This test is most commonly performed on newborns or infants with hydrocephalus, or males suspected of having a hydrocele. The test may also be performed on breast tissue to detect lesions and/or cysts. In newborns, a bright halogen light may be used to transilluminate the chest cavity if it is suspected they have a pneumothorax.

Transillumination through the chest is only possible on small newborns. Diagnosis of pneumothorax is always confirmed by chest X-ray. In general, transillumination is not a particularly good test for any of these disorders, and further tests, such as an X-ray or ultrasound, are needed to confirm the diagnosis.

Areas filled with air or fluid that is not supposed to be in that location have increased light transmission and transilluminate when they should not. For example, in a darkened room, a newborn infant's head can be seen to light up brightly when transilluminated if there is excess fluid present (suggesting hydrocephalus).

Results specific to the breasts include:

• If there is a lesion and bleeding has occurred, those internal areas will be dark to black, because blood does not transilluminate.
• Benign tumors tend to appear red.
• Malignant tumors are brown to black.