apparatus for determining inter - pen offsets in a multiple pen ink jet printer including a piezoelectric ink drop detector having a piezoelectric detector film having one or more openings formed therein . a carriage position sensor indicates the position of the carriage at the time a first ink drop is detected from each of the ink jet pens as the pens are scanned across an opening , whereby the sensed positions for the respective pens provides information indicative of inter - pen offset in the scan direction . for determination of inter - pen offset in the media scan direction , the piezoelectric film includes a plurality of openings , whereby the detect / no detect patterns for each of the pens provides information indicative of the inter - pen offsets .