Here at Rapid Eyewear, I'm always happy to answer technical questions from customers, either on the phone or by email. One of the most commonly asked questions, not surprisingly: "What are the various lenses for?" We do give a brief explanation of this in our internet product listings, but to use a metaphor, I will try and put a bit more meat on the bones for you here.
A helpful aspect of sunglasses standards is the fact most use the same internationally recognised lens 'categories'. There are in fact, five of them, and all appear in ISO, US and Australian standards. They are as follows...
These lenses, usually yellow in colour, offer the lowest level of filtering. They actually reduce the amount of light allowed to pass through by less than 20%, in which case you might be wondering why bother? In actual fact, category 0 lenses have a number of benefits:
1). Category 0 lenses still offer 100% UVA / UVB protection, protecting your eyes even in relatively low poor light conditions
2). By adding an anti-glare coating to the rear surfaces, cat 0's can be used to reduce the glare of artificial lights such as car headlamps and street lights*
3). By specifically filtering blue light, in even in fairly dark conditions amber cat 0's can improve contrast and your ability to define objects at distance
4). Being made from impact resistant polycarbonate, category 0 lenses can be used as safety lenses in sports even when there isn't much sunshine around
*For more information on the benefits of category 0 lenses for use when night driving, please see my previous blog article specifically on this subject
It should be noted that yellow category 0 lenses have the artificial effect of making everything look brighter when you put then in front of your eyes - it is literally like turning a light on. This is slightly misleading as sunglasses lenses cannot physically improve the amount of light you can see - any sort of filtering can only reduce light, but when you wear cat 0's it doesn't look like that! This effect is caused by the fact blue, or dark light, is filtered, leaving the brighter part of the spectrum behind. Category 0 lenses are NOT 'light enhancing' and any claims made by manufacturers or sellers to the contrary should be treated with great caution.
Category 0 lenses should be used when there is very little light available, and in darkness when you want to reduce the glare of artificial lights.
Typically allowing around 60% of light to be transmitted through it, a category 1 filtered lens is designed for use in conditions where the light is quite dull. They can be utilised when you have, for example, low sun in the evenings or mornings, or a thin cloud layer. here at Rapid Eyewear, we don't manufacture cat 1's because we feel that cat 0's and cat 2's (see below) cover the bases pretty well.
An extremely useful lens, and my personal favourite, is the cat 2. Probably the most versatile lens on the market, when it comes to sports and driving, for me, a pair of sunglasses with cat 2's are a 'must have' accessory. Light transmittance for this one is typically down to 30%, but even at that level, you will find you can see very clearly in light cloud or hazy conditions; at the same time, even in relatively bright sun, a cat 2 will seriously reduce the harshness of bright light. The great thing about that is, if you're driving, cycling or taking part in some other sporting activity where it isn't practical to keep swapping your lenses, with cat 2's fitted you have all the bases covered and you won't get caught out not being able to see.
personally, I always keep two pairs of Altius aviator sunglasses for driving in my car: one is the red cat 2 version, the other a pair of cat 3's. On a bright sunny day the cat 3's are the way to go of course, but if I can sense the sun will keep coming and going during a journey, I wear the cat 2's. They are also ideal if you start an evening journey in relatively bright conditions, but knowing it will become darker, and possibly with low sun, as you progress. I also used the cat 2's a lot during my time playing cricket; I would find myself fielding for two hours, during which time the light could change several times - again, the cat 2's could deal with the lot.
As with cat 0's and cat 1's, cat 2's filter blue light, improving depth perception and contrast which is very important for playing field sports in particular. All Rapid Eyewear sunglasses with interchangeable lenses include this extremely useful tint.
Whether you buy sunglasses in Poundland or Sunglasses Hut, you'll almost certainly get a pair with cat 3's fitted and that's because it is the standard tint for use in sunshine. The light transmittance with this one is down to approximately 15%, but that's enough on bright days and you can use these for driving when it is nice and sunny. The lenses can be a variety of colours, with grey being the most common but they can also be blue and red.
Now we're at the dark end of the lens scale. Cat 4's only allow a paltry 5% of light to reach your eye; if you think that's not enough, you'd be right for most applications but they are useful for specific applications such as climbing at altitude, or walking / hanging out in parts of the world where the sun can be particularly severe such as Australia, Africa and the southern states of America.
It is important to note that you must never, ever use Cat 4's for driving or cycling; they are simply too dark to be safe on the roads, even in the aforementioned parts of the world where the sun can be especially intense. You may also find them too dark for 'field' sports where you need to track a fast moving ball.
That concludes our look at the various tinted lenses available for sunglasses. At Rapid Eyewear we offer a wide variety of sunglasses with interchangeable lenses for different sports and leisure applications, as shown here.
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