After correct watering, the next most important factor for successfully growing orchids is providing them with the correct kind and amount of light. Ideally, potted orchids would sit in a sunny window or another area where they would receive at least four hours of sunlight a day. Some species can tolerate more direct light for longer periods than others, but this is a reliable rule.

If you have no sunny windowsills for your orchids, however, you can still persuade them to flourish by providing them with light from artificial sources. You can use common fluorescent light bulbs, which are inexpensive and give off a cool light. Some growers have had good results using one cool-white fluorescent bulb and one warm-light fluorescent bulb, which will offer your plants more balanced light. Incandescent bulbs will also work, but since they emit more heat than do fluorescent bulbs, you will need to be careful to avoid getting your plants too hot.

Different species of orchids require different lighting conditions. You will probably have decent success without getting investing a lot of energy in technicalities, but if you want to be more precise, you can check orchid growing guides and websites to find out how many foot-candles of light different species prefer before you set up your artificial lights. You can determine how many foot-candles the spot you’re considering for your plants will get by using an inexpensive light meter to measure the light levels there. Then you can adjust the lighting accordingly by using bulbs with lower wattages, or by using fewer light bulbs in your lighting fixture.

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A foot-candle is a measurement of illumination. It is defined as the illuminance of a surface area that is one foot from the source of the light. One foot-candle equals one lumen per square foot. Full sunlight has a foot-candle rating of about 10,000 foot-candles. On an overcast, the number of foot-candles emitted drops to closer to 1,000. Foot-candle ratings in interior spaces vary depending on the source of the illumination (natural versus man-made) and whether the surfaces are also proximate to windows.

Industries ranging from lighting manufacturers to horticultural business to museums and art galleries use foot-candles in the course of doing business. Lamps and lighting fixtures are rated according to the number of foot-candles they emit, and architects and interior designers use those designations to figure correct lighting configurations for work areas and public spaces. The optimum level of foot-candle exposure for plants varies from species to species, so horticulturists must pay attention to this factor when planning gardens or adjusting light levels in greenhouses. Curators and art handlers in museums and galleries regulate light levels in their facilities because exposure to high levels of light can permanently damage art works and historical documents.

The level of foot-candles in any space is easily measured with a hand-held light meter, available from most camera stores or office-supply retailers who sell photographic equipment. Specific techniques for using light meters may vary somewhat, but generally speaking, the user points the light meter at the surface in question and the meter produces a reading of the intensity of the light on that surface in foot-candles.