Lens Tech: Photochromic Lenses

From glassware to Cr-39 plastic to the lightest in polycarbonate and high index, ophthalmic lens technologies have advanced in a number of ways to provide more comfort. After visiting your optometrist for your yearly eye exam, finding the right pair of glasses means considering many choices. The prescription from the doctor, the type of frame, and the shape of the lenses all need to be considered when making a decision for your next pair of glasses.

With photochromic lenses, patients are given a feature in their glasses to protect their eyes from UV radiation from the sun.

Photochromic lenses have the ability to change color from a clear lens to a dark color tint when exposed to the sun, and change back to clear once indoors. This added feature to the matrix of the lens provides some protection by absorbing the UV radiation from the sun. The sun changes the compounds embedded into the lens to block some of this harmful radiation.

Constant exposure to these harmful rays of energy can lead to several visual problems such as cataracts, pterygium, and squamous cell carcinoma.

Protection is crucial to the health of the eye over a period of time. The signs of ocular disease from the harmful rays of the sun can vary depending on the onset exposure. Starting at a young age there should always be some type of protection from the sun, such as sunscreen, shade, or sunglasses.

Photochromic lenses do not replace a pair of sunglasses because they do NOT provide 100% protection from the UV radiation.

Understanding how these lenses work takes a little knowledge of the electromagnetic spectrum and chemistry. All radiant energy can be thought of as being a type of wave, and is categorized in the electromagnetic spectrum. Some energy waves such as radio waves are on the lower end of the spectrum, while others like microwaves and x-rays are on the higher end of the energy spectrum. The part of the spectrum that is visible to the human eye is 380 nanometers (nm) to 760 nm which is known as the visible spectrum. For example between 650-750 nm, the human eye transmits this wavelength and converts this signal at the retina to the brain, perceiving the signal as a red color. The part of the spectrum that the photochromic lenses rely on to change color is the non-visible spectrum, which make up the ultraviolet (UV) radiation with wavelengths that range from 290-380 nm on the spectrum.

When fabricating the lenses, special photochromic dyes are embedded into the matrix of the lens. These dyes are made of inorganic isomer molecules, molecules that have the same chemical components but exist in different arrangements and are not made up of carbon or hydrogen. When the lenses are clear, the dyes are in their inactive state and the molecules are in an arrangement not seen through the lens. Once these dyes absorb UV radiation from the sun, the molecular structure changes shape and the molecules arrange themselves to an active state, causing the lens to change color.

Since the active state is unstable for the molecule to hold, reverting back to the molecule’s inactive state when indoors allows for the lens to be clear once again. These lenses will not change when indoors because these inorganic molecules do not change shape when hit with the illumination of indoor lighting. These dyes can be any color and any degree of darkness. The most commonly used photochromic dyes are oxazines, Fulgide Derivatives, and Napthopyran, all of which have advantages and disadvantages, such as faster darkening of the lens and quicker reversion of the lens to it’s clear state.

Photochromic lenses do not change tint behind the windshield of a car because the windshield already provides some protection from UV radiation.

Consult your optician after seeing the doctor when picking out your next pair of glasses. Photochromic lenses are a unique product that have the benefit of protection from a potentially harmful source of UV rays. This technology has advanced thoroughly from it’s first release in the 1960s, and continues to advance to fit our needs. In the 40 years since these lenses have been on the market to consumers, theses lenses have filled a niche in the optical world that provides patients a unique feature to their new pair of glasses.

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