The Ultimate Guide on How Photochromic Lenses Work

1. What is Photochromic Technology?

First, when you look at the word photochromic, it is made of two words:

• Photo, which refers to light
• Chromic which refers to the  color

It follows that, a photochromic material is one that undergoes a reversible change in color (tint), on exposure to a certain threshold of ultraviolet radiation (UV).

I am sure you have seen such materials.

Here is a good example:

As you can see, the color of this polycarbonate photochromic sheet and film, changes on exposure to ultraviolet radiation.

Normally, this change is due to molecular reaction of photochromic compounds in these materials.

That is, a certain threshold of ultraviolet radiation initiates a reversible change in the molecular structure of photochromic compounds.

It is this reaction that causes the lens to darken.

The intensity of the tint and uniformity will depend on the chemical composition of these molecules on the lens.

You will learn more about these later in this guide.

Also, you can learn more about the history and advancements of this technology from this free Complete Guide Book to Photochromic Technology.

What is a Photochromic Lens?

These are lenses that remain clear/nearly clear indoors (in the absence of specific threshold of UV) and darken/change color when outdoors (on exposure to specific threshold of UV).

In most cases, it takes about 30 seconds for the lenses to tint fully and about 2 to 5 minutes to change to the original color.

So, the change in photochromic molecule structure will result in the following phenomenon:

This illustrates how a photochromic lens changes both indoors and outdoors.

I know you could be wondering what really makes up these photochromic chemicals.

Don’t worry, I will explain that shortly.

For now, I want you to understand that photochromic lenses are made from many materials.

However, the most common photochromic lens materials are glass and polycarbonate sheet (plastic).

In the recent past, polycarbonate sheet has become a popular material among many photochromic lens manufacturers.

Of course, this is due to shatterproof and other desirable features of polycarbonate sheets.

Since Roger Araujo, made the first photochromic lenses, there has been a lot of research and innovations going on.

So, companies in the photochromic lens industry try to modify these accessories to suit specific needs of their clients.

For this reason you will realize that the modern photochromic lenses feature organic molecules as opposed to silver compounds.

I will explain this in the next section.

How Photochromic Lenses Work (The Reversible Molecular Reaction You Should Know)

I am sure you know that the working principle of photochromic lenses depends on the fact that:

• Photochromic materials tint or darken on exposure to a specific threshold of UV
• In the absence of UV, the tint reverses back to the original color of the photochromic material (becomes clear).

In this section, I want to discuss this subject in detail.

That is, I will focus more on the structural/molecular changes that take place within the photochromic materials.

This way, I am sure you will understand how photochromic lenses work.

To make it easier for you to understand, I will discuss the two types of photochromic lenses separately.

Here is all you need to know:

· How Glass Photochromic Lenses Work

The first photochromic lenses were made from glass.

In these lenses, the main photochromic elements were mainly silver compounds.

These silver compounds were either silver chloride or silver halide.

The compounds were embedded within the glass substrate.

This substrate could vary in size thou the most common size was about 5nm.

In this case, the tinting was due to a chemical reaction, which I will explain shortly.

Here is how silver compounds react to create a photochromic effect:

Silver Chloride:

To make a glass photochromic lens, you need to dope silver chloride with copper.

Normally, a number of manufacturers use borosilicate glass to produce these photochromic lenses.

The process involves adding silver chloride to the melt after which, cooling takes place.

However, at this stage, the glass is bright blue and is not yet, photochromic.

You must therefore subject it to heat treatment to a temperature of about 600°C to obtain photochromic properties.

The duration of the heat treatment and temperature will determine the sensitivity of the photochromic molecules.

Now, when you subject glass photochromic lens to a short-wave light, it will begin to tint gradually.

This implies that, the silver chloride will begin to break into silver and chlorine.

This is what leads to the darkening process.

You can see the chemical reactions below showing the oxidation and reduction processes.

Oxidation and reduction of silver chloride

Again, when you remove the UV light, silver will begin to combine with chlorine. It is at this stage that copper plays an integral role.

That is, the presence of copper (I) chloride will help to reverse the process.

You can see this in the equation below:

A reaction that helps the photochromic glass retain its original color

It is a process we refer to as a photo-oxidation of photochromic glass.

The fact that this process is reversible makes it useful in making photochromic lenses.

Now, let’s look at case two of photochromic effect on lenses.

· How Polycarbonate Photochromic Lenses Work

The commercialization of plastic photochromic lenses has seen a number of manufacturers opt for polycarbonate sheets as a better alternative.

By adopting polycarbonate materials, they can manufacture a range of photochromic lenses with varying designs and shapes.

WeeTect photochromic lenses

But more interestingly, the chemistry behind the working principle of plastic photochromic lenses is quite different from that of glass.

These photochromic lenses depend on carbon based compounds.

So far, there have been significant improvements in this industry with the aim of producing fast reacting and effective photochromic surfaces.

For instance, initially photochromic lens manufacturers were using pyridobenzoxazines.

This is basically an organic dye.

Today, they use naphthopyrans, with others opting for indenonaphthopyrans.

So, let’s see how these organic compounds react to achieve photochromic properties.

