The fast development of new technologies for enhancement of daylight penetration through the building envelope has led to many innovative solutions of facade designs. One major mistake in many of these developments is that manufactures seek to increase the total amount of daylight through a given opening, which is almost against the physical laws. For instance, the transmission of diffuse daylight through conventional, fully opened venetian blinds is as low as 50 %, and most of the “innovative” systems have a lower transmission. With the new glazing technologies of highly insulating windowpanes, there is no longer any thermal reasons for limitations of the window area, so one obvious way of getting more daylight is to increase the window size. Therefore, today the major challenge is to control the direct solar radiation and to improve the distribution of daylight in the spaces behind the facades. Many of the daylighting systems tested in Task 21 do in fact improve the daylight distribution and control (at least to some degree) the direct solar radiation. However, for almost every system there are strong limitations to how they can be installed and under which conditions they can perform well. And that is how it must be. The problem is that this fact is not well understood by design practitioners. While there are few problems of technical character, there are many related to the practical application of most of the daylighting systems on the market. Appropriate design and application of the systems depend upon the designer’s appreciation of the limitations to the systems. The most important limitations are, in short:

• Most daylighting systems prevent an unobstructed view to the outside

Consequently, when using such systems, a distinction should be made between the view window and the daylight window. This also means that most systems can not work “on their own”, but must be supplemented by a shading device for the view window.

• When hit by direct sunlight, many systems reach extremely high luminance values, causing severe glare problems

Consequently, such systems should be used only so that the high luminance spots or stripes are out of sight, i.e. normally these systems should be used above eye height for a standing person. Again, a separate solution for protection against sunlight must be found for the view window.

• Many systems that redirect sunlight causes bright spots of high luminances on the room surfaces, which may cause problems with high contrasts, glare, or reflections in computer screens.

Consequently, from the occupants point of view, such systems only perform well, if the bright areas are out of sight. This means that very careful studies of how the sunlight will be redirected over the day and over the year are necessary.

• Fixed systems designed for use of diffuse daylight may serve as a shading device but few (and very expensive) systems will improve daylight penetration in the room.

Systems designed for diffuse daylight or predominantly overcast conditions