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In 1666, Isaac Newton ‘... procured me a Triangular glass-Prisme, to try therewith the celebrated Phaenomena of Colours’ and opened up a new era into the scientific investigation of light (1). It was not until 1801 that Johann Ritter discovered the ultraviolet (UV) region of the solar spectrum by showing that chemical action was caused by some form of energy in the dark portion beyond the violet (2). In the previous year 1800, Sir William Herschel had demonstrated the existence of radiation beyond the red end of the visible spectrum, a component now known as infrared radiation (3). These three components of the solar spectrumªultraviolet, visible and infraredªare referred to collectively as optical radiation. But it is the ultraviolet rays, comprising about 5% of terrestrial sunlight, which hold the greatest interest in photodermatology. It is common practice to talk of ultraviolet light, or UVL. This is incorrect; the term light …