What Is Transillumination?

Medically Reviewed by Carmelita Swiner, MD on July 07, 2023
3 min read

Transillumination is when a doctor shines a light through a body part to look for abnormalities. It can be done with other tests to diagnose certain conditions.

Read on to learn more about what transillumination is, why it’s done, and more.

A transillumination test is used by doctors to identify abnormalities in a body cavity or organ.

The doctor will dim the lights and use a bright light directed towards a specific body part to see what’s underneath the skin. Areas the light is usually used on include:

  • Scrotum
  • Head
  • Chest of a newborn or premature infant
  • A woman’s breast

The light can also be used in some parts of the abdomen to see parts of the intestine and stomach during a colonoscopy and endoscopy.

Transillumination is a quick and simple way of finding out if you have various conditions. If doctors see something off, they will order more tests to get a clearer picture of any conditions you may have.

This test is completely painless and needs no preparation.

A transillumination test is a way of diagnosing certain conditions. It’s often used along with other procedures and tests to confirm a diagnosis. For instance, transillumination may be used with ultrasound and a biopsy.

The following conditions may be visible through transillumination:

  • Breast cysts or lesions.
  • Hydrocele (fluid-filled sac) in the scrotum or testicle tumors.
  • Spermatocele, which is a cyst that develops in the epididymis (the small tube at the top of the testicle that transports and collects sperm).
  • Cracks in teeth that may not be visible with the naked eye.
  • Eye issues such as pigment dispersion syndrome (PDS), scleral tumor, and ocular injuries.

Transillumination is also used to diagnose several conditions in newborn babies:

  • Macrocephaly in newborns (large head, which is often a sign that there’s an underlying brain condition).
  • Meningocele in newborns. This is a birth defect where a sac sticks out from the spine.
  • Meningomyelocele, also known as neural tube defect, in newborns. This is when the spinal canal and backbone don’t close properly before the baby is born. It’s a form of spina bifida.
  • Subdural effusion in newborns (when cerebrospinal fluid is trapped between the surface and outer lining of the brain).
  • Hydrocephalus (a condition where cerebrospinal fluid builds up in the brain) in infants or newborns.

A bright halogen light may be used to transilluminate a newborn’s chest cavity if there are signs of air around the heart or a collapsed lung. Transillumination through the chest can only be done on small newborns.

Transillumination tests alone aren’t accurate enough to rely on. Further tests, such as ultrasound, CT, or X-ray, need to be done to confirm the diagnosis.

After transillumination, your doctor will note if you have normal or abnormal results.

Normal findings depend on what area of the body you’re looking at. Generally, a body part with a normal result won’t light up under the light.

Areas that are filled with abnormal fluid or air will light up under the light. For instance, the head of a newborn who possibly has hydrocephalus will light up during the transillumination process.

On the breast. Abnormal results on the breast look as follows:

  • Malignant or harmful tumors will be brown to black
  • Benign tumors will be red
  • Internal areas will look dark or black if there’s a lesion and bleeding, since blood doesn’t transilluminate

On the scrotum. If you have a spermatocele in your scrotum, transillumination will reveal the mass is fluid-filled rather than solid. The light will go through the fluid-filled spermatocele, whereas a solid testicle will not transmit much light.

The transillumination will indicate you have a spermatocele as opposed to something else. If the spermatocele gets too large, your doctor may suggest surgery to remove it.

Eye conditions. If you have pigment dispersion syndrome (PDS), the doctor will be able to see how the pigment is breaking free from the iris and deposited on the back of the cornea, which is the outermost part of the eye’s lens.

If you have a scleral tumor, transillumination will let the doctor see where the borders of the tumor are. Your doctor will also be able to see the extent of the tumor and how far it’s progressed. Transillumination will help the doctor to plan a surgical procedure to get rid of the tumor.

If you have ocular injuries, transillumination will help the doctor see the extent of your injuries and what to do with them.