Which tool can be used to evaluate the quality of X-ray images?

A densitometer is the correct choice for evaluating the quality of X-ray images because it measures the optical density of the film or image. This measurement is crucial in assessing the contrast and overall quality of the radiographic image produced by the X-ray machine. By quantifying how much light is absorbed by the film, a densitometer helps operators ensure that the images have the appropriate density for accurate diagnosis.

The other tools mentioned are not suitable for this specific purpose. An oscilloscope is used for visualizing electrical signals, particularly their voltage over time, which is not applicable to the quality of X-ray images. A colorimeter measures the color of light and is typically used in color analysis, not in radiography. A thermometer measures temperature and has no relevance to the evaluation of X-ray images. Therefore, the densitometer stands as the specialized instrument for evaluating optical properties in radiographic imaging.