an 11-inch radius is needed to cover the 14 x 17 area. To determine x-ray coverage,

use the following formula: the tangent of the target angle (tan) times the focus-film

distance (FFD) equals the radius of the area covered (RC) or tan X FFD = RC.

Example: If the target angle is 20 and the FFD is 40 inches, the radius of the covered

area would be 14 inches, since the tangent of 20 is 0.364.

Figure 3-14.

Target angle and x-ray coverage.

a. To prevent relay damage due to arcing, an x-ray exposure is normally

synchronized to the line voltage so that it begins and ends when the sine wave is at

zero value. With this system, in a single-phase generator the shortest exposure

possible is 1/120 second since the sine wave reaches zero value every 1/120-second.

With a grid-controlled tube, the exposure is also synchronized with the line voltage, but

it does not necessarily begin and end at zero value. It can be synchronized to a

particular portion of the sine wave.

(1) Figure 3-15 shows two waveforms with exposures of 1/120 second on a

conventional times tube and 1/360 second on a grid-controlled tube. Since the

conventional exposure should begin and end at zero value, it must encompass one

complete pulse. On the other hand, the grid-controlled exposure indicated here

includes only the middle third of the pulse. Even shorter exposures are possible with

grid-controlled tubes, and this makes them desirable for examinations requiring very

short exposures.

MD0950

3-19