Fluorescent Lamp Artificial Daylight Units for Colour Matching

The colour and colour rendering characteristics of natural daylight are considered together with the requirements for artificial daylight sources for critical colour matching purposes. The development of artificial daylight units utilising tubular fluorescent electric discharge lamps is shown to follow two main lines, namely, (a) a combination of fluorescent powders in either cold or hot cathode fluorescent lamps, and (b) a combination of radiation from fluorescent lamps and groups of incandescent tungsten filament lamps. Practical tubular fluorescent daylight colour matching units using cold cathode tubes are in use in industry for critical colour matching purposes, whilst experimental hot cathode lamps have been used in some prolonged field tests. An alternative method being investigated is to combine blue fluorescent lamps with incandescent tungsten filament lamps. This provides a closer approximation to black body spectral distribution than at present possible with fluorescent lamps alone, provided care is taken to ensure adequate mixing of the light, and adjustment is made for the differential life behaviour of the component lamps. The outstanding advantage of artificial daylight units incorporating tubular fluorescent lamps is their high efficiency (of the order of 30 lm/w.) which enables a very much higher illumination to be obtained over large working areas than has previously been practicable with filtered tungsten light or light from daylight units incorporating CO 2 tubes.