Light is essential for plant growth and can be one of the most limiting factors for success with houseplants. Both the amount and the quality of light are important.
Light intensity refers to brightness and is measured in foot-candles, which is defined as the amount of light that one candle produces, measured one foot away. Outdoors, the amount of sunlight that is cast may be 10,000 - 12,000 foot-candles. Indoors, the amount of light that enters the room is only a small amount. While most plants will perform best with bright light conditions, some plants can tolerate moderate to low light conditions. While light intensity is measured in foot-candles and can be evaluated by using a light meter or photographic exposure meter, you can also follow these general guidelines.
Low Light (75 foot candles)
North window: a few feet away; East/West window: 3-10 feet away; South window:15-20 feet away
Medium (average) Light (150 foot candles)
North window: directly in front; East/west window: a few feet away; South window: 3-10 feet away
High or Bright Light (300 foot candles)
East/west window: directly in front; South window: up to 5 feet away
Direct (1500 foot candles)
South window: directly in front
Keep in mind that light conditions change throughout the seasons. Several factors can influence your light intensity and duration. For example, the sun is higher on the horizon in the winter (increasing the brightness indoors) and lower on the horizon in the summer (decreasing the brightness indoors); surrounding trees and shrubs may block sunlight with their leaves in summer, but allow for more light during the winter after their leaves have fallen off.
Light duration refers to the number of hours of light that falls on a plant in 24 hours.
In low light conditions, plants can benefit from increasing the duration of light. Duration of 14 - 16 hours is sufficient. Do not expose plants to more than 16 hours of light since they need a rest period too. Also, be aware that flowering response in some plants is influenced by day-length and should be considered when supplementing the available light.
Light quality refers to the wavelengths of light. Plants need blue and red wavelengths for photosynthesis. For flowering, they also need infrared light.
Incandescent lights: mostly red, some infrared, low blue
Fluorescent lights: vary according to manufacturer
Cool white: mostly blue, low in red
For foliage plants, using cool white fluorescent bulbs works well. For blooming plants, use incandescent or special "grow light" bulbs.