Within the visible light spectrum, red and blue are the two critical colors that power photosynthesis the most. Red light promotes blossoming and fruiting, while blue light promotes vegetative growth in the stems and leaves.Greenhouse gardeners use high intensity discharge (HID) lights to supplement proper light levels during the shorter days of winter. HID lights use two main types of bulbs, high pressure sodium (HPS) and metal halide (MH).High pressure sodium lights produce more red light while the metal halide produces more blue light. Some light ballasts can utilize both types of bulbs, which allows gardeners to use MH bulbs during the vegetative growth stage and then to switch to HPS during the blossoming or fruiting stages.LED lighting is an alternative grow light technology that has emerged due to its potential for lowering operational costs, lowering heat output, bulb longevity, and highly selective light quality. However, more research must be completed before a conclusive comparison between HID and LED lights can be made.