In a darkened room, a bright light (eg. powerful torch) can be shone through some body parts to determine their structure. This is transillumination. The most common use of transillumination is to shine a torch through the scrotum of a patient with a hydrocele to locate the testis within the fluid filled scrotum. In some situations the skull of an infant can be transilluminated through the gaps between the skull bones to outline the brain in cases of hydrocephalus.