Light is essential for cactus plants, as you probably know. Tremendous differences occur in plants, based on their type.
Look at the plant to get an indication as to its origin and its light preferences.
Succulents that have spines need bright light to grow well but they will exist with less. Confined in pots next to a window they may sunburn.
Protect them from concentrated rays of the sun. Succulents without spines do well in bright to filtered light with protection from the very hot sun.
Light and Day Length
Cacti and other succulents do well in 16 hours of fluorescent light mixed with incandescent light in a ratio of 10 watts of fluorescent to one watt of incandescent light.
If you want flowers, you will have to do a little studying on your plants' individual requirements. Some need to be kept cool for a while and others need a good rest with plenty of darkness before they will flower.
Christmas cactus, Schlumbergera bridgesii, for example, should be kept rather dry -- below 60 degrees Fahrenheit and given eight hour days (16 hour nights) to encourage blooming. Set your timer or cover the plant each night with an opaque, black cloth.
When we refer to types of light, we generally refer to "bright," "filtered," and "shady." Here's what those definitions mean:
Bright. This means bright light or full sun with no curtains or blinds between the plant and the window. On the other side of the window, there is no tree, sign, or building to obstruct the light from the outside. In bright light, when you hold your hand a foot over a sheet of paper, you get a clear, sharp shadow.
Filtered. This means sunlight which is diffused by a glass curtain in the window. In filtered light, your hand makes a fuzzy, but distinguishable shadow when held a foot above a sheet of typing paper.
Shady. This means no direct sun or other light, and your hand held a foot over a sheet of paper will produce nothing more than a blob. All plants that tolerate shade need high humidity.
Although cacti are known for surviving in the dry, desert heat, they also need water to flourish. In the next section, learn about the water and humidity requirements of cactus plants.