Museums exhibit many materials such as coloured textiles, papers and paints that fade when exposed to light. The rate of fading depends on two factors: intensity of illumination, and duration of illumination. The CCI Light Damage Slide Rule, which accompanies this note, is a basic care-of-collections tool that enables museum staff to regulate intensity and duration of illumination in order to preserve the colour in artifacts for as long as possible. By relating intensity of light falling on an artifact, to the length of time it is exposed to light, the Slide Rule not only demonstrates the damage that will result from a particular combination of intensity and exposure, but illustrates the results if one or both factors are reduced. The Slide Rule can be used alone; in combination with the CCI poster on light damage to colours; or in conjunction with other data on the lightfastness of colours. This note describes the features of the CCI Light Damage Slide Rule, and provides step-by-step instructions for its use. The Light Damage Slide Rule has two sides, serving two functions: 1. to calculate light damage to an artifact (Side One); and 2. to assist in selecting and using appropriate bulbs in museum displays (Side Two).