Different kinds of sunglass lenses
The colour of sunglass lenses can vary by style, fashion, and purpose. The colours recommended for general use are green, grey or brown. These colours avoid or minimise colour distortion, which could be dangerous in some situations eg driving a vehicle. Grey lenses are considered to be neutral since they do not enhance contrast or distort colours. Whereas, brown and green lenses cause some minimal colour distortion, but have contrast-enhancing properties.
In conditions of medium and lower light conditions red lenses are better because they are good at enhancing contrast. Red lenses cause colour distortion. Similarly, orange and yellow cause colour distortion but they have the best contrast enhancement and depth perception. You will often see golfers and shooters wearing yellow lenses.
Fashion colours such as blue and purple lenses offer no real benefits. Some sunglasses have graduated colouring in the lens with the top darker, fading to clear in the lower section. These lenses protect from the bright sky at the top.
Mirrorshades are another fashion kind of sunglasses. A mirrored coating is applied to the lens that reflects some of the light when it meets the lens before it is transmitted through the lens. These sunglasses are particularly useful in very bright conditions. The mirrored coating is applied after the lens is manufactured and so can be applied to any colour lens.
Some lenses are polarised to reduce glare caused by light reflected from polarising surfaces such as water or from polarised diffuse sky radiation.
Another kind of lens is the photochromic lens. These lenses gradually darken with bright light and lighten with darkness. Photochromic lenses work by changing their chemical structure after absorbing UV light. The light causes the photochromics to absorb colour and then change back to clear when the UV source is removed. If the user wants the lens to change from one colour to a different colour they can purchase lenses which combine a permanent pigment with the photochromic.
Sunglass lenses can be made from several different materials, glass, plastic (most often acrylic), polycarbonate, and CR-39. For the best optical clarity and greatest scratch resistance, glass lenses are first choice, but they are heavy and can be broken. Although plastic lenses are lighter they are more susceptible to scratching. Polycarbonate and CR-39 are the most popular choices because polycarbonate lenses are lighter than plastic, and are shatter-resistant, and CR-39 lenses are light, highly scratch-resistant, and have low transparency for UV and infrared radiation.