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.
Target angle and x-ray coverage.
3-13. GRID-CONTROLLED TUBE
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