We’ve all experienced being temporarily unable to focus our eyes and see clearly when stepping out into the sunshine.  It’s frustrating, disorienting, and can be dangerous.   However, what’s even more frustrating is having to switch between clear and gray safety glasses when heading outside. During these transitions, you may find yourself forgetting one pair or the other, then get stuck indoors or outdoors without the correct lens. 

Photochromic lenses for glasses were originally introduced in 1966 by Corning Glass Works.  There it was discovered that when silver-based crystals were applied to the molten glass, the finished lenses would darken in response to UV light in just a few minutes.  The more intense the UV exposure, the darker the lenses would become, and they would continue to adjust in various light conditions.  The lenses are clear (or nearly clear) indoors and darken automatically when exposed to sunlight.  You can see how this type of lens technology would be advantageous for someone wanting to eliminate the need for carrying both a clear and gray set of safety glasses.    Since these lenses were developed, light-responsive technology has seen significant improvements in performance.  As a result of these improvements and MCR Safety's ability to improve manufacturing efficiencies, they have developed their first affordable photochromic safety glasses.  Below is everything you need to know about MCR Safety’s photochromic lenses.

Photochromic Lenses

You may hear safety glasses referred to as having "auto-tinting lenses",   "light-adaptive lenses", “transition lenses”, or "variable tint lenses".  If you’re considering photochromic lenses, or already have a pair, you may be wondering exactly how they become darker or lighter.  The safety glass achieves its photochromic properties by embedding molecules into the lenses, which allows for reversible darkening capabilities.  These molecules change shape when activated by the sun's ultraviolet radiation, causing them to automatically darken.  As the presence of UV light decreases, the lenses will adapt and become clear again. 

Photochromic lenses darken faster than they turn back to clear.  If you have a pair, you’ve probably noticed that darkening generally happens pretty quickly.  However, the chemical change of the molecules back to their inert, clear form happens more slowly.    MCR covers more on VLT and the specific transition times next.

MCR Safety’s Technology and the Importance of VLT

When thinking about any pair of safety glasses, it’s essential to pay attention to the amount of Visible Light Transition (VLT) offered.  The amount of visible light that passes through each lens affects one’s vision and is necessary to consider when purchasing safety glasses.  Different applications and environments call for different lenses, such as an amber lens being required for low-light environments.  The American National Standards Institute (ANSI) provides guidelines for clear- and filter-plano (non-prescription) lenses in its Z87.1-2015 edition. 

Here are the standard VLTs for both clear and gray safety glasses:

• 90% of visible light is transmitted

• 13% of visible light is transmitted

When considering safety glasses with photochromic lenses, it is also essential to know how long it takes to transition between clear and gray and the amount of VLT offered by the lenses as time progresses. 

When transitioning from clear to gray 

As you can see from the numbers below, it does not take long when transitioning from clear to gray:

• At 0 seconds, there is 84.8% VLT

• At 5 seconds, there is 48% VLT

• At 10 seconds, there is 28.5% VLT

• At 15 seconds, there is 20.5% VLT

• At 20 seconds, there is 18.4% VLT

• At 25 seconds, there is 16.8% VLT

• At 30 seconds, there is 15.8 % VLT

VLT, when a lens is at full gray, is more of a medium gray due to it not reaching a complete 13% VLT.  

When transitioning from gray to clear

One thing stands out from the numbers below: it takes longer to reach clear going from a gray lens.  Here are the VLT measurements as they transition:

• At 0 seconds, there is 14.4% VLT

• At 10 seconds, there is 20.3% VLT

• At 30 seconds, there is 33.7% VLT

• At 40 seconds, there is 39.6% VLT

• At 50 seconds, there is 44.7% VLT

• At 60 seconds, there is 48.9% VLT

• At 70 seconds, there is 53.5% VLT

• At 80 seconds, there is 57.1% VLT

• At 90 seconds, there is 60.1% VLT

• At 100 seconds, there is 62.7% VLT

• At 110 seconds, there is 64.9% VLT

• At 120 seconds, there is 66.8% VLT

• At 130 seconds, there is 68.4% VLT

• At 140 seconds, there is 69.8% VLT

• At 150 seconds, there is 71% VLT

• At 160 seconds, there is 72% VLT

• At 170 seconds, there is 72.8% VLT

• At 180 seconds, there is 73.5% VLT

To reach an 84.8% VLT clear lens, a lens will take a total of 5 minutes of transition time. So, anyone needing fast-acting clear glasses should maybe think twice about selecting a photochromic lens.

Temperature Dependency – Two Important Factors

Photochromic lenses are temperature-dependent, which means transitional performance varies.   So, it’s important to remember these two factors when thinking about transition time:

• Photochromic lens performance is affected by local hot temperatures and humidity, meaning the safety glass' lens will not be as dark.

• Photochromic lenses for glasses are more effective in colder temperatures, meaning the safety glass lens will get darker.

As discussed in the following article, photochromic lenses perform perfectly at around 40-45°F, on a clear day and with low humidity. However, once temperatures reach 100°F, transmittance performance is cut essentially in half. This means it will be difficult for transition safety glasses to achieve the same darkness as standard gray safety glasses worn in warm temperatures. On the flip side, these lenses darken faster in colder temperatures. 

Auto-Tinting Advantages and Disadvantages

MCR Safety's goal is to educate, which means they want users to take in all pertinent information before deciding. There are benefits and drawbacks to photochromic lenses, so you will need to decide whether these glasses are the right choice for you. In some cases, photochromic lenses are not ideal, primarily if most work is performed only indoors. For example, the U.S. Army has declared safety glasses with photochromic lenses may not be worn indoors due to the slow rate of change from gray to clear.

Advantages of Photochromic Lenses

• Provides comfortable all-day vision, and as the daylight levels change, these glasses change with time.  

• Allow you to navigate the workplace with one set of safety glasses. Glasses automatically adapt to changes in light conditions.  You can wear them both indoors and outdoors, rather than needing to switch between two pairs of glasses (this also means you’ll be less likely to lose them).

• They offer 100% protection against harmful UVA, UVB and UVC rays.

Disadvantages to Photochromic Lenses

• Even when fully activated, the lenses don’t become as dark as traditional gray safety glasses.

• The amount of time needed to darken is impacted by the temperature and humidity, performing best in colder temperatures.

• Why don’t they work in my vehicle?  Because windshields are designed to block most UV light, the lenses won’t darken because they can’t be activated.

MCR is not trying to discourage you here, just laying out what you need to consider before purchasing.  The last thing they want is an upset user, only because they failed to lay out everything one should know. 

One more "pro" before moving on: these glasses are great for sports activities, such as bicycling. Early morning rides, before the sun is up, require clear glasses. However, once that sun starts to come up, you need to transition into a gray lens. This means they're also great for riding motorcycles. Don't get caught riding at dusk without MCR Safety's new photochromic lens!

Previously Featured on MCR Safety's blog.