Thermotrope window (image 1 of 2). Researcher Helen Rose Wilson holding an Interpane thermotrope window. A thermotrope window will darken when the temperature reaches a certain point. This will reflect any sunlight which would normally enter a building. This is useful for controlling room temperatures. The glazing is based on a heat reactive layer. Below a pre-set temperature the layer will remain clear as seen in the image. Above this temperature the layer will become cloudy, thus reflecting the sunlight. This glazing was developed at the Institute for Solar Energy (ISE), Freiburg, Germany. See photo T830/103 showing the window when it is opaque.