Local distortion“ or colloquially called „Roller waves“ is the current term to describe a geometrical imperfection over a short distance (300 mm) along the glass surface. Measurements of such imperfections can be easily produced with a feeler gauge and a straight ruler or you can buy some more sophisticated measurement equipment. The measurements are accurate and can be easily repeated, satisfying the production and QM department.
Standard EN 12150 & 1863 states that over a distance of 300 mm you can have a maximum deflection of 3,00 mm.
However this does not tell us what the naked eye can see.
What the naked eye can see is a wavelength with its corresponding amplitude. And this is where millidiopter can be used. A metric measurement of the optical power bringing the wavelength and wave depth into consideration. So by its nature millidiopters are a much better unit to describe the optical quality of glass.
Let’s look at the details of millidiopters. How can we measure them? How do they work in transmission or reflection? What about the viewing angle and the distance to the reflected objects?