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Colour filters are just what they say they are…filters

… they selectively block parts of the available light spectrum and let the rest through.

Above: A prism separates and makes visible part of the energy from the light source.

Above: A ‘colour’ filter selectively blocks parts of this visible spectrum to produce the desired ‘colour’  light.

The colour the audience will see depends what reflects this energy is reflected to the eye. If there are colours similar to that of the filter in the path of the light part of the energy will be reflected and ‘seen’. If there are no complementary colours to reflect the energy most will be absorbed depending on the nature of the material it is falling on ie. fabric will absorb most of the energy, a painted gloss surface may reflect some allowing the surface to be seen.

To explore more about colour:

• as one of the four useable properties of light

• and how to use it

The filter material most commonly used in the theatre is plastic, typically polyester which is coated with a dye or polycarbonate where the raw material is mixed with the dye and extruded as a  body coloured sheet.

The type of base material; the dye; the amount and distribution of the light beam along with the luminaire design will determine the life of the filter. Being a thermoplastic the filter base material will deteriorate with heat, the softening and collapsing of the sheet will speed up this process. Selecon has devoted considerable resources to ensure our luminaires deliver the best possible filter life. In particular:

• Selecon ellipsoidal reflector designs ensure that the energy is spread as evenly as possible over the light beam avoiding the concentrations of energy which can melt filter and damage patterns in seconds.

• Dichroic filters remove much of the heat (infra–red) energy from the ellipsoidal beam.

Our ellipsoidal lens system designs avoid placing the filter at the second focal point of the reflector.

Selecon Cyc units all feature specially designed colour frames which both grip the filter and isolate the filter frame from the heat of the luminaire body.

Fresnels and PCs include carefully detailed filter mounting positions which guide air over the filter to exhaust the heat away as quickly as possible.

The Rama range of 1000W/1200W Fresnels and PCs feature a front assembly manufactured from engineering plastics which are poor transmitters of heat. Heat from the luminaire body which used to be transmitted into the filter frame is now effectively confined to the body extrusions.

Plastic filters will deteriorate with use, they are inexpensive and should be replaced as soon as they show signs of deterioration such as:

• Change in the colour change (fading or colour shift)

• Damage such as dirt or scratching which will significantly reduce and alter the quality of the light output

• Heat deformation, wrinkles or folds formed into the sheet

Tungsten halogen light sources generate significant heat (80%), much of which is collected with the visible light and passed through the filter. Saturated colours (deep colours) may filter over 90% of the visible energy from the beam. The energy blocked by the filter is lost as heat, combined with the heat in the beam the filter plastic is placed under considerable stress.

Dichroic glass filters offer great longevity as an alternative to plastic filters but are only available in a limited range, they are not effective with cyc floods and they are expensive, typically outside the budget of most productions.