Photochromic lens are covered in a photochromic optical dye. In a normal state- a colourless state, the particles within the coating are separated into two halves, and sit perpendicular- at opposite angles to each other. When these particles react to UV light, the two halves in the colourless state join together to become one larger particle and absorbs the UV light to present colour. Tints of grey, green and brown are available, Grey is the most popular tint worldwide, while brown is preferred by many Europeans. In a simple sense, UV light triggers particles within the coating to gain colour- tint to the lens. In traditional photochromic lenses, the reversed colour change reaction is to an extent driven by temperature - Thermochromic. Heat drives the largest particles with colour to be broken back down into two colourless halves. As UV light often accompanies heat, the performance of photochromic lens can be hindered in very hot conditions.
The brand Transitions are leading the way with technological advances. The new Transition Xtractive photochromic glasses also react to natural light, through the blending of photochromic optical dyes. By reacting to natural light, these new type of photochromic glasses ease driving by reacting to natural light, as discussed, the windscreen of cars block the majority of UV rays, therefore by reacting to natural light as well, these glasses will tint. Transitions Xtractive, on average achieve 53% level of tint while in the car, which is an increase based upon traditional photochromic lens. Furthermore, Transitions Xtra have a low level of tint at around 20% in their normal state to ease the harshness of artificial light from inside. Transitions Xtractive lenses also have minimised the impact of heat on photochromic lens, by improving their effectiveness by 17% on temperatures over 32 Degrees Celsius.
Photochromic optical dye is a key and well known example of how colour changing products can aid everyday life. Photochromic lenses are highly popular which is presented due to the fact a new pair of transition glasses is sold in the world every second- 125,798,400 pairs a year.