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Tempered and Heat Strengthen 
Quality Standards


Deviation in flatness spanning the entire pane of glass. Commonly seen in tempered glass and IGUs. 

Allowed within reason. Not measurable on site or in an IGU. Positive and deflection is a natural occurrence with seasonal and barometric pressure cycling.


Localized deviation in flatness that can look like ripples across the glass, or pockets of indentations. 

Allowed and very common in tempered glass. Not measurable on site or in an IGU.

Stain Pattern

AKA iridescence: An optical effect that results from the tempering process appearing as a pattern of dark spots on the glass. The intensity of the issue increases when viewed at steep angles to the glass and with polarized sunglasses. 

Allowed. This optical issue is characteristic of tempered glass and cannot be completely controlled. It will vary from pane to pane.

Quality standards for tempered and heat strengthen glass

Visual Standards - Bowing

  • Use a "true straight edge, wire, string or monofilament fishing line."

  • Measure each of the 4 sides for overall bow on the concave portion of the glass within 1" of the glass edge and parallel to the edge.

  • Also check the approximate horizontal and vertical centerlines for overall bow. 

How to measure bowing on tempered glass
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