Ct

[sandeepgoodbole]

What are the minimun CT requierments for CR 1.49, 1.56, 1.6, 1.61, 1.67,1.74, Poly, Futurex, Trivex. Is it true that more CT needed for Arc as compaired with Hard Coats?

[RT]

In the U.S., there are no minimum center thickness requirements, per se. There is the Food and Drug Administration (FDA) drop ball test for determining impact resistance (21 CFR 801.410(c)(3)). Since there is a correlation between lens CT and passing the drop ball test, most labs will set a minimum CT for each lens material. But it is not the center thickness that is important, it is whether or not the lens can pass dropball.

However, there are certain hard coats that can be applied that can significantly change performance. For example, CR39 (1.499) is typically ground to a minimum CT of about 2.0 mm. However, application of a cushion coat can reduce the required CT to as low as 1.3 mm and still pass drop ball.

Conversely, application of an AR coat can decrease impact resistance. Mid-index materials like 1.56 generally require extra center thickness if an AR coat is applied. The culprit here is the heat involved in AR coat application. It is possible that you could use a cushion coat to offset the effects of the AR. 1.67 materials, for example, typically use a cushion coat to achieve a minimum CT of 1.0 mm. Without the cushion coat, most labs require a minimum CT of 1.5 mm. That's why it's important to know your lab's process...that extra 0.5mm could negate any thickness advantage to using the higher index of refraction.

For some materials, like polycarbonate and Trivex, the issue with minimum center thickness does not relate to impact resistance. These materials are flexible, and a lower center thickness can yield lenses that aren't particularly rigid. At low (1.0 mm) center thickness, there are problems with low yield in the laboratory, and problems with lens retention in the frames. Also, it is hard to make hard/AR coats that can withstand the flexing of a thin lens. Crazed AR is all too common on thin center poly lenses.

Unfortunately, there is not one compact answer to your question, as the lens material is not the sole factor that determines minimum center thickness reqiurements. And outside of the US, impact resistance requirements vary...meaning that something that isn't done in the US can be done elsewhere.

[ILT Client Service]

The minimum thickness for lenses, besides the material index, depends on a few other elements:

- Respective country’s requirement.
In the USA, it’s common that they make the AR coated lenses with a minimum thickness of 2mm to pass the FDA drop ball test. In other some other parts of the world, this may not be necessary and consumers prefer the lenses to be as thin as possible.

- Frame requirements. For eg, Rimlon frames usually require a thicker edge for grooving.

- RX Laboratory’s machines and tools to achieve the desired thickness.

[AMIGODIANA]

ILT say is right