It’s that time when my eyes are telling me that I should go get my eyes tested. I’m getting a headache from wearing my glasses for an extended period of time and without them it is as if my eyes have been wrapped with greased cling wrap and I can’t see anything beyond 5m clearly. I hadn’t realised how bad my vision was until recently and I have started researching which lenses to get in my new pair of glasses.
I am also a photographer though these days I have had little time to spend taking photos but my obsession for clear vision knows no bounds. I want lenses that can give me this and after a year of two pairs of glasses, one for everyday use and a pair of sunglasses, I have come to a point where I just want one pair of glasses that does both. Something like this.
I’ve never had glasses with lenses that change colour. Well there is the pair of sunglasses bought in a daze after a late night flight across Australia and in all my time wearing them, I have never noticed them going darker. At least they didn’t cost me a fortune but this is different. I need to be able to see clearly, not just hide my inability to cope with bright sunlight.
Glasses with lenses that darken upon being exposed to UV rays from the sun and then returning to their undarkened state indoors were invented in the 1960s. Early models of these photochromic lenses were slow to darken or lighten and sometimes much to the inconvenience of the wearer, become stuck midway. These days, things have improved with increased speed in the changing of colour and these lenses can be made from glass or plastic.
Glass photochromic lenses have silver halide crystals embedded in the glass. When the crystals are exposed to UVA light, (found in sunlight), the colourless silver halide crystals combine with electrons in the glass to form elemental silver. The silver is visible and because of this, the lenses appear darker. When the silver halide is no longer exposed to UVA light, the reaction is reversed with the silver returns to its ionic state and the colour of the lens lightens.
In plastic lenses a dye is used. When the photochromic dye molecule is exposed to UV light, the molecule rearranges itself to absorb visible light. This leads to what we observe as a darker lens. The dye in the first generation of plastic photochromic lenses come from a family of dyes known as blue pyridobenzoxazines, but today, napthopyrans are used. The improvements in the types of dyes used has led to faster transitioning times of the darkening and lightening of the lenses. However, one disadvantage with with plastic photochromic lenses, the dye molecule can degrade over time.
The chemicals used in plastic photochromic dyes are incompatible with common plastic lenses so competing companies have developed plastics that are compatible with the dyes. Plastic lenses are are made of polycarbonate and thiourea which disagree with photochromic dyes. Thiourea can cause the dyes to break down and the structure of polycarbonate makes it difficult for embedded dye molecules to react rapidly. What happens in the construction of a plastic lens is that the dye is sandwiched between coatings of polyurethane on the surface of the plastic lenses forming a physical barrier between the plastic and the dye.
When it comes to driving, I have found that friends and families have one common complaint irrespective of whether the lenses are glass or plastic, is that sometimes they fail the darken resulting in much squinting at the glare of sunlight. This is because more windscreens of cars have UV absorbers preventing the activation needed for the darkening of the lenses. Research continues into improving this.
Another quirk of photochromic lenses is that in the cold, they can darken. This is because the chemical process that results in the lightening process is dependent on a warm temperature. The higher the temperature, the less dark photochromic lenses will be which means that on a hot summer day, photochromic lenses don’t reach their full darkness. Though if you’re someone that hits the ski slopes, this is ideal because in cold conditions, photochromic lenses will reach complete darkness in sunny conditions.
I’ll keep you updated on what I do in the end when it comes to my glasses. I do need to consider the disadvantages of wearing photochromic lenses as well as finding out how much these things cost. I don’t imagine them to be inexpensive.