Tempered or toughened glass has been processed by controlled thermal or chemical treatments to increase its strength, both physically and thermally when compared to normal glass. Tempered glass is made by processes which creates balanced internal stresses. The greater contraction of the inner layer during tempering induces compressive stresses in the surface of the glass balanced by tensile stressed in the body of the glass. A strain pattern can be viewed from tempering when observed with polarized light or by using a pair of polarizing sun glasses. Sterling has perfected the “Maltese Cross Strain Pattern” which indicates a very hard temper with compressive stress exceeding 100MPa. This tempering formula is used for heat absorbing filters that experience extreme heating and cooling fluctuations.
Sterling is unique in that we have an in-house Tempering Department. Since the 1950’s Sterling has gained national and international recognition for tempering both large and small optical components. Sterling has been successful in tempering Optical Filters as thin as 1mm. We have six tempering and sagging ovens in our Tempering Department. By far the most common optical component that requires tempering is a Heat Absorbing Filter.
Image: Compression pattern of the 8" diameter BK7 lens that was used to film the wreckage of the RMS Titanic. The ship is lying at a depth of 12,555 feet. The compression pattern is viewed under cross polarized filters.
The first patent for tempered or toughened glass was held by a chemist named Rudolph A. Seiden from Austria. The patent was issued in the early 1900’s. However, the underlying mechanism though not know at the time, the effects of tempering glass has been known for centuries. In the 1640’s Prince Rupert of Bavaria brought the discovery of what are now known as “Prince Rupert’s Drops” to the attention of the King. These teardrop shaped bits of glass were produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it and, in fact’ tempering it. The teardrops were often used by the King as a practical joke where he would conceal them in his hand and then drop them when pretending to cry.
Sterling Precision Optics can make a Condenser Lens from Heat Absorbing Material. This combination eliminates the need for an extra lens element in your system. The element is then tempered to withstand the heating and cooling cycle.