Photochromic lenses are lenses that darken and lighten in response to changes in light. These lenses make it easier to see in low light conditions and even help protect your eyes from UV rays. This means they can darken automatically when the sun goes down or darken indoors and lighten when they are outdoors. If you are outside without any direct sun, they should still maintain their color regardless of how much time passes. Photochromic lenses differ from sunglasses because they have a fixed tint that protects the eye from glare but cannot change. In contrast, photochromic glasses adapt quickly to the available light type and adjust accordingly.
Most people with achromatopsia utilize photochromic lenses. They are also used for people with low light sensitivity who need to shade their eyes from the sun. Photochromic lenses can be made from glass, polycarbonate, or plastic. Photochromic lenses help reduce eye strain and eye damage by blocking up to 100% of harmful UVA/UVB rays, and they are very comfortable to wear because you do not need different glasses for different lighting conditions, protect the eye from high risk of cataracts and block out as much light as possible, and they are inexpensive since it has the properties of both lenses (standard lens and sunglasses).
Photochromic lenses can be divided into two categories—those that fade gradually over time (called gradual-fading lenses) and those that fade abruptly after exposure to a certain amount of light (called sudden-fading lenses). Gradual-fading lenses will take longer to change color than sudden-fading lenses; however, they tend to have a lower initial cost. Sudden-fading lenses often have higher initial costs but may last longer and hold their color better. Both lenses will perform well in low temperatures. Most manufacturers recommend replacing the lenses every three years. However, the lifespan of your lenses can vary depending on several factors, including environmental conditions, cleaning practices, and the care and maintenance of the product.