Wednesday, 11 July 2012

Photochromic glass

Today I write about photochromic glasses . In simple terms, the photochromism is a mechanism where the device darkens with the sun, and then returns to the initial state if the exposure vanishes. At the transparent state is characterized by a transmission in the visible of about 80-90%, which decreases progressively during the change up to 10-15%. [1]
In more technical terms, the photochromic materials have the ability to reversibly change their absorption characteristics in response to the wavelengths of radiation (especially ultraviolet light). This phenomenon is obtained using iron oxide with fluorides (or chlorides) of silver or copper.

An animation of the process is here.

The phenomenon was observed as early as 1960 by using a Corning silica glass doped with silver, but the difficulty in producing large areas and his "uncontrollability" has allowed the use of this phenomenon only to visual lenses, in devices for cars .
An interesting case is the anti-glare rearview mirror that some manufacturers offers, since it's an example of a user controllable photochromic device (UCPC defined = user controlled photo-Chromic). [2] It's able to change the color even if the radiation is low, or almost completely absent (as we have seen, the glass isn't transparent to UV) with a very good response times. In reality, a UCPC is very similar to a glass of electrochromic type, which will be analyzed in another post.

Here is an example graph of transmittance (I don't have real experimental data).

Red line = clear glass
Blue line = tinted glass 

The complete discussion with the data is in [2].


[2] Gimtong Teowee, Todd Gudgel, Kevin McCarthy, Anoop Agrawal, Pierre Allemand, John Cronin, "User controllable photochromic (UCPC) devices", Electrochimica Acta 44, (1999), 3017-3026.

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