Gasotrope window (image 2 of 2). View of a man holding a gasotrope window. A gasotrope window will darken when a current of carbon dioxide gas is passed through the panes. This will reflect the sunlight which would normally enter a building. In the image the window is in its darkened state after the carbon dioxide has been passed through. The carbon dioxide current is manually controlled and can be applied to suit the degree of darkening required by the user. This glazing was developed at the Institute for Solar Energy (ISE), Freiburg, Germany. See photo T830/104 showing the window when it is transparent.