Sustainability Issue #2 April 2010

Today's computerised office work poses much greater challenges to the creation of a good working environment without glare than the paper based work of earlier days. This is a consequence of both a changed field of vision and the fact that the screen is a luminous surface. The introduction of the computer on the desk has made us raise our eyes from the desk and to look more or less straight ahead. This means that  now we have almost always a window in the surrounding field of vision, around the screen. At the same time, we have also experienced quite a long lived architectural trend with large glazed facades, which means that the screen almost inevitably has a window behind it regardless of how we furnish the room.

If the computer screen is at an angle of about 45 degrees to the window, glaze is worse than if the screen is placed at ninety degrees to the window.

Light disturbs visibility

The fact that the screen shines at a relatively constant luminous intensity is at least as large a change as the increased field of vision. The electric light and especially the daylight or sunlight that falls on the screen must not be too bright because we can then no longer distinguish the contrasts of the screen and its visibility vanishes. This is fundamentally different from paper that reflected all light that fell on it. With paper, it was to a much higher degree the contrast between the letters on the paper and the colour of the paper that determined the contrast and our ability to see and interpret the text. Lighting that was too bright  did not eliminate visibility in the same way.

The consequence has been that the more or less completely glazed facades almost inevitably cause glare during part of the year unless the facade is fitted with some kind of antiglare protection. The sky is so light during large parts of the year that it blinds us, and this happens in all directions! Modern solar control glass is effective in reducing solar thermal load, but is by no means sufficient to eliminate glare. Antiglare protection is recommended even for facades facing north since the sky is often too bright to look at. A neighbouring building to the north can also cause glare when the sun is reflected, especially if it is light in colour.

The increasingly common, more or less completely glazed facades almost inevitably cause glare during part of the year unless the facade is fitted with some kind of antiglare protection. The sky is so light during large parts of the year that it blinds us, and this happens in all directions! Antiglare protection is recommended even for facades facing north since the sky is often too bright to look at. A neighbouring building to the north can also cause glare when the sun is reflected, especially if it is light in colour.

A venetian blind with its adjustable slats has superior capability to regulate the quantity of light completely steplessly. The user can him/herself set the blind and reduce the brightness of the window or the sky as preferred.

Curtains and venetian blinds

In the market there is a tremendous number of fabrics of different densities and colours which reduce light to different degrees. Fabrics are used in many forms of chiefly interior curtains such as roller blinds which can be removed or pulled down as required. Instead of a fabric a classic venetian blind can be chosen. A venetian blind with its adjustable slats has a superior capability to regulate the quantity of light completely steplessly. The user can him/herself set the blind and reduce the brightness of the window or the sky as preferred. If antiglare protection is placed on the outside, it works as a very effective solar screen in order to prevent excess temperatures. The antiglare protection or solar blind should be removable. In order to maximise daylight in the room, we can then take it away on the many dark and cloudy days during the winter months.

Daylight in the room

Daylight in office environments has been studied at Lund University Faculty of Engineering in a number of projects. One project concerned glazed office buildings in which different glazed areas were compared with regard to both energy use and access to daylight. In office rooms lit from the side, the daylight factor rapidly decreases as one moves away from the window into the room. The daylight factor provides a measure of the illuminance due to daylight as a percentage of the illuminance outdoors. The daylight factor can be increased by increasing the glazed area and by placing the windows higher.

Glare

In a typical office building from the 90s, windows were provided as holes in the wall and the glazed area was about thirty per cent of the facade area. In modern architecture in the 2000s, we have seen both fully glazed facades and facades in which sixty per cent or more is glazing. The study shows that the daylight factor is increased by a relativelty large amount when the window percentage is raised from thirty to sixty per cent, while an increase from sixty to one hundred  per cent adds very little. The reason is that light that is admitted below desk level is largely lost on first reflection from the floor. If attention is to be paid to the brightness of the window and visual comfort, there must be effective antiglare protection that can be used as needed in all three trypes of facades. For the two larger areas, access to daylight in the room is at the same time  reduced so much that it is not much greater than in an office with thirty per cent of the facade glazed, with the venetian blind open. This shows the difficulty in increasing access to daylight without creating more glare.

Visualisation of a south facing office room with thirty, sixty and one hundred per cent glazed facade without solar protection. The daylight factor increases by a relatively large amount when the window percentage is raised from thirty to sixty per cent, while an increase from sixty to one hundred  per cent adds very little. The reason is that light that is admitted below desk level is largely lost on first reflection from the floor.

Coupled windows

In another project, an ordinary interior venetian blind was fitted with a very precise motor and control for the slat angle with regard to solar height, so that direct sunlight is always prevented. Since the solar height in summer is high, the blind then has quite open slat angles which allows a sight of the surroundings, while in winter they must be almost completely closed to prevent direct sunlight. Our conclusion, however, is that the angles in the summer are not large enough, i.e. the venetian blind is too open, to prevent  also glare from the sky or the blind itself. This applies most when solar intensity is high, which is the situation that  normally also provides the highest levels of illumination. If the blind control is adjusted slightly so that it closes a little more when sunlight is strong in the summer, the proposed controlled venetian blind has a very good chance of working well both as solar protection and antiglare protection if it is placed between the panes in a coupled window. It has therefore great potential in renovating old offices which often have coupled windows.

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