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.