Now you can temper certain types of lenses with a two hour process. This is an alternative to the well known 16 hour process and was designed in response to the optical industry’s ongoing need for better service. This new process works on four types of Corning photochromic lenses:
• Corning PhotoGray Extra,
• Corning Photogray II,
• Corning PhotoBrown Extra
• Corning PhotoBrown II.

The 2 hour process has three significant benefits:
• It reduces the actual time for ion exchange by 88%.
• It provides as least three times the capacity of the 16 hour process.
• It eliminates the need for mixing 2 different salts together when first loading or when replenishing the chem tempering system.

Corning conducted extensive in house and field testing of a wide range of photochromic dress eyewear lenses made from the four brands mentioned above. The field tests involved seven optical laboratories which strengthened half of their photochromic lenses using the 2 hour process and the other half using the 16 hour process. Over a twelve week period more than 27,000 Rx lenses were drop-ball tested. Breakage ran at the rate of 0.57% for the 16 hour process and 0.51% for the 2 hour process. In house testing by Corning also confirmed that finishing or handling defects can cause lenses to fail testing even when properly tempered by either process.

Corning also states that, “When replacement of a singlephotochromic lens is necessary, Corning recommends that the old lens be restrengthened along with, and by the same method as, the new lens. Matching the thickness of the old lens is suggested. If the old lenses were anti-reflective coated then both lenses should be replaced. These procedures will minimize the possibility of color mismatch between the lenses.”

The 2 hour process should only be used for the four Corning brands listed above.
Do not use for crown lenses, fixed tint lenses, or other types of photochromic lenses.

The exisiting 16 hour photochromic process can be used for all existing
photochromic lenses for which chemical tempering is appropriate.
Do not use with crown lenses or fixed tint lenses.

The existing 16 hour crown process can be used for all existing crown and fixed tint
lenses for which chemical tempering is appropriate.
Do not use with any type of photochromic lenses.

Failure to comply with these uses and limitations may result in lenses having as lower impact resistance.

Source: Corning Technical Bulletins. Corning, PhotoGray Extra, Photogray II, PhotoBrown Extra, and PhotoBrown II are trademarks of Corning Inc., Corning. NY.