MAGIC GLASS

The sci­ence be­hind auto-dark­en­ing lenses

Cycle World - - Components - By STEVE AN­DER­SON

Arthur C. Clark fa­mously said that any suf­fi­ciently ad­vanced tech­nol­ogy is in­dis­tin­guish­able from magic. That cer­tainly ap­plies to face shields, gog­gles, and glasses that au­to­mat­i­cally darken. That sor­cery has ex­isted for decades, but it has been refined and is more widely avail­able now than ever. This change of lens shade as light in­creases is called pho­tochromism, and it de­pends on a re­versible chem­i­cal re­ac­tion that takes place when a ma­te­rial em­bed­ded in glass or plas­tic is exposed to cer­tain wave­lengths of light.

Glass pho­tochromic lenses were first of­fered in eye­glasses in a process in­vented by Corn­ing Glass Works in the 1960s. Corn­ing infused glass with a sil­ver halide (gen­er­ally sil­ver chlo­ride), and then, af­ter cast­ing, pre­cisely heat-treated the lens blanks. Sil­ver chlo­ride mol­e­cules were trans­par­ent af­ter the heat treat­ment, as was the lens, but ex­po­sure to ul­tra­vi­o­let (UV) light caused the sil­ver chlo­ride to de­com­pose into its con­stituent el­e­ments. In bright sun­light, the lens filled with mi­cro­scopic par­ti­cles of el­e­men­tal sil­ver, dark­en­ing in the process. Re­move the UV light, and the sil­ver would again re­act with the ad­ja­cent chlo­ride still trapped in the glass, and be­come trans­par­ent once again. Vary­ing the per­cent­ages of sil­ver chlo­ride and glass cre­ated lenses of dif­fer­ing dark­en­ing char­ac­ter­is­tics.

All chem­i­cal re­ac­tions fol­low cer­tain laws, and one of those is that the speed of re­ac­tion in­creases as tem­per­a­ture rises, and falls as tem­per­a­tures de­creases. On very hot days, the re-re­ac­tion of sil­ver and chlo­ride would oc­cur so quickly that the lens wouldn’t fully darken—the chem­i­cal re-re­ac­tion raced ahead of the light-in­duced de­com­po­si­tion. The in­verse would oc­cur on ex­tremely cold days. Also, be­cause the pho­tochromic ad­di­tive was dis­persed evenly through­out the lens, lens dark­ness would vary with lens thick­ness, which could be an is­sue with large-cor­rec­tion pre­scrip­tions. The re­ac­tion was not quite per­fectly re­versible ei­ther, so over a large num­ber of dark­en­ing cy­cles the lenses tended to not come back to their orig­i­nal trans­parency.

One is­sue with these lenses for driv­ers is that car wind­shields have been engi­neered to fil­ter out al­most all of sun­light’s UV com­po­nents. In a modern ve­hi­cle, the clas­sic sil­ver chlo­ride pho­tochromic glasses just stay clear, no mat­ter how bright it gets. Of course, that’s not an is­sue for a mo­tor­cy­clist.

En­gi­neers have de­vel­oped new pho­tochromic ad­di­tives with su­pe­rior per­for­mance that can be more read­ily used on mo­tor­cy­cle-hel­met faceshields or gog­gle lenses. These

are or­ganic pho­tochromic mol­e­cules (in­dolino-spiron­aph­thoxacins, for one fam­ily) that are ap­plied to a plas­tic lens through one of two tech­nolo­gies. One is a heat-in­duced in­fu­sion process, where the outer 0.006 of an inch of an al­ready fully formed plas­tic lens or faceshield has the pho­tochromic mol­e­cule dif­fused into it. Work­ers can then ap­ply nor­mal scratch-re­sis­tant coat­ings.

Tran­si­tions, a com­pany spe­cial­iz­ing in pho­tochromic tech­nol­ogy, has a dif­fer­ent process it de­scribes as Trans-bond­ing, for those lens ma­te­ri­als that are not com­pat­i­ble with the in­fu­sion process. Trans-bond­ing cre­ates a thin layer of pho­tochromic ma­te­rial di­rectly on the sur­face of the lens. In ei­ther case, the wide ar­ray of or­ganic pho­tochromic ad­di­tives has al­lowed modern lenses to re­spond more quickly, to be less af­fected by tem­per­a­ture, and to be of­fered in a wider ar­ray of col­ors. They even re­spond to blue light as well as UV, so they can once again be ef­fec­tive in an au­to­mo­bile.

There is at least one com­peti­tor to pho­tochromic lenses: elec­trochromic. If you’ve had the op­por­tu­nity to ride in the lat­est Boe­ing, the 787, you’ll no­tice its side win­dows don’t have shades. In­stead, they have a switch. Flip the switch, and the win­dow dims from trans­par­ent to a deep-bluish cast in the first gen­er­a­tion, and to al­most black in the most re­cent it­er­a­tion. This tech­nol­ogy re­lies on a gel cap­tured be­tween two sheets of glass that changes trans­parency in re­sponse to a small elec­tric cur­rent.

Elec­trochromic faceshields re­quire a power source but of­fer much faster mode switch­ing than pho­tochromic faceshields. The E-tint Akira fam­ily of mo­tor­cy­cle-vi­sor in­serts claims to switch trans­parency lev­els in one-tenth of a sec­ond. It can be ex­pen­sive, as in the AGV faceshield that re­tails for al­most $100 more than a Bell pho­tochromic ex­am­ple. No one said magic comes cheap.

Pho­tog­ra­phy by DREW RUIZ

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