To the human eye, light can appear both very bright and very colorful. Imagine a sunny landscape or a storefront lit by a neon sign; they are bright and colorful! However, a camera captures a range of contrast that is much narrower and not as intelligently selected, so that the sunny landscape or neon-lit storefront can look washed out. This problem of poorly controlled contrast is especially bad in cheap cameras or cameras that have small sensors, such as webcams. As a result, bright light sources tend to be imaged as big white blobs with thin rims of color. These blobs also tend to mimic a lens's iris—typically, a polygon approximating a circle.
The thought of all lights becoming white and circular makes the world seem like a poorer place, if you ask me. Nonetheless, in computer vision, we can take advantage of such a predictable pattern...