This covers normal light bulbs and the higher power types such as photofloods, and also many microscope illuminators. These types of light are much redder than daylight, so for colour work you need either a colour compensating filter or a film intended for tungsten light. If you vary the intensity with a rheostat, the colour of the light changes, getting redder as it becomes dimmer. Normal light bulbs are usually both too dim and too red. Photofloods are much brighter (at the cost of an expected life of 3 hours), and colour films are available designed for them. They are also very hot, and will quickly cause plants to wilt and insects to die. Microscope illuminators usually produce a very uneven light, but can be used with care. Focus at dim settings and only turn them up high to measure exposure and take the photograph, or bulb life will be short. The manufacturer will tell you how to adjust the light to correct the colour for tungsten-balanced film. Tungsten lighting needs mains electricity.