The overall darkness (blackness) of a picture is defined by its density. The difference in luminance and darkness between regions on a radiograph is referred to as contrast.
Radiographic contrast is defined as the difference in densities between different areas on a radiograph. A film with very light and very dark areas gets a high contrast. As there are few shades of gray from one extreme to the other, contrast or a short gray scale is appropriate. A radiograph with numerous shades of gray is known as a poor contrast or long gray scale radiograph.
Using a low kilovoltage to generate x-rays will usually result in a radiograph with a high contrast. This is due to the fact that low energy radiation is more easily attenuated. As a result, with low energy radiation, the ratio of photons passed through a thick and thin region will be larger.
The beam quality is controlled by the kilovoltage. The kVp governs the speed of electrons moving from the cathode to the anode and influences the x-ray beam's penetrating capabilities.
The total energy contained in the x-ray beam in a particular area at a given point is described as intensity; it is the product of quality and quantity.
Most dental x-ray equipment has a range of 65-100kV. Lower settings may not penetrate the tissue sufficiently, resulting in high contrast photos; higher settings may increase the radiation the patient gets, resulting in low contrast images.
To reduce the intensity of the second shot to half the value of the first, at least 3.0 mm of aluminum must be used. If this can be accomplished with less aluminum, it is judged that the tube's radiation output at that KVP is insufficient, and servicing is necessary.
