Thermochromic windows are designed to block solar gain. A drawback is that they reduce visible light transmission. Electrochromic glazing changes optical properties when an electric current goes through the unit. A thin metallic film is deposited on the glass similar to low emissivity coatings. Another technique involves sandwiching a liquid quartz film between two layers of glazing.
Photochromic materials change their properties in response to light.
Photo gray sunglasses are the best known example. When photochromic materials change their transmittance, the absorptivity is increased, thus causing the glass to absorb more heat. On sunny cold days, they absorb solar heat and room source heat and then radiate some heat back to the surroundings. On sunny hot days, they do not reject as much solar heat as reflective windows. Evacuated Glazing Evacuated, sealed insulated glazing is designed to achieve higher levels of thermal performance by using a vacuum to inhibit any kind of conductive or convective heat losses.