· How Does the Naphthopyrans Produce Photochromic Effect on Plastic Lenses?

First, I want to state that the structure of these carbon based compounds may vary broadly.

This will depend on what the manufacturer opts to use for its products.

However, how photochromic lens works will still remain the same.

Let’s review some of the basic facts about this plastic photochromic lens:

An illustration of how naphthopyrans cause the photochromic effect.

Whenever you expose these plastic photochromic lenses to a certain threshold of ultra-violet radiation, the chemical bonds in these compounds will be broken.

This will result in a complete change in the molecular structure.

What happens is that, the weak bonds will break and the molecules will rearrange to form species that absorb light at stronger wavelengths.

As a result the lens will begin to tint.

When you remove or in the absence of the ultraviolet radiation, these carbon compounds will retain their original structure.

As a result, the plastic lens retains its original color.

Again, here, the change in structure manifests itself in the form of the surface tinting and retaining its original color.

So, the bottom line is:

Even though both plastic and glass photochromic lenses will tint and retain the original color on exposure on exposure to the UV radiation, the chemistry behind every process is clearly different.

Again, don’t confuse photochromic lenses and polarized lenses.

You should note that polarized lenses are permanently dark or tinted.

Therefore, they don’t change color whether there is UV light or not.

Now, why should you invest in technology now that you know how photochromic lenses work?

Benefits of Photochromic Lenses

Now that you have learnt how photochromic lenses work, it will be important to know the exact reasons for investing in this accessory.

Below are some of the benefits of photochromic lenses:

Reducing Unnecessary Expenses

Buying a lens that you can use both indoors and outdoors is cost effective in the long-run.

One such example is a photochromic lens.

This lens will automatically adjust depending on the intensity of the surrounding ultraviolet radiation.

How photochromic lenses change from clear to dark

Basically, this will save you from investing in two lenses or sunglasses.

In fact, it is for this reason that most people opt for photochromic lenses and not polarized lenses.

Guarantees UV Protection

Photochromic lens provides an ultimate protection against the harmful ultraviolet radiations such as the blue violet light.

Obviously, you don’t have to strain the natural defense mechanisms of your eyes to stay free from harmful ultraviolet radiation.

Normally, the effectiveness of these lenses will depend on the quality of photochromic molecules on the material.

Photochromic Lenses are Convenient

The truth is that you will only have a pair of sunglass or lens for both indoor and outdoor activities.

This will be far much better than having two or more regular glasses.

Also, it will be even hectic to change/switch sunglasses, assuming you are in an outdoor activity that requires much of your attention.

No More Eye Problems

Photochromic lenses will protect your eyes from any form of ultraviolet radiations.

This will reduce chances of suffering from eye problems due UV radiation, such as blindness or cataracts.

Possible eye problems due to UV radiation

Consolidating Photochromic with other Coating Technologies

Notably, to improve performance of your lenses, you can use photochromic alongside other lens coating technologies.

That is, you can have hydrophilic anti-fog, anti-abrasion, UV resistant of augmented reality technologies, just to mention a few.

But again, even though there have been significant improvements in the photochromic technology, you still have to deal with certain challenges.

Here are some challenges that most photochromic lens manufacturers have to deal with:

Slow Reaction Time

As I indicated while discussing how photochromic lenses work, there could be a lag in:

• Oxidation and reduction reaction  in the case of glasses or,
• Slow change in the structure of carbon compounds in the case of polycarbonate lenses

All these will depend on the quality of photochromic molecules.

In most cases, the lenses may tint really quick, may be a few seconds.

However, the reverse may take quite long – about 5 minutes.

It will mainly depend on the surrounding temperature, which tends to affect the reaction time of lenses.

This can be dangerous if you enter a dark room immediately.

People Suffering from Extreme Light Conditions

At times, photochromic lenses may not be dark enough to protect your eyes from extreme light conditions.

This may require that you have an additional protective coating to block the extra ultraviolet radiation.

Apart from these two, you will find that these lenses may not be effective in cars.

This is because the windscreen will naturally block most of the light that can activate the photochromic elements.

I am sure by choosing high quality lenses that protect photochromic compounds from breaking down and ensures faster activation your eyes will always be safe.

WeeTect photochromic product

1. Photochromic Visor Insert 

2. photochromic visor

3. photochromic goggle lens

4.photochromic sheet and film

Conclusion

As you can see, the working principle of photochromic lenses is based on invisible chemical reactions or change in molecular structure. Their effects manifest in the form of color change. Again, to benefit from all these, you need to buy high quality photochromic lenses from reputable and trusted manufacturer.

Always feel free to contact us in case you have questions about photochromic lenses. At WeeTect, we have years of experience and know-how in various photochromic technologies.

And, in case you want to learn more about photochromic technology, here are some useful links:

• Self-darkening Eye Glasses– Source: Chemical & Engineering News
• Photochromic Lens– Source: Wikipedia
• A Complete Guide Book to Photochromic Technology–Source: WeeTect
• Evaluation of the Performance of Photochromic Spectacles– Source: NCBI
• Introduction to Organic Photochromic Molecules– By Wiley VCH