Clarity

**edKENdance** · 08-23-2001, 08:31 PM

I have often heard that glass is of a higher optical quality then plastics. Most of the people that I sell to prefer glass as it is of higher scratch resitance. My question, is the optical clarity of glass perceptable on a perceivable level or is it only a technical difference. Also, I know that glass is more scratch resistant then plastic but is that only a scientific norm as opposed to good treatment with the proper lens cleaning tools? Glass or plastic in the hands of an idiot is just as likely to scratch I'm sure. What would you consider to be a good life for a frame. I have some customers with 20 year old frames and some with 2 week old frames that decide they have to change.

**optigoddess** · 08-23-2001, 09:19 PM

I have heard somewhere that the life expectancy of a mid-line frame is 2 years. In my experience, I have found that inexpensive frames usually last about 1 year.

**Jim G** · 08-24-2001, 10:21 AM

If you would define "clarity" as Transmittance and Residual Color (yellow), the following stats apply: Glass - Transmission = 91.6%; Yellowness index = 0.5%. CR-39 plastic is 92.4% and 0.7%. Poly is 90.3 & 1.8%. Spectralite is 91.2% & 2.2%. MR-6 (1.60) is 91.7% and 2.0%. (All figures from the OLA "Spectral Transmission of Lens Materials" guide.)

What's that all mean? Not much. Were you to place the various lenses on a white piece of paper, you might be able to discern some differences. However, the user would unable to tell the difference.

Re: scratch resistance, there really is no comparison. Glass is the winner by a HUGE margin...tho, the SRC's are getting better.

Glass is used (exclusively?) in all high tech optical instrumentation not necessarily because of it's "clarity" but because of other, physical characteristics (rigidity, change due to temp, scratch resistance, etc.) tho it is always AR coated to improve it's "clarity."

**chip anderson** · 08-24-2001, 12:10 PM

Yes, Glass has a much greater optical clarity. You will note that no one ever made a fine microscope or telescope out of plastic. The surface of glass can be ground and polished to a much finer surface. Plastic, even when molded against glass just will not accept the surface quality of glass.

I am sure that your plastic lens manufacturer will give you a lot of arguement to the contrary, but this is the way it is.

Just as many contact lens manufacturers will claim that the optics of gas permeable plastics are comprable to Polymethylmetraclate, it just ain't so. You will often find that the patient you just couldn't make visually happy in a gas permeable plastic will be thrilled with PMMA. If you have the skills required to fit PMMA and are not afraid of having the rest of your colleges frown on you.

:cheers:

Chip