This story was posted on cycleworld.com
Arthur C. Clark said any advanced technology is inseparable from magic. This certainly applies to face shields, goggles and goggles that darken automatically. This witchcraft has been around for decades, but it has been perfected and is more widely available than ever before. This change in shading as the light increases is called photochromism. It depends on a reversible chemical reaction that occurs when a material embedded in glass or plastic is exposed to certain wavelengths of light.
Photochromic glass lenses were first introduced in glasses using a process invented by Corning Glass Works in the 1960s. Corning infused glass with a silver halide (usually silver chloride), then, after casting, accurately process the lens blanks. Silver chloride molecules were transparent after heat treatment, similar to the lens, but exposure to ultraviolet (UV) radiation caused the decomposition of silver chloride in its various components. In full sun, the lens is filled with microscopic particles of elemental silver, which darken. Remove the UV light and the silver will react again with the adjacent chloride still trapped in the glass and become transparent again. The variation in percentages of silver chloride and glass created lenses with different darkening characteristics.
All chemical reactions follow certain laws, and one of them is that the reaction rate increases as the temperature increases and decreases as temperatures decrease. In very hot weather, the reaction of silver and chloride would occur so rapidly that the lens did not darken completely – the chemical reaction proceeded in advance of the light-induced decomposition. The opposite would happen on extremely cold days. In addition, since the photochromic additive was uniformly dispersed throughout the lens, the blackness of the lens would vary with the thickness of the lens, which could be a problem with the high correction requirements. The reaction has also not been perfectly reversible, so that over a large number of darkening cycles, the lenses tend not to regain their original transparency.
A problem with these lenses for drivers is that car windshields have been designed to filter almost all UV components from the sun. In a modern vehicle, classic silver chloride photochromic eyewear stays crisp, no matter how bright it is. Of course, this is not a problem for a motorcyclist.
Engineers have developed new high performance photochromic additives that can be used more easily on motorcycle helmet face shields or goggles. These are organic photochromic molecules (indolino-spironaphthoxacins, for a family) that are applied to a plastic lens through one of two technologies. One is a heat-induced infusion process, where the photochromic molecule is diffused into the already fully formed plastic lens or facial mask (0.006 inch). Workers can then apply scratch-resistant coatings.
Transitions, a company specializing in photochromic technology, uses a different process, Trans-Bonding, for lens materials that are not compatible with the perfusion process. Trans-Bonding creates a thin layer of photochromic material directly on the surface of the lens. In both cases, the wide range of photochromic organic additives has allowed modern lenses to react faster, be less temperature sensitive and be available in a wider range of colors. They even react to blue light as well as UV rays, allowing them to become effective again in an automobile.
Photochromic lenses have at least one competitor: electrochromics. If you had the opportunity to drive the latest Boeing, the 787, you will notice that its side windows are not shaded. Instead, they have a switch. Switch the switch and the window goes from transparent to a deep blue dominant in the first generation and almost black in the last iteration. This technology relies on a gel captured between two sheets of glass that changes the transparency in response to a low electric current.
Electrochromic face shields require a power source but offer much faster mode switching than photochromic face shields. The family of motorcycle visor inserts E-Tint Akira claims to change transparency level in one-tenth of a second. This can be expensive, as in the AGV facial shield that costs almost $ 100 more than a photochromic example of Bell. Nobody has said that magic is cheap.