An optometrist explains: how much heat can eyeglass lenses tolerate?

Glasses may seem durable, but both the lenses and the frame can be very sensitive to heat. Many people who wear glasses may be familiar with the feeling of their glasses no longer being clear after a trip to the sauna. The same can happen if you forget your sunglasses in the car for too long on a hot summer day or while on holiday. In such cases, the damage is usually irreversible.

Most of today’s glasses are made of plastic. Such materials start to soften in the temperature range 90–150 °C but can deform already at much lower temperatures.

Even more delicate than the material of the lenses are the coatings applied to the lenses to protect the plastic, enhance comfort and improve clarity. The heat resistance of the coatings is in the range 80–100 °C, but temperatures as low as 60 °C can be sufficient to cause damage.

When lenses are exposed to high heat, a dense network of tiny cracks can form in the coating, distorting vision and causing halos. In this case, you will feel as if the glasses are dirty and can no longer be cleaned. In addition, the heat weakens the bond between the coating and the lens, which causes the coatings to peel off the lenses.

On a hot, sunny day, plus glasses can also act as a magnifying glass, concentrating the sun’s rays and setting fire to the spot where the rays congregate.

Optometrist Gerli Jones gives recommendations for how to prevent heat damage to your glasses:

1. Glasses should be washed with lukewarm water, as hot water will damage both the frame and the lens.
2. Direct heat sources such as saunas, hairdryers, campfires and barbecues should be avoided.
3. When opening the oven, let the initial heat and steam escape.
4. Don’t leave your glasses in the car on a hot summer day as temperatures in the car can reach 60–70 °C, which can damage both the lens and the frame itself.
5. It’s also a good idea not to leave your glasses outdoors in the sun on a dark background.