The flatness of tempered glass is mainly characterized by two technical indicators: overall bow and Local bow. 

Overall bow: Place a ruler against two edges of the glass or in diagonal direction and use a feeler to measure the gap between the straight edge of the ruler and the concave surface of the glass. The overall bow is the value of the height of the arc divided by the chord length.  

1.Height of the arc 2.Chord length 3.Tempered glass.

Local bow is measured with a ruler parallel to the edge and 150 mm from the edge of the glass. The measurement length is 300mm. The waviness is the value of the height of trough or peak measured by a feeler divided by 300mm. 

1.Ruler.2.Height of the trough or peak 3.Tempered glass.