The purpose of low emission coating is reduction of heat loss through the windows by reflection of infra-red radiation back into the room. The first low emission coatings were applied onto the soft glass surface during the cooling phase of float production process, and are called "hard" or "on-line" coatings. They were more durable, but also more noticeable and not as effective as modern coatings, which are applied in "off-line" process and called "soft". Compared to "hard" coatings they are almost invisible and are very effective in prevention of heat losses through the large glazed areas of modern buildings. The most popular are Low-E 1.1 and Low-E 1.0 coatings, which provide U value respectively 1.1 and 1.0 W/m²K in double insulating glass units, and 0.5 W/m²K in triple insulating glass units. There are also special Low-E coatings for passive house glazing, with improved solar energy transmission, intended for use in triple insulating glass units only.