Photochromic behavior of tungsten acid and other tungsten chemicals is described in number of different articles.

You can start withhttp://www.jmst.org/fileup/PDF/2006243.pdf; http://www.sciencedirect.com/science/article/pii/S025405841500036X; http://ir.lib.szu.edu.cn:8080/online-view/244041/271/1/9-磷钨酸光致变色玻璃涂层的制备与表征(英文).pdf; etc. 

or You can use some informations written in:

Paul Monk, Roger Mortimer, David Rosseinsky Electrochromism and Electrochromic Devices, Cambridge University Press 2007