Dental X-rays. These show teeth (white) from both the upper and lower jaw. The X-ray film is mounted in a device that is clenched between the teeth, positioning the X-ray film parallel to the teeth. An X-ray machine sends X-rays through the teeth to hit the film. This is known as exposing the film, and this image of the teeth is created on the film as the teeth block varying amounts of X-rays. This allows a dentist to assess the internal integrity of the teeth, and to check for any signs of tooth decay. The bright white areas in the teeth may be fillings or crowns that repaired previous areas of tooth decay. The X-ray second from bottom shows a dark area (far right), a possible tooth cavity.