The transitioning that a Photochromic lens undergoes when exposed to sunlight (or Ultraviolet Light in general) can seem like magic.  One second you are indoors with your clear glasses, the next moment you are outside and have sunglasses-like shades on your face without changing frames. 

It is a quite fascinating phenomenon and it has left many wondering the exact magic trick behind this rapid change in color density. 

This guide carefully explains the science behind the change in color a photochromic lens undergoes when exposed to sunlight as well as their many benefits.

What Are Photochromic Lenses?

A Photochromic lens is a lens that stays clear indoors away from light and darkens upon exposure to ultraviolet light from the sun. They serve a dual function of both glasses and sunglasses. 

The first photochromic lenses were made from glass and were patented in the 1960s. Over time, plastic lenses were developed in the 80’s and 90’s and are now the go-to and popular choice among modern users.

How Do Photochromic Lenses Work?

Photochromic lenses contain light-sensitive molecules that shift their structures when they are exposed to UV rays from the sun. This change in structure allows them to be able to absorb more light and concurrently give the lenses a darkened appearance.

Once the light source is taken away, the structures shift back to their initial conformation, and the process is reversed thereby giving the lens a clear appearance again. 

These molecules are sensitive to the presence of UV light and the amount of UV rays passing through them. This means that their structural change is faster in the presence of more UV light and this change is slow when they are exposed to a minimal quantity of ultraviolet rays. 

This explains why your photochromic lens may not darken as much when you are in a shade as opposed to being directly hit by the sun. However, this doesn't mean you need to stand directly in the sun to receive daily doses of UV rays. Ultraviolet light penetrates the clouds and your photochromic lenses will darken just fine under overcast skies.

Do you want finer details on the working mechanism of photochromic lenses? The exact working mechanism is dependent on the material used to make the lens. This is explained below.

How Do Plastic Photochromic Lenses Work?

Plastic photochromic lenses are made from carbon-based compounds and this is what is reactive to light and changes structures in the presence of UV light. They are also called photochromic dyes. 

Their actual chemical names are quite long like pyridobenzoxazines and indenonaphthopyrans - you don't need to bother with them. However, it is important to know that these organic chemical compounds react faster and change color quickly than molecules used in glass photochromic lenses. 

Once exposed to light, the chemical bond in these dyes break causing a structural transformation that allows them to absorb more visible light. 

The more the light absorbed, the darker the lens becomes; and there is the magic trick. Once UV light penetration reduces, these dyes go back to their initial structure and the lenses become clear as day all over again.  

Plastic photochromic lenses are more common than their glass counterparts and research is still ongoing on how to make these plastic lenses even better.

How Do Glass Photochromic Lenses Work?

Glass photochromic lenses have several minute amounts of silver halide scattered inside them. The popular halide used is Silver Chloride. Once these chemical compounds are exposed to ultraviolet rays, the silver ion compound gains an electron to become an ionic (or elemental silver). These released silver absorb visible light. Thus, the more silver released, the darker the lens becomes as it absorbs more visible light.  

Many photographic films employ this same silver halide chemical mechanism. Exposure to light causes the images on them to darken. However, photochromic lenses are different in that they revert back to their original clear appearance when the ultraviolet light source is taken away.

Once UV rays are withdrawn, a second compound in the glass (usually copper chloride) accepts the electron back from the silver metal. This causes the reaction to be reversed and the lens becomes clear once again.

While photochromic lenses made from glass are not common anymore, this first set of technology paved the way for the current trend employed in light-responsive glasses.

Parameters Used In Photochromic Lenses And What They Mean

There are various parameters used in the photochromic lens industry. It is important to understand these parameters as they allow you to pick the right lens for use. The various parameters include:

1. Material: 

The material of a photochromic lens can vary depending on the manufacturer. The glass and plastic materials have been covered in previous sections. However, there is also the polycarbonate material which is very effective and widely used.

Examples of materials you will find when shopping for a photochromic lens include: CR-39 (Columbia Resin 39), photochromic oligomer, PMMA (Polymethyl methacrylate), MR-8, MR-7, and PC. 

Usually, polycarbonate is preferred because of its strength and resistance to impact. However, glass and plastic have their advantages over polycarbonate. For example, MR-7 is loved for its thinness and lightweightness.

2. UV Cut-off:

This is the wavelength below which the substance or chemical used in the photochromic lens can absorb all the UV rays. Therefore, the higher the UV cut off the better it is at blocking UV rays. 

Most photochromic lenses at Maat Optical have a minimum UV cut-off of 360, while some can go as high as 410 which is very good. 

3. Option Color:

Photochromic lenses come in different colors which can include blue, yellow, clear, brown, gray and so on. While one color might have an advantage over the other, they are mostly selected based on personal preference.

4. Abbe Value:

The abbe value is the measure of the extent to which light is separated or dispersed when it passes through the lens. The higher the abbe value, the better.

Plastic materials have been found to have the best abbe value while polycarbonate materials have been found to have the worst. An abbe value greater than 50 is very good.

Other parameters of a photochromic lens include power range, coating and specific gravity.

How Quickly Do Photochromic Lenses Work?

Photochromic lenses darken in 30–60 seconds on average when exposed to ultraviolet light. To clear, they take a bit longer: 2–3 minutes is normal. 

Scientists are still working with photochromic dyes to quicken the reactivity of the lenses. In the future, we could see faster and faster activation and clearing times.

How Does Temperature Affect Photochromic Lenses?

The rate at which photochromic lenses darken or clear up is impacted by temperature. When it’s colder, the lenses are apt to darken a bit more fully but take a longer time to clear up. When it’s warmer, they may not darken quite as much but will clear up more quickly, as the molecules within them are more reactive.

Do Photochromic Lenses Wear Out?

The more photochromic lenses are exposed to sunlight the less they become reactive to light and the longer they take to darken when exposed to sunlight. They may also develop a yellowish tint with use over time. 

Eventually, your photochromic lenses will become less reactive to ultraviolet light and will take longer to shift between their two states. They may also take on a yellowish tint that signals their age. Most photochromic lenses will perform well for three years or more, however, so they’re still a long-term investment.

Conclusion

Photochromic lenses are of great value to everyone as they protect your eye from harmful UV rays.

Photochromic lenses are able to do this due to the presence of some specific chemicals.

Photochromic lenses don’t last a lifetime, however, you can get a very durable and authentic photochromic lens from Maat Optical.