how do transitions work?

[foureyes]

probably a simple question for most of you- i know they are activated by UV light, but i'd like to know what chemical reactions actually occur.

thanks!

[rinselberg]

Thank you for an excellent question.

Transitions is a brand name. "Photochromic" is the generic term for lenses that darken when exposed to an increasing amount of light energy and fade when the ambient light energy is reduced.

The first photochromic lenses were glass lenses and used relatively simple silver/copper compounds (halides) to achieve their photochromic effect - not altogether different from the chemistry of the first photographic films for cameras.

In order to endow plastic lenses with similar photochromic properties, chemical engineers turned to more complex organic (carbon-based) photodyes, starting with the Photolite lens in 1982.



The photochromic effect is described by the organic chemical equation at the bottom of the excerpt (above).

Ultraviolet energy (as from bright sun exposure) drives the organic reaction to the right, breaking a carbon-oxygen bond and changing the shape of the molecule to a more open form that absorbs a band of light frequencies within the visible spectrum. This is the darkening effect.

Ambient heat energy, which increases as the temperature increases, drives the organic reaction to the left, favoring the closed, transparent form of the photodye. This is the fading effect. This is the reason that plastic photochromic lenses (including Transitions) darken more completely in bright sunlight when the weather is cold, and do not darken as completely when the weather is warm or hot. As the temperature decreases, there is less ambient heat energy to drive the equation towards the left. In other words, less heat energy to counteract the darkening effect of UV radiation. Therefore, as the temperature decreases, the chemical equilibrium is shifted towards the right, converting more of the transparent molecules into the light absorbing molecules and making the darkening effect more complete.

Photochromic lens developers have been hard at work "tweaking" the chemistry ever since. Even trying to say the names of the chemicals involved is a mouthful. For example, here's the title of a recent research report with the objective of improving the organic photochromic technology:

Synthesis and Photochromic Behaviour of Naphthopyrans, Pyranoquinolines, Pyranoquinazolines and Pyranoquinoxalines

Perhaps more about the exact chemistry of the Transitions photodyes could be discovered by a search of the online patents database. But the basic principles would be more or less the same as is illustrated by the simpler example of the Photolite lens of 1982.

Lenses that depend only on UV light energy to darken are not fully functional inside a car, where the glass windshield functions as a UV filter and blocks the UV light from passing inside.

Younger Optics is addressing this need with a new line of Drivewear lenses that darken in response to visible, as well as UV light. These lenses are also polarized. They are sun/outdoors specialty lenses and do not fade all the way to an almost clear state like Transitions lenses and the like.

The development of Drivewear lenses has been achieved by combining new additions to the Younger Optics proprietary NuPolar polarizing technologies with new Transitions-based photochromic technologies.

Sources:
Photochromic compounds: Chemistry and application in ophthalmic lenses
Synthesis and Photochromic Behaviour of Naphthopyrans, Pyranoquinolines, Pyranoquinazolines and Pyranoquinoxalines


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[foureyes]

thanks

thanks for such a thorough response!

one more thing-
how does the composition of the drivewear lens in order to enable it to darken in visible light as well?

[Jim Schafer]

Thanks Rinsie, don't you sleep!

and here is a bit simpler explanation of what happens when you walk outdoors wearing a pair of photochromic eyewear.

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 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.


Forrest Blackburn, Ph.D.
PPG Industries Inc.

[Jim Schafer]

one more thing-
how does the composition of the drivewear lens in order to enable it to darken in visible light as well?

Some Photochromic dye compounds activate in the visible range of light too. This is how a product like DriveWear activate in a car.

Compounds can be clear to a color, one color to another color and both UV, VL sensitive and don't forget heat activated (thermalchromic).

Jim

[foureyes]

thanks, jim

i doubt they will ask me stuff like this on my ABO exam (taking it this weekend) but i'd rather know as much as i can. plus it is really fascinating.
thanks for taking the time to answer my questions, i really appreciate it.

[Curious_007]

and here is a bit simpler explanation of what happens when you walk outdoors wearing a pair of photochromic eyewear.

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.
<clip>
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.
<clip>


Forrest Blackburn, Ph.D.
PPG Industries Inc.

Thanks for that explanation. Very interesting! What wavelength and intensity UV light is required for a reaction to occur? Near-UV (~385nm) LEDs are available for pennies. They're being sold for use in various low-end consumer products such as penlights for viewing the anti-fraud controls in currency, credit cards, and some state-issued ID cards. Would the low-level output from one of these devices be enough to trigger the photochemical change to darken the lens? I'm a geek - I'll admit it - but I can envision mounting a couple of these UV LEDs in my pickup's sunvisor to "turn on" my Transitions lenses when I needed a little darker tint while driving without having to put on my clip-on sunglasses...

[Jim Schafer]

Hi 007,
I am pretty sure the intensity needed to darken the lens suitable for driving would be fairly strong, but the 385 nm you speak of is a tad to high.

The UV wave lengths that trigger the main reaction in Transitions lenses are below the 385 nm range these lights radiate, there may be some low level activation but not much.

You may want to check out the threads on DriveWear. It is a newly released specialty lens that has generated quite a bit of excitement.

Thanks for posting and best regards,
Jim