|Iridescence in Tempered Automotive Glass|
NOW YOU SEE THEM – NOW YOU DO NOT! What are we talking about? We are talking about the strange discoloured blotches that can sometimes be seen in all tempered glass, but are particularly prevalent in tempered automotive glass as shown in the picture 1 of a car rear window.
Picture 1 Iridescence in a tempered vehicle backlite
These marks are called iridescence and are the result of varying levels of stress in the glass. We see them because the high stress points in the glass twist the light, and if the light is polarised (or the glass is viewed through polaroid sunglasses) then we can detect the different shades produced by the twisting. We don't see the spots in normal lighting conditions because normal light is not polarised. However, all reflected light is polarised, so when reflected light falls on the glass then we see the strain pattern in the glass. In this technical article we will review the iridescence in detail.
WHY DOES THE 'DOT' STRESS PATTERN OCCUR?
The 'Dot' stress pattern is deliberately introduced into the glass by the glass processor during the tempering process in the quench. To find the reason why they introduce the dot pattern into automotive tempered glass we need to look at the standard that must be passed by sample glasses after processing.
All automotive glass quality standards state that in the event of a glass failing, the particle count must be within certain specifications. The particle count and description for allowed shape and size this is dependent on the standard in question. Now to achieve the particle count we need to understand what happens when a tempered glass fails.
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FOR FURTHER INFORMATION CONTACT OUR SALES AND OPERATIONS AT:
Safety Glass Experts International Oy Ltd
Palatsinraitti 1 E 92
FI-33120 Tampere Finland
Phone: +358 400 979 300
Fax: +358 2 6518 2539
Photos: Safety Glass Experts International Oy Ltd
Last review: September, 2011
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