transitions and cool temp.

[OPTIDONN]

Why do lower temp. make transitions react quicker?

[Jim Schafer]

Hi Optidonn,
Lower temps slow the chemical reaction, leaving more molecules open (in the darkend or colored positions) than closed (in the clear position).

In other words, in colder temps, every photochromic lens darkens oudoors in response to sunlight. To date, the challenge for photochromic lens manufacturers has been to create a lens that can darken quickly and sustain an optimum level of darkness at high temperatures, when darkness is most important for comfort.

Here is a nice write up from my PPG photochromic synthesis research buddy Forrest Blackburn P.hD. on photochromics and temperature, a little technical but an easy read; he did a pretty good balance on this not so simple subject.

Photochromism 101

When a photochromic lens is exposed to ultraviolet light (UV) wavelengths present in sunlight, the resulting photochromic reaction causes some of the photochromic molecules to rearrange into an activated form that absorbs visible light. As trillions of these reactions take place, the lens darkens. When UV light is removed, a chemical reaction driven by ambient heat reconverts the activated photochromic molecules to their original, clear form and the lens fades.



The amount of darkening is a function of the competition between the activating and fading chemical reactions, the equilibrium condition established between them, and the specific photochromic molecules and lens substrate. The more activated photochromic molecules that are present, the darker the lens becomes. The degree of darkness depends on the level of available UV radiation driving the activation reaction and the temperature of the lens, which drives the fade reaction.



The level of UV radiation is what primarily controls the activating photochemical reaction in a photochromic lens. The more UV present, the more photochromic molecules are activated and the darker the lens becomes.



Generally, the level of UV radiation is highest at midday and during the summer. Over the course of a few weeks of sunny days in any given season, the level of UV light from midday sunlight does not change significantly. The temperature, however, may fluctuate by as much as 30°F/ 17°C over the same period.



Temperature is what primarily controls the chemical reaction that causes a darkened photochromic lens to fade to its clear state. Lower temperatures slow the fading reaction, causing the number of activated photochromic molecules too build up over time and establish a new equilibrium that favors activated molecules. With more activated molecules present, the lens is darker. Higher temperatures create a faster fading reaction, so fewer photochromic molecules are activated and the lens darkens less.



On hot, sunny days all photochromic lenses will be less dark than on sunny, cool fall days.
regards,
Jim