As a result of heat-treating glass, optical distortions may arise from out-of-plane deformations of the glass surface. Currently, there are no industry-wide standards that specify acceptable values for these optical distortions; however, there are existing methods/instruments to measure them. The methods/instruments referenced in this document are designed and intended for in-plant use only. These methods for measuring distortion cannot be applied in the field on vertical glazing and cannot be used on insulating glass units. This document was published in April 2013.