An X-ray is an image made up of several white, grey and black overlapping shadows. The image that you see, depends on how many X-rays are able to pass through and hit the film, the more dense objects (e.g. metal) let fewer beams pass through and the whiter the image appears in that area.
A whiter area is known in dentist speak as a ‘radiopaque
’ area. Blacker areas on the film have not stopped any X-rays (e.g. air) and these areas are known as ‘radiolucent
’. If the image is grey, then some of the X-rays but not all, passed through. Of course X-rays are not just white, black and grey but a spectrum of shades (depending on how many X-rays pass through) that go to form the image.
This is important to know as the dentist may refer to the area as 'an area of darkness' but will often forget he is talking to a patient and not a radiologist and say “I can see a periapical radiolucency associated with that tooth”, meaning, ‘I can see an area of darkness around the end of that tooth,’ indicating infection into the nerve, a classic sign that the tooth needs root canal
or to be extracted.
The darkness of the image is also affected by the strength of the X-ray beam itself, the type of film used and the way in which the X-ray is developed. All these aspects are controlled by the dentist to ensure a clear and accurate picture of the area in question (as always, at the lowest possible dose).