T.O. 33B-1-1
6-91
the distance of the light to the card is to the distance of the light to object. This image is a true projection. If the source
has finite size, the shadows cast will not be perfect projections, but will have surrounding areas that will be out of
register, producing a gray cast of unsharpness, which is, called penumbra. Diagrams B through D in Figure 6-42 show
the effect of changing source size, altering the relative position of source, object and recording surface. From these
examples, it will be seen that the following conditions are desirable to produce sharp shadow images:
a. The X-ray source should be as small as possible.
b. The X-ray source should be as far from the object as possible.
c. The recording surface should be as close to the object as possible.
Figure 6-42. Geometrical Factors
6.8.4.1.1.1
There are other factors affecting detail. They include motion, screens, film and scatter. If the source, object, or film
move independently of each other or are not in phase, blurring will result. Rigid supports for all three elements must
be used to prevent this blurring. Since characteristics and conditions of film, screens, and scatter are also related to
film contrast and density, they will be discussed later in subsequent paragraphs.
