An artificial source of light is required when growing indoors. Plants require a certain colour spectrum of light to promote optimum levels of photosynthesis. A plant in a vegetative state will perform best under a blue spectrum of light, while the same plant in flower will perform best under a red spectrum of light.
Light spectrum colours are measured in nanometers. Intensity of light is measured in lumens. There are various types of lighting used for indoor growing including Fluorescent, LED, High Intensity Discharge (HID), Metal Halide and High Pressure Sodium (HPS). For more information about lighting, check out our Beginners Guide to Hydroponics or The Fundamentals of Hydroponics.