HOME >> Example of defects in your lenses: You found thin lines (cracks) in your spectacle lenses!?


I have found lines crossing through my lenses even though it has not even been a year since I purchased my eyeglasses. Are my lenses defective?

It seems that you do not notice them in the daytime, but notice them when the light from oncoming vehicles at night are dispersed.
After close examinations, we can see that these lines appear all throughout your lenses.

In such cases, we ask you to bring them to the eyeglass shop where you purchased them and have them take a look as we cannot assess the situation or cause behind it over the phone.
The results of the inspections showed that heat was the main cause.

The photos below are of some of the lenses that have been inspected in the past and the photos of the three lenses in the row (exterior, partial enlargement, and distortion confirmed) are of the same lenses.


The cracks in these lenses become clearly visible after a few days, and particularly so around their centers, where stress is concentrated, during the initial stages. Common characteristics that these lenses share are that lines can be found all throughout those that are damaged, these lines are in the coated surfaces, the lines are webbed and not like those formed when wiping, and that they are highly distorted.

This is an explanation on the mechanism behind such conditions.
Plastic lenses have their base materials at their centers. There are hard coat layers for protecting the base materials, antireflection coating above them, and water-repellent coat layers on the outer surface.
The base materials are organic and expand at high temperatures, but inorganic antireflection coating does not expand as much as the base materials do.
Therefore, when base materials expand due to the effects of heat, they form cracks in the coating like those in the ground because the coating cannot keep up with the expansions.
We were once criticized by someone in the automobile industry who said that we in the eyeglasses industry have it easy because we do not have to manufacture products that can withstand all environments from cold regions to tropical areas like they do. This is true to some extent as most of our products are not very resistant to heat.

The basis of our determination that it was damage from heat

The photos below are of lenses that have been heated at 60℃, 65℃, and 70℃ for twenty minutes at a time.
The changing of the colors in the distortions shows how they gradually expand.
Lenses must be heated up to 60℃ or more and their base materials must be expanded as shown in the bottom photo to become heavily distorted as shown in the top photo. We have determined that damage from such heat is the cause of cracking.

Actual cases

However, there is a question that still remains.
It is whether there is exposure to so much heat in our daily lives.
There was an experiment that we once conducted.

We measured the temperature inside a car that had been parked for an hour in the daytime before summer came when the temperature was 23.3℃ outside.
The dashboard was 74.5℃ and the areas around the back seats and inside the dashboard were 60℃ or more.
This means that storage inside a vehicle is dangerous even if it is not mid-summer or in an area not on the dashboard or even if it is for a short period of time.
There are cars with sunglass holders, but please think of them as temporary storage spaces to use when driving.

Other factors that appear in daily life are radiant heat from barbeque hot plates, grills and charcoal fire - all of which can be 70℃ or more.
For example, it is extremely dangerous to place your eyeglasses down near a grill at a barbeque restaurant even for a moment.
There is also the possibility of hot air from hair driers and heaters, and other things that may not be seen on a normal basis such as bonfires.
We usually do not sense danger even when temperatures are 60℃ or more like how we can bear saunas of nearly 80℃, but bathwater of 50℃ or more feels extremely hot to us because water and air have different heat conduction. Therefore, there is the possibility of not even remembering any instances of exposure to heat.
That is why most people say that they have no recollection of any such circumstances when asked.

This phenomenon known as thermal cracking tends to occur repeatedly in lenses of the same person.
Please remember that if something like this occurs, your problem will not be solved even if you switch lenses or manufacturers.
We understand that this may sound to some like we are trying to avoid responsibility for delivering low quality products or that we do not care, but it is clear from the photos that they have been affected enough by heat for them to expand. We believe that identifying the environments and conditions they are in is the fastest way of finding a solution.
We hope that this information can serve as reference for you.