The photochromatic formula

As early as the beginning of the 19th century, scientists were aware of the light-sensitive characteristics of certain chemicals such as silver halides. This led to the beginning of the photographic process, of which the basic chemistry has changed very little until the recent introduction of digitally formed images. Minute silver-halide crystals, usually silver chloride or bromide, are suspended in an emulsion. When exposed to light ie long-wave ultra violet and the blue end of the visible spectrum, the crystals separate into their ions: Cl- and Ag+. The chloride ion, through further exposure, loses its negatively charged electron to the silver ion to produce dark particles of metallic silver. This basic photographic concept was used in the development of photochromic glass. However, with spectacle lenses it was essential that the process could be reversed in order for the spectacles to be worn indoors or in low lighting conditions. To achieve this, both copper and silver halides were added to the glass. The chlorine atoms are converted back to the chloride ion by the copper, which also undergoes a chemical change. When the light source is removed the copper changes the metallic silver back to its positively charged ion. The silver and chloride ions now reform back to silver chloride. The copper chloride is also reformed. With the obstacle of reversal removed, the mid 1960s saw the introduction by the Corning Glass Works of the first photochromic lenses. The original lenses darkened to a brown colour (photobrown) but these were soon followed by a photogrey option. Other companies such as Chance-Pilkington (Reactolite) and Carl Zeiss (Umbramatic) later produced their own range of glass photochromic lenses, these lenses differing only slightly in their colour, reaction rates and sensitivity to certain wavebands of the visible spectrum. Up until the mid 1980s photochromic lenses were only available in glass, which meant heavier, uncomfortable lenses especially for patients with higher prescriptions. With the decline of glass for other lens types in favour of newer plastics materials, the industry looked to follow suit with photochromic lenses. The main problem facing development scientists was that silver halide chemistry is impossible in a plastic lens. The search for a plastics material, with similar reversible light-sensitive behaviour to glass, took developers down the organic chemistry path. Compounds, known as spiropyrans, were created and were found to react in a way to give the same appearance as the glass counterpart. Improvements to the performance of plastic photochromic lenses continue to this day although the same basic photochemical technology remains. This involves the action of ultraviolet (UV) light breaking the bond between the spiro carbon and the oxygen, creating an open form compound which strongly absorbs in the visible region, reducing light transmission through the lens. Reversal to the original closed form takes place when the UV source is removed. Figure 1 shows the general reaction of UV radiation on the photochromic material. These plastic lenses can be created in three different ways (Figure 2). The more common method, used by Transitions, is a process known as 'imbibing' or absorption of the photochromic material into the outer surface of the lens to a depth of about 0.15mm. Lenses may also be dipped into the material to give a required depth onto the surface of the lens. A third method, used by Rodenstock in its ColorMatic lenses, is known as 'Casting' or 'In-mass' where the light-sensitive material is cast into the lens material at the time of manufacture.