While there's plenty of information available on plant lighting, there's no resource available that focus on the fundamentals of how plants utilize light, types of plant lights available and how the sun compares to commercial indoor plant lights.

Light consists of many units of energy called photons. Light is both visible and invisible. Invisible light includes far-red and ultraviolet rays (UV) - both of which affect plant growth. The colors in a rainbow includes all of the light we can see (visible light). The light emitting from the sun is bent by raindrops in the air which separate "white light" into all the visible colors.

The visible light spectrum is made up of several colors including red, orange, yellow, green, blue, indigo, and violet.

How do plants use light?
Light is absorbed by chlorophyll, the green pigment on the leaves of plants and then converted into energy by a process called photosynthesis. Plants only utilize two of the colors in the visible light spectrum - red and blue. The other visible light colors that are reflected by plants includes green and far-red. Plants use far-red, however, to determine the amount of competition in their environment (other plants). The more plants around, the more far-red light is reflected. High levels of far-red light is an indication of competing plants. This tell the plant to grow taller, allowing it to tower over other plants to receive more light or energy.

Sunlight
While indoor horticulture lights have made significant improvements and progress, the sun remains the king of plant light. The sun, an atomic furnace that converts mass into energy, turns 657 million tons of hydrogen into 653 tons of helium. The other 4 million tons of mass is released as energy into space. Earth only receive about one two-billionths of the released energy. During mid-summer, plants enjoy a feast of over 100,000 lux (10,000 fc) of illumination.

Indoor Plant Lights
High Intensity Discharge (HID) lamps are the absolute best choice for growing plants indoors. They offer the highest lumen to provide sufficient amounts of light to plants. The best HID lamps for growing plants are Metal Halide (MH) and High Pressure Sodium (HPS).

All HID lamps including MH, HPS and Mercury Vapor requires one to several minutes to reach it's maximum output levels. If turned off, the arc tube must be cooled to a specific temperature before attempting to restart.

HID lamps are generally not very energy efficient like fluorescent lamps. They also burn at a higher temperature than fluorescent tubes - making grow rooms hot.

MH lamps are generally used during the plant's growth phase. Fluorescent is also an acceptable choice for growing small plants and is generally used during the growth phase. HPS lamps are used to flower plants because it promotes flower growth and development leading to an increased yield.