filter. Rosco’s Super Heat Shield™ is designed to absorb and deflect heat from the color filter, minimizing the effects of high temperatures. Super Heat Shield features a proprietary plastic substrate that can withstand a continuous operating temperature nearly twice that of polyester or polycarbonate plastic. It protects the color filter by blocking the convected heat, however, it does not block infrared energy.
Properly used, Super Heat Shield can increase the lifespan of a color filter in very hot fixtures without reducing light transmission. It must be placed so that there is an air space on both sides of the filter to allow the convected heat to escape. Do not sandwich the color filter together with the Super Heat Shield in one holder. This will cause premature failure of the filter.