The shadow of an object under an electric light is particularly dark in the middle, called the umbra, and the gray parts around it are called the penumbra. These phenomena are closely related to the linear propagation of light. It is obvious that the larger the area of the light source body, the smaller the umbra. Fluorescent lamps have a larger luminous area than incandescent lamps, and the umbra of objects under fluorescent lamps is very small. Therefore, the shadow under fluorescent lamps is not obvious, while the shadow under incandescent lamps is very clear. When we light up an incandescent lamp around us, the umbra will completely disappear and the penumbra will also be invisible. People have made shadowless lamps based on the above principles.

A shadowless lamp is a circular arrangement of several lamps with high luminous intensity on a lamp panel, forming a large area of light source. In this way, there will be different angles of light shining on the operating table, which does not produce obvious umbra and ensures sufficient brightness of the surgical field of view, so it is called a "shadowless lamp".