The intense UV rays of summer are uncomfortable, especially for our eyes. Many presbyopia people would like to wear bifocal sunglasses to protect their eyes. The others may choose photochromic glasses because they are more convenient. So which ones are better? What are the differences between photochromic reading glasses and reading sunglasses?
What are photochromic reading glasses?
Photochromic spectacles turn dark when exposed to UV light but return to a clear lens when no longer exposed to this light. The tinting effect is caused by the silver chloride or other halides in the lens, so the lens is transparent when not exposed to UV light. But it reacts when exposed to UV light and absorbs the visible light to give the lens a dark color. The chemical reaction is reversible so that the lenses will return to their original transparent state when they are no longer exposed to UV light. Therefore, people can wear photochromic reading glasses indoors or outdoor and do not need to change other reading glasses frequently.
What are the differences between photochromic reading glasses and bifocal sunglasses?
The biggest difference between them is that the lens of bifocal sunglasses cannot change to clear indoors, which is only suitable for intense sunshine outdoors. People can wear photochromic reading glasses indoors because they will return to transparency when there is no UV rays.
However, the change of photochromic substances from dark to light is a thermal reaction. The dark lens will become a little lighter in a higher temperature environment. Therefore, photochromic lenses are not as dark as ordinary sunglasses at high temperatures. That means they cannot entirely replace sunglasses in the hot summer. Conversely, a darker effect can be obtained in a cold environment, so photochromic lenses are more suitable in snow than in summer outdoor activities.
To sum up, photochromic reading glasses are more convenient than sunglasses if people do not want to change the glasses frequently when going in and out. But they couldn’t replace sunglasses when the temperature is too high.