Something that emits light is known as a light source. Light sources can be divided into natural light sources, such as the sun, stars, lightning, and bioluminescence, and artificial light sources, including incandescent lighting, fluorescent lighting, and sodium lamps. They can also be categorized by their light intensity characteristics, i.e., constant light sources that emit the same amount of light over a fixed period of time (for example, the sun and incandescent lighting) and light sources that vary over time. Fluorescent lighting may appear to be constant, but it actually changes in accordance with the frequency of the power source. The human eye is just not capable of detecting such fast variations.