Densitometer

A densitometer is used to study the relationship between the intensity of the exposure of the film and the blackness after processing (sensitometry). In order to do this, two pieces of apparatus are needed: an aluminum step wedge, sometimes called a penetrometer, and the densitometer. The steps involved are:

.First, the film under investigation is exposed through the aluminum step wedge at some standard technique (for example, 70 kVp with 2.5 mm A1 total filtration). When processed, the x-ray film will have areas of increasing density corresponding to sections of the step wedge with decreasing thickness. The step wedge is fabricated so that the intensity of exposure to the film under each step can be determined.

 .The processed film is analyzed in the densitometer, a device that has a light source focused through a pinhole with a light-sensing device positioned on the opposite side of the film. The x-ray film is positioned between the pinhole and the light sensor, and the amount of light transmitted through each segment of the radiograph is measured. These data are recorded and analyzed and when plotted result in a characteristic curve.

.It is not the absolute exposure that is of greatest interest but rather the change in density over each exposure interval.

. The useful range of radiographic densities is approximately 0.25-2.5. However, approximately 75% of all radiographs show image patterns in the range of 0.5-1.25 optical density.