The Calibration Package for PSM is a product offered by Optical Perspectives Group (OPG) that provides a NIST traceable square wave grating with 4 lp/mm spacing for calibrating the lateral scale of the PSM and other optical instruments. The package includes a 64 mm square glass substrate with a 25 mm square chrome square wave line pair pattern, a calibration certificate showing traceability to NIST, and a sheet with the actual calibration data. The square wave grating is used to ensure that the magnification is set correctly for crucial measurements.