I recently went into a lab that puts a backside coating on their high index and spectralite lenses, even on their high index photochromic lenses, I don't think this is a needed application and would like some more thoughts on this. Rich R
I recently went into a lab that puts a backside coating on their high index and spectralite lenses, even on their high index photochromic lenses, I don't think this is a needed application and would like some more thoughts on this. Rich R
Exactly what 'high index photochromic' lens are you talking about?
In my opinion, backside coatings are desirable on any non-glass lens material - even CR39. And they are absolutely essential on any lens that will be A/R coated.
[Note that the combination of a backside hard coat and A/R will affect the impact performance of a lens. Before applying a backside hard coat, be sure to check with your A/R coater to make sure this combination will pass FDA requirements.]
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Steve
OptiBoard Administrator
[This message has been edited by Steve Machol (edited 08-06-2000).]
Helo Steve, when I mentioned the high index photochromic, I was talking about the Hoya Bios progressive lens, thanks for your feed back. Rich R
Rich,
I agree with Steve totally about backside hard coating on any lens requiring SAR. I have found through experience that SAR works best with the backside coating.
I personally backside hard coat all materials but CR. I backside hard coat all Transitions and Sun Sensor lenses.
Jerry
Caveat first. We are both a lab and we own Ultra Optics, a company that supplies the majority of labs in the U.S. with backside coating machinery and chemistry.
The following is the criteria we use in determining whether we backside coat.
First, for AR, we use lenses with both front and backside coating. If we can't buy 'em with a front and backside coating, we buy the front coated and then coat the back in the lab. Testing has shown that a lens so treated with perform significantly better than a "naked" lens...even CR-39.
We have not found a surfaced mid index material that will consistently pass drop-ball at 1.5 c.t. when both a scratch coat and an AR coat are used. We require a minimum 2.0 c.t. for that combination.
As to when should I backside coat a lens, we test every material for scratch resistance as compared to uncoated CR-39, which has become a de facto standard. If the scratch resistance is worse than uncoated CR-39, the patient will be unhappy. Our experience is that every mid- and high index material, except Spectralite, requires the coating. Some other mid-index manufacturers have stated that their product does not require a coating; we disagree.
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