Lens Shrinkage
The CR-39® lens that I recently sent to my local lab for AR coating was initially fitted snugly into a metal frame. When it came back from the coating process it was about .25mm too small for the eyewire. The lab told me that the size reduction was caused by the lens not being cured sufficiently during manufacturing. How could this have happened to a stock lens I bought from a reputable supplier?
Your lab is likely correct about this. When lenses are manufactured, they go through a curing cycle known as “degassing.” In this process, the soft, moldable lens material expels the gases trapped inside and becomes hard.
Manufacturers compensate the lens’s curvature and diameter for this process so the finished product turns out well within power, thickness, and diameter specifications. During preparation for being placed into an AR-coating chamber, lenses are subjected to a number of steps. including an intense cleaning in an ultrasonic cleaner, a drying process, and a curing process. The drying process may include exposure to nitrogen gas vapor (in the range of -320°). This intense drying removes the possibility of minute water vapor on the lens’ surface. The curing process is often done by heating the lens. This step ensures that lens has been completely degassed. Either of these processes can cause a lens to shrink.
Because the lens will be subjected to strong forces in the coating chamber, it is imperative that the lens be ultra clean, super dry, and completely cured. Even a minor flaw in these areas may cause a coating failure.
— William Underwood, ABOM
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