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Rosco 3204 Half Blue CTB Lighting Gel Mired Shift -68 Boosts 3200K sources to 4100K. A partial conversion to compensate for varying daylight conditions and voltage reduction, or to boost aging HMI lamps and yellowed soft lights. Deep-dyed base. (Transmission = 52%).
Deep-Dyed Deep-dyed color filters, like Roscolux, 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 molecules below the surface. Compared to surface coating, more extreme temperatures are required to cause the dye particles to sublimate through the surface. Deep-dyed filters are, therefore, more resistant to fading than surface coated.
HOW COLOR FILTERS WORK Filters create color by subtracting certain wavelengths of color. Thus, a red filter absorbs blue and green, allowing only the red wavelengths to pass. The process is subtractive, not additive, so the light source must emit a full spectrum.
The Rosco swatchbook provides detailed information on the spectral energy curve of each filter. The curve describes the wavelengths of color transmitted through each filter. For example, Supergel 342 transmits approximately 40% of the violet and blue energy of the spectrum and 75% of the orange and red energy. It absorbs all energy in the yellow and green range.
Durability The life of color filters depends on many variables: the color, the instrument and lamp used, the dimmer level a filter generally runs at, 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 it's melting temperature faster. When darker filters are needed try choosing filters that transmit high amounts of the 700 nm range. You can find this information by looking at the Spectral Energy Distribution (S.E.D.) curve located in the swatchbook for each Supergel color filter. Filters than transmit high levels at 700 nm may also transmit high levels in the infrared range above 700 nm.(See the Supergel swatchbook for information on how to read S.E.D. curves.)
To prolong the life of a color filter, align your ellipsoidal lamp to a flat field focus. (Get rid of the hot spot.) You can increase the distance between the lamp and the filter by using a top hat or barn door. In extreme cases, try Rosco Heat Shield or Thermashield to prolong the life of your filters.
Never use a plastic filter directly in front of an open faced lamp. This will nearly always cause premature failure because the heat is trapped and it has nowhere to go except to the plastic filter. Always allow a suitable air gap.