For many years the 400-watt high-pressure mercuryvapour lamp, type MA/V or MB/U (or in the Netherlands, HO 2000) has been the standard light source for supplementing daylight. More recently however the development of the mercury-fluorescent lamp has provided a source with a better spectral composition and a higher light output. Certain of these lamps have given rise to difficulties with crops, leading to the introduction of special versions for horticultural use. Fluorescent tubes have been less popular in Britain although they give good results and could be competitive economically.
For promoting long-day effects the incandescent lamps used to provide a night break have given good service, but the other light sources may have advantages under some conditions.