Surgical shadowless lamps are used to illuminate the surgical site to provide optimal observation of small, low-contrast objects at different depths in the incision and body cavity. Since the operator's head, hands, and instruments can all cause disturbing shadows to the surgical site, the surgical shadowless lamp should be planned to eliminate the shadow as much as possible and reduce color distortion to a low degree.

In addition, the shadowless lamp must also be able to continue to operate for a long time, without dissipating excessive heat, because overheating will make the surgeon uncomfortable, and will also dry the tissues in the surgical area, the shadowless lamp is not actually "shadowless", it just lightens the umbra, so that the umbra is not significant. Shadows are made of objects that shine on light.

The shadows are different everywhere on Earth. If you look closely at the shadow under the electric light, you will also find that the middle of the shadow is particularly dark and the surrounding area is slightly lighter. The particularly dark part of the middle shadow is called the umbra, and the dim part around it is called the penumbra. The occurrence of these phenomena is closely related to the linear propagation of light.

If you place a cylindrical tea cone on the table with a candle lit next to it, the tea cone will cast a clear shadow. If two candles are lit next to the tea tube, two overlapping shadows will be formed. The overlapping part of the two shadows is completely black without light hitting it, which is the umbra; The local area that can be illuminated by only one candle next to the umbra is a half-light and half-dark penumbra.