Infrared Filter Material

Last updated June 2006

Cave photographers often make use of an infrared filter over a camera- mounted flashgun. The purpose of this is to allow the infrared light to trigger a slave unit, whilst avoiding illuminating the foreground of the shot with visible light.

Optical quality infrared filters are expensive but, of course you do not need an optical quality filter, because the material is placed over the light source, not the lens. Most organic dyes are transparent to infrared so, in theory, any piece of translucent coloured plastic will fulfil the task - e.g. a green sweet wrapper and a red sweet wrapper should look black (or, at least, a muddy brown) when superimposed, but will probably be almost transparent to infrared. Photographers often make use of a piece of unexposed, developed, slide film. Chromogenic monochrome film will also work, but note that conventional silver halide film will not work, because it is loaded with silver halide particles.

A good source of infrared filter material that is available in larger quantities than sweet wrappers and the tail-ends of slide films is the lighting 'gels' used in the theatrical and movie industries. These gels are used in front of theatrical lights and, in a thinner version, are used to cover room windows during filming, particularly to correct the colour balance between daylight and tungsten lighting.

I obtained a roll of a 'neutral density' gel filter from www.directlighting.co.uk. The item number was 211 0.9 Neutral Density 1 x 1.35m, which is a gel with a density of 0.9. This means that it transmits 12.5% of the visible light falling on it [see footnote], which is equivalent to an attenuation of three photographic stops. It is, however, almost transparent to infrared. (Thanks to Henry Lyall of CREG for measuring the IR properties for me). Used over a camera-mounted flashgun this gel will reduce the foreground illumination by three stops, whilst retaining its ability to trigger an infrared slave unit. If three stops is not enough attenuation, doubling the material will give you six stops; tripling it, nine, and so on.

update June 2006: I no longer have any stock of this material - sorry.

Footnote For the technically-minded, or those who think Ive made a typing mistake - density is measured on a logarithmic scale. Three photographic stops is a light ratio of 8 times (i.e. 2^3). To get the density, we take the log (to base 10) of 8 to get 0.9. So a filter density of 0.9 lets through 12.5% (1/8th) of the light. Similarly, a filter of density 0.6 corresponds to two stops of attenuation and lets through 1/4 of the light.

Instructions supplied with filter material

Use this filter over your flashgun to reduce its brightness. The filter allows infrared light to pass, so your slave units can still be triggered; but it blocks most visible light. This allows you to control the illumination in the foreground of your shot.

This filter material will work with Firefly and Gibson slave units.

This filter is an "ND 0.9" type, with an optical density of 0.9. This means that it lets through an eighth of the visible light and has the same effect on your flashgun's illumination as if you were to reduce your camera's aperture by three stops. This should be enough to eliminate any foreground lighting but, if in doubt, you can double the filter.

Some cautions...