Photochromic glass lenses have been around since the early 1960s, introduced by US company Corning. However, it wasn’t until the mid-1980s when the first plastic photochromic lens was made.
What makes plastic photochromic lenses work?
Plastic photochromic lenses work through a reversible chemical reaction, where a colourless molecule is converted to a coloured form by the application of light energy.
However, it’s not just any light energy, but a specific component of sunlight, UV energy that causes the activation. Thermal energy or simply heat drives the photochromic molecules back to the clear state.
The UV region is made up of the invisible, higher energy wavelengths of light energy. UV energy is divided into three categories; UVA (320-400nm), UVB (280-320nm), and UVC (100-280nm). UVA energy is the lowest energy portion of the UV range (measured in Hz) and it is this zone that causes the millions of photochromic molecules in photochromic lenses to react.
In the colourless state, these molecules consist of two smaller, extended conjugated system chromophores, held out of plane of one another. Upon UV activation, an electronic
rearrangement occurs, resulting in the molecule forming one large, extended conjugated system chromophore that absorbs in both the UV and the visible portion of the spectrum.
Factors that affect the performance of photochromic lenses
Many people believe that photochromic molecules operate on a simple principle – that is that they are either ‘on’ or ‘off’. Unfortunately, photochromic molecules are very complex and exhibit a range of performance based on their environment and how they are energised. Photochromic lenses are affected by the level of UV radiation and temperature.
This raises the question of what causes UV radiation to vary. The answer is that latitude, altitude, time of the year, time of the day and orientation of the lens all affect how much UV is available to cause the molecules to change colour.
UV radiation varies considerably based on your location on the globe. The further away from the equator, the lower the UV levels. UV energy also varies with the season, which is why we experience temperature changes. These charts show the difference in UV levels across Australia between summer and winter.
UV energy also varies considerably based on the time of day. We’ve all heard that we should avoid direct exposure to the sun between the hours of 10am and 2pm. The graph below clearly shows why. The blue line is the solar spectrum at noon; the red line is the spectrum at 9am.
If we look at the UV portion of the spectrum, we can see that the UV energy doubles between 9am and 12pm.
Lens orientation plays a role in lens performance too. It’s not just how much UV energy is available, but how much that actually strikes the lens that will determine the darkening. You can see that the total energy reaching the lens is only about one-fifth of the direct energy.
We need to also consider reflected UV energy and altitude. For example, substantially more UV is reflected off snow than off grass. A person wearing a hat while golfing may not experience as much lens darkening as a person snow skiing. The hat blocks the direct solar energy and the grass surface is a poor reflector of UV. The overall energy reaching the lenses will be reduced under these conditions.
Snow is an excellent reflector. A person snow skiing, even while wearing a hat, will have considerably more UV energy reflected up toward their lenses.
As you increase altitude, you reduce the amount of atmospheric filtering of the UV energy. In basic terms, the higher you go, the more UV energy is available. For every 300-metre increase, you increase the UV by 5%.
Photochromic lenses really don’t activate in a car, and there are two reasons for this. Due to the design of today’s modern windscreens, virtually no UV reaches the driver. All manufacturers use a UV absorbing layer in the windshield to prevent the dashboard components from degrading. Because cars are enclosed, there is a significant reduction in the total light energy entering the car as well.
The blue line shows the sun’s intensity outside of the car when facing the sun. The green lines shows the sun’s intensity inside the car, directly at the windshield. Note how the UV component is completely eliminated by the windshield glass.
The red line shows the remaining sunlight that actually reaches your face in a normal driving position. It’s important to note that this represents only 1/50th of the solar energy that was available outside of the car. While some view this as a problem, one can still argue that photochromic lenses are great for everyday use and can recommend a second pair of sunglasses to be used in extreme conditions or for driving.