Finding the cause of scratched lenses

Every three years, a glance at my lenses shows me that I'm either doing something wrong when cleaning them - or that the scratch-free surface of the sinfully expensive lenses isn't as great as promised in the advertising brochures.

So I went to the optician - and took the opportunity to have my glasses measured again. And lo and behold: for the first time, they actually got a little better.

The friendly optician explained to me that the cracks on the surface could not have been caused by cleaning with tissues, glasses cleaning wipes or under running water. Rather, the glasses had probably become too hot during cooking or baking. The temperature differences could cause microcracks.

Of course, this explanation leaves me somewhat perplexed - after all, I rarely hang my bespectacled face in the oven. It couldn't have been the deep fryer either, because I don't own one. When I open the oven, I don't want to completely rule out the possibility of my glasses getting a little hot. Opening the dishwasher also causes a brief surge of heat. But the explanation, which I don't quite understand, awakens my research instinct. Something inside me urges me to investigate this somewhat abstruse statement further.

There is one homework assignment to do first: I can still get a fair idea of the geometry of the edge of my lenses, but I'd like to be a bit more precise about the inner and outer radius. Otherwise there is a risk that someone will doubt my calculation.

A brief study of the basics of optics shows: My -5.5 diopters correspond approximately to the radii R₁ = 0.0833 m and R₂ = -0.0493 m for a highly refractive plastic with a refractive index of 1.67. And yes, you have read correctly: The radius R₂ is negative. As an engineer, I've never seen a negative radius before and was therefore surprised - but it seems to work in terms of the formula. Just as important: the radii must be given in meters - otherwise they don't match the diopters. The outer dimensions without rounded corners are 65 mm × 35 mm, the thickness in the middle is stated in the spectacle passport and is 1.2 mm. Well - and the layer thickness is 4 μm, at least in the calculation model. According to brief research, this seems quite realistic.

So, we have the geometry. What is missing are the material properties of the glass (or plastic) and the coating. We need the following for both materials: modulus of elasticity, transverse contraction coefficient, specific heat capacity, density and coefficient of thermal expansion. The tensile strength of the coating is used for evaluation. If you are interested in more detail, I will be happy to send you the values I used. However, the average reader of my blogs does not like tables or formulas - so I have refrained from using them.

Armed with a heightened awareness and an internal stopwatch, I have observed how long I spend with my face in the critical area of the open dishwasher, stove or pan - or how long I clean my glasses under warm running water.

I have thought about three load cases that interest me and that largely describe the short life of my glasses:

LF1: Washing under warm water

Water temperature: 40 °C
Heat transfer coefficient: 1000 W/(m²K)
Duration: 10 seconds uniform exposure from all sides

LF2: Opening the dishwasher - water vapor rises

Steam temperature: 90 °C
Heat transfer coefficient: 100 W/(m²K)
Duration: approx. 1 second - only on the outer surface of the glasses

LF3: Leaving glasses in a hot car

Homogeneously heated to 70 °C

The second load case also roughly covers cooking scenarios. I usually avoid splashes of fat with hot oil on my face.

The heat transfer values are almost perfectly estimated based on my 36 years of experience in simulation at Merkle CAE Solutions, while the time data - as mentioned - comes from real tests at the sink and in the kitchen.

The results clearly show that uniform, high temperatures cause the most damage to the lenses. The coating has a lower thermal expansion than the plastic. If it is only heated briefly from the front, this does not hurt it as much. When washing under warm water, on the other hand, the heat transfer is much better, the lenses become warmer overall - and this increases the tensile stresses in the coating.

If the glasses are left in the car, tensions above the tensile strength of the coating occur from a temperature increase of 30 °C. The result: micro-cracks. The result: microcracks that are distributed relatively evenly across the surface.

I will probably need new lenses again in three years' time. But I'll set the water temperature a little cooler when I clean them - but I'm more of a spray glasses person anyway, as it removes grease and streaks better.

Leaving the glasses in a hot car or taking them into the sauna is probably unnecessary. In future, I will also be more careful when opening the lid of the oven, grill or dishwasher.

The dark feeling that the coating will simply scratch over time - no matter what the friendly optician says - unfortunately won't let go of me completely. We'll see...

PS: As always, I look forward to your feedback. Are there any everyday topics that you would like to know more about with the help of simulation at Merkle CAE Solutions? Feel free to write to me!