When the contrast ratio between the facade and the window is too high, a transient region can be inserted between the window and the facade. But, if the transient region does not increase the amount of light on the facade, then the contrast ratio between the facade and the window is still too high. The transition in brightness is then more gradual, but within the field of view the contrast ratio is still too high. Why would this gradual contrast then be more comfortable? It could be because of the rapid movements the eye makes to observe the entire area that is relevant to a person’s interest. Scanning the area of interest without the transition region, the eye encounters a large contrast difference for which the eye needs to adapt itself. And this happens with every scan the eye makes. For an area of interest surrounded by a transition region, there are two possible reasons why the visual comfort is larger. The first is that the eye has enough time to adapt to the lower luminance value while scanning the transition region. The adaptation time should then determine the size of the transition region. The other possibility is that the brain signals the eye to stop the scan at the transition region so that the eye does not need to adapt to the large luminance ratio of the surrounding area. This is the case when the adaptation is very fast. Of course it could also be a combination of both possibilities. See for more information on the human visual system the NEN 3087 norm [4].

A transition region is nothing new. The use of stained glass transition band was common in the 19th century. Transition regions through the use of net curtains in dwellings were, and in certain regions still are, also very popular. The size of the transition region, however, is expected to be somewhere between 5 to 10 cm based on personal experience, but further research is necessary to give a more detailed estimate.