Transillumination

Transillumination is the transmission of light through tissues of the body. A common example is the transmission of a flash of light through fingers, producing a red glow. This is due to the fact that red blood cells absorbed other colors of the beam and transmitted only the red component.

Applications
Transillumination is a major application of visible light in medicine. It is mainly used by pediatricians to shine light in bodies of infants and observe the amount of scattered light. Since their skeleton is not fully calcified, light can easily penetrate tissues. Common examples are diagnosis of:

Hydrocephalus (water head)
Light penetrates to the inside of the skull of the infant. If there is an excess of cerebrospinal fluid (CSF), light is scattered to different parts of the skull, producing patterns characteristic to hydrocephalus. The device used in this operation is a Chun gun that uses a 150 watt projection bulb as a light source.

Pneumothorax (collapsed lungs)
Bright light penetrates the thin front chest wall and reflects off the back chest wall to indicate the degree of pneumothorax. To treat it, a physician insert a needle attached to a syringe into the area of collapse to remove the air between lungs and chest wall, causing the lung to reinflate.

Other studies include the sinuses, the gums, the breasts and the testes.