Compared to round collimator, use of rectangular collimator reduces the patient's skin surface exposed by:
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. 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 34
15%
30%
45%
60%