A complain regarding PALs' center thickness

[Danicris Lim]

Good day optiboard friends!



I would like to ask your opinion regarding a letter of complaint my colleague, who’s working in a lens laboratory, received from his client. The client’s complaining of a thicker center thickness of an Image PAL transitions. The complete prescription was as follows:



OD: -3.00 Dsph

OS: - 3.00 Dsph

Add: +2.25 Dsph

(rimless frame)



Can we really expect an Image PAL transitions (index of refraction:1.50) to have a thinner center thickness though the lens undergo a process of photochromism? Isn’t it a wrong notion to compare an Image PAL transitions to a Hoya GP that is not a photochromic lens with regards to its center thickness?



Attached herewith is a copy of the letter of complaint.

I am hoping to receive your comments very soon regarding this matter.


Thank you very much!



Danicris Lim

[HarryChiling]

First I would not have done a rimless frame in plastic, second if he would like a thinner lens and it can be acomplished I would remake it. I would also point out that the lens center thickness must be considered for the reading segment as well and any yoke prism. So the answer may not be as straight forward as the patient may think. As for the transition lens it would not make a difference except for the fact that the index of a transition lens is slightly higher than a CR-39 lens due to different material makeup. So technically the transition should be slightly thinner but it is not true that the center would be thinner, that can only be ground down to a specific thickness before excedding the ANSI threshhold for safety, you would see the difference in thickness at the edges.