Energy for photosynthesis is provided by light from the sun or from artificial lamps. Photosynthesis can also take place under other sources of light like tube light. But the intensity of light from these sources need to be adjusted so that photosynthesis proceeds properly.
In winter months the low natural light intensity is the limiting factor for plants actively growing in a heated greenhouse. Intensity can be increased by using artificial lighting, which can also extend the length of day, which is short during the winter, by supplementary lighting.
Lamps are chosen for increasing intensity, and therefore more photosynthesis. Since the photosynthetic green pigment chlorophyll absorbs mainly red and blue light and reflects more of the yellow and green part of the spectrum, it is important that the lamps used produce a balanced wavelength spectrum to include as high a proportion of those colours as possible, in order that the plant makes most efficient use of the light provided.