Lights which are turned on and off a lot wear out faster. Three to six inch sections on both sides of the tube dull out from deposits after a short term of use. Growers figure the effective length of a 4 ft tube as 3 feet 4 inches and of an 8 ft tube as 7 feet. Light Spectrums and Photosynthesis Each source of light has a characteristic spectrum, which is caused by the varying wave lengths of light therein. Fluorescents and other electric lights emit different shades of light to grow marijuana. To our eyes midday summer sunlight looks neutral, incandescent lights have a reddish tint, fluorescents vary in spectrum according to their type, MH lamps a have a blue coolness to them, and HPS lamps look pink-amber.