<prompt> Compared to round collimator, use of rectangular collimator reduces the patients skin surface exposed by:15304560  <response>: 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 patients skin surface exposed by 60 over that of a round 7 cm PID. Reference: White and Pharoahs Oral Radiology Principles and Interpretation 7th edition page no 34 