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Hong Kong Builder
(b) Situations requiring special sources or arrangement of sources.
Artificial daylight.
It seems probable that the difference in the apparent colour of objects when viewed by artificial and natural light has been noted by critical observers from the earliest times. Certainly such discrepancies have been widely known ever since the inception of current artificial light sources, and the recent development of the use of electric discharge lamps has provided examples of this phenomenon which are probably more striking and more widely commented upon than any previously accessible to the general public.
There are many industrial purposes for which ordinary forms of artificial light are unsuitable owing to the necessity for a colour rendering which is similar to that obtained in daylight, and there are many others where although daylight colour rendering is unimportant, differences in colour which are difficult to detect in normal artificial light must be rendered more obvious. Since the daylight appearance is generally considered satisfactory in such cases, any form of special light which simulates this more closely than ordinary artificial light is generally referred to as "artificial daylight," though it would probably be more correct to limit the term of lights under which daylight colour rendering is reproduced with some considerable degree of fidelity.
Fig. 3 Carbon dioxide discharge tube for artificial daylight.
or
The method of obtaining a colour-matching light from tungsten lamps by means of glass filters specially coloured reflectors, though widely employed, is not completely satisfactory where extreme accuracy and constancy is desired, since however good the filter or reflector may be, the system is still dependent on the tungsten filament lamp. The changes in the colour of the light emitted by the lamp during ageing and due to quite normal voltage fluctuations cannot be allowed for in the filter, and may produce sufficient difference in the quality of the artificial daylight to cause discrepancies in matching operations performed at different times. For situations requiring light sources which will give approximately daylight colour rendering to objects without, however, having to satisfy the most stringent requirements of colour-matching, such fittings can give very satisfactory results. They have the advantage of being the cheapest lighting system to instal capable of giving approximately daylight colour rendering.
It is possible that fluorescent discharge tube lighting
Fig. 4 A Cold Cathode carbon dioxide tube artificial daylight installation in a hosiery mill.
may be found more suitable than colour-corrected tungsten filament lamps for many situations. Very good colour rendering can be obtained under correctly chosen combinations of tubes, with a very high luminous efficiency.
For the most critical colour-matching work the best source is a cold cathode carbon-dioxide discharge lamp. Although its efficiency is low the lamp is almost completely independent of variations due to ageing or to mains voltage fluctuations, and avoids the use of filters and reflectors of special colour. Although the spectrum of the light from such a tube is essentially a line spectrum and therefore different in form from that of natural daylight, the lines are so closely spaced that both the colour of the source and the colour rendering of objects under this form of artificial daylight agrees extremely well with the result in the obscured north sky daylight usually employed.
Critical control of the gas pressure is essential for colour constancy, and this has been obtained auto- matically by means of a Moore valve operated by a solenoid connected in series with the electric supply to the tube. A fall in pressure causes an increase in the current flowing, raises an armature and exposes the tip
Fig. 5 Lighting with fluorescent discharge tubes in an aircraft gauge room.
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