Surgical shadowless lamps are used to illuminate the surgical site to optimally investigate small, low-contrast objects at different depths in the incision and body cavity. Since the operator's head, hands, and instruments can cause disturbing shadows to the surgical site, surgical shadowless lamps should be designed to eliminate dark shadows as much as possible and minimize color distortion.

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, but also make the arrangement in the surgical area boring, the shadowless lamp is not actually "shadowless", it only reduces the umbra, so that the umbra is not obvious. Shadows are made of light shining on objects. The shadows everywhere on Earth are different. If you examine 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 gray 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 around 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 umbra; Where only a candle around the umbra can reach, it is a half-light and half-dark penumbra. If you light three or even four candles, the umbra will gradually shrink and the penumbral part will show many layers. This is also true that objects can generate shadows composed of umbras and penumbras under electric light.