To shine a bright beam through a body organ or cavity to detect diseases or other abnormalities; when pus or a lesion is present, the transmission of lucidity is diminished or absent: The test is transilluminated primarily on newborns or infants with hydrocephalus, or males suspected of having a hydrocele (accumulation of serus fluid in a saclike cavity).

The examination may also be transilluminated on breast tissue to detect lesions and cysts.

In newborns, a bright halogen ray may be used to transilluminate the chest cavity if they are suspected of having a pneumothorax.

Transilluminating through the chest is only possible on small newborns.

Bodily areas filled with air or fluid, that is not normal to that location, have increased lucid transmission and so they should not be transilluminated.

In a darkened room, a newborn infant's head can be seen to shine brightly when transilluminated if there is excess fluid present; thus, suggesting hydrocephalus or an abnormal buildup of cerebrospinal fluid (CSF) in the ventricles of the brain.

One of a range of new digital technologies that is helping to improve diagnostic techniques is the "Difoti" (digital imaging fiber optic transillumination). One application of the "Difoti" is when dentists position a wand or thin stick-like instrument above each tooth and as light passes through the enamel, any cavities or other irregularities show the patterns and the information is captured by the wand’s sensor and transmitted to a display.