The ability of a material to resist scratching has always been a main concern to the eyeglass wearer. Scratches on the lens reduce light transmission, impair vision, distract the wearer, and negatively affect the impact resistance of the lens. and in glass render the lens unsafe. For these reasons, scratch resistant coatings have been applied to ophthalmic lenses for many years. The measure of their effectiveness has been somewhat confusing since every manufacturer claims their coating to be the "hardest".

There are many laboratory tests for scratch resistance, but there are four that are generally accepted as valid for lenses. They are:


  • Bayr test - The subject lens is placed in oscillating sand for a specific amount of time. It is then tested on a haze meter and given a value. The ophthalmic range for the Bayer test goes from raw CR-39 at 1 to crown glass at 10.
  • Tumble test - Developed by American Optical, the subject lens is put in a container with an abrasive compound and rolled, so as to tumble the lens in the abrasive for a certain amount of time. Once again the lens is checked in a haze meter for abrasion.
  • Steel wool - In this test, different grades of steel wool are passed across the surface of a lens. The result is generally checked visually but can be checked by a haze meter. This is the test that is most often done in the field, as it requires the least sophisticated equipment.
  • Eraser stroke test - An eraser is rubbed on the surface of the lens under a steady pressure. The lens is visually checked and the number of strokes required to rub through the scratch coat layer is noted.

Currently the Bayer test is considered the most reliable and repeatable of these four. It's also used in many other industries, so it is considered a legitimate scientific test.