Controlling excessive glare
Glare is created when areas that are too bright are located within the field of view, or when the contrast ratio is high. Nevertheless, glare caused by daylight differs from glare caused by electric light sources in terms of the size, complex luminance distribution and acceptance of the users (e.g. people tend to be more tolerant of glare in a daylit environment)⁴.
While high levels of daylight in a classroom provided via large windows and skylights are optimum, they still need to be controlled to avoid the excessive glare that can come from direct sunlight. This issue becomes increasingly acute with today’s widespread use of interactive whiteboards and projectors in classrooms.
One important aspect to consider when controlling glare and contrast is the orientation of the windows. Larger expanses of glazing should ideally be facing north, in order to allow diffused daylight to penetrate as much as possible throughout the day/year.
Another effective solution for glare control is the installation of easily operable opaque blinds to control daylight levels. An alternative to blinds is permanent external shading, but this may still require the use of additional shading devices, depending on individual circumstances and requirements.
Other solar protection devices such as curtains, roller blinds etc., made in textile, film or perforated opaque materials can be used. These will all reduce the occurrence of glare to differing degrees, depending on: optical properties of the material; orientation of the window; geographical location and annual sunshine hours; glazing area and transmittance of the pane and the distance of the user from the light source.
The material properties and levels of glare protection for these type of solar protection devices are defined in European Standard EN 14501 Blinds and shutters, and the European Standard EN 17037 Daylight in Buildings.
Equally important is the issue of contrast. When considering both glare and contrast, the difference between absolute brightness and relative brightness is critical. Consider how high-beam headlights can be blinding at night, while hardly noticeable during the day. Similarly, a strong source of light will cause much less glare and contrast in a generally brightly-lit room compared to, say, a single large window with direct sunlight in an otherwise dimly-lit room.
A rule-of-thumb guideline of the “luminance ratio” between visual task and near surroundings (e.g. adjacent or periphery wall surface) is recommended not to exceed 1:10 within the field of view. Today, and with modern open plan offices, and especially with windows in the visual field, the ratio can be 20:1 or higher between a window and adjacent surfaces. This luminance ratio is an expression of the ratio between the luminance within the central vision and the peripheral vision of the surroundings³.
Windows vs. skylights
Good daylight distribution across a room is often best achieved by using several different sources of daylight, such as a combination of skylights and windows, and this can also ensure reduced levels of glare and contrast. However, let’s not forget the importance of a view: "When we are sitting next to a window, we would rather tolerate a high amount of daylight and enjoy the view outside than draw the blinds down and use artificial lighting.⁴
For spaces where even large areas of window glazing will not allow enough daylight to penetrate, or where their installation is simply not possible – such as very large classrooms, lecture theatres or indoor common areas in the centre of a building – skylights are a great option to consider. Operable skylights strategically located in the overall building design allow plenty of daylight to penetrate in the dark winter months, while also providing fresh air throughout the year, improving indoor air quality and helping regulate temperature.