How Colour Filters are Manufactured, How to Read an SED curve, Durability
In a body-coloured filter like Supergel, the colorant is integrated with the polycarbonate plastic substrate. The process starts with powdered resin and dye being fed into an extruder. Under intense pressure and heat approaching 600ºF, the drive screw combines the melted resin and dye into a coloured "honey". This coloured mixture is pushed through a die which forms it into a coloured plastic film 61cm wide. This thoroughly coloured film is then sealed between two layers of clear polycarbonate which have been extruded on the top and bottom of the coloured core The excellent performance of this engineered filter on a hot theatre light is a result of both the higher temperature resistance of the polycarbonate base polymer combined with the unique extrusion process which seals the colour core between the clear layers. For the colour to fade by dye sublimation, the dye molecules must migrate out of the body-coloured internal layer through the clear sealing layers. As a result, body-coloured filters are the most durable fade-resistant.
Deep-dyed colour filters like Roscolux and Cinegel begin with a roll of clear polyester. The film is passed through a bath of heated solvent suffused with dye. The solvent causes the film to swell expanding the polymer structure and allowing the dye molecules to penetrate the surface. The film is then washed and the polymer contracts to its normal form, trapping the dye below the surface. With the dye inside the plastic, more extreme temperatures are required to cause the dye particles to sublimate out of the film. Deep-dyed filters are, therefore, more resistant to fading and may be used on hot lights.
The easiest method of producing a colour filter is to "paint" the colour on one or both sides of a clear plastic base. Most of Rosco's E-Colour+ range as well as Lee Filters® are manufactured using this process. Polyester film is commonly used as the plastic base of a surface coated filter since easily accepts the solvent-based coloured lacquers. Using simple air drying to cure the coating, no heat is involved in the process. Surface coating is one of the oldest methods of manufacturing colour filters but it results in a product that is more susceptible to fading and deterioration from the extreme heat found in luminaires.
The spectral energy distribution curve of each filter describes the wavelengths of colour transmitted throug the individual filters. For example, Supergel #312 transmits approximately 80% of the red and green energy above 500nm in the spectrum but only a small amount of deep blue/indigo. The result is a strong, clean yellow colour with just enough blue to prevent it from becoming an acid yellow like Supergel 10.
The Transmission Percentage refers to overall light transmission that is allowed to pass through each individual filter, based on a Tungsten light source, Illuminant A.
The life of colour filters depends on many variables: the colour, the instrument and lamp used, the dimmer level, and the amount of time the light is running. For these reasons it is impossible to assign a "life" for each filter. However some basics knowledge and experience can help with estimates. Dark green and dark blue filters usually burn out the fastest because they absorb the most infrared energy. Absorbing the extra infrared energy causes the plastic to reach its melting temperature faster. When darker, more saturated colours are needed try to select a colour that transmits high amounts of energy above the 700 nm range. You can find this information by looking at the Spectral Energy Distribution (S.E.D.) curve located in the swatchbook or click on the graph next to each colour, below. 700 nm may also transmit high levels in the infrared range above 700 nm.
To prolong the life of a colour filter, align your ellipsoidal lamp to a flat field focus, in order to eliminate the hot spot. Increasing the distance between the lamp and the filter by using a top hat or barn door will also reduce the heat on the filter, extending it's life. In more critical situations, use Rosco Heat Shield or Thermashield to prolong the life of your colour filters.
Never use a plastic filter directly in front of an open faced lamp. Always allow a suitable air gap between the exposed lamp and the filter, 2-3 (50-75mm) inches is recommended.