Compared to round PID, use of rectangular PID reduces the patient's skin surface exposed by:

There are several means to limit the size of the x-ray beam. First, a rectangular position-indicating device (PID) may be attached to the radiographic tube housing. Use of a rectangular PID having an exit opening of 3.5 cm × 4.4 cm (1.38 inches × 1.34 inches) reduces the area of the patient’s skin surface exposed by 60% over that of a round (7 cm) PID. White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition