Plants differ and vary in the amount of light needed to promote growth and flowering, and they have been classified according to their light requirements: plants that flower in response to longer days than nights are called long-day plants; those that respond to shorter days than nights are short-day plants; those that will flower and fruit under a wide range of day lengths are called indeterminate or day length-neutral; those that respond to days and nights of approximately equal length are called intermediate.
The addition of artificial light (preferably from fluorescent lamps) to a greenhouse is helpful in several ways. Its control can hasten or retard the development of plants at the gardener's whim; it provides light when sunlight is absent during rainy and stormy weather; it extends day length during short winter days; the space available for growing can be increased considerably by installing fluorescent lamps on the underside of waterproofed benches, thus fully utilizing the under-bench space for growing; it can turn a north-facing greenhouse that would normally accommodate only foliage plants into a flowering greenhouse.
Several excellent books and websites exist on the subject of Fluorescent Light Gardening and are available to help you with the mechanics of installation and to guide you in selection and use of the correct kinds of fluorescent lamps for growth of plants. More recently, HID's and LED's are being used for illumination purposes, and you can find excellent information on these types of lighting systems as well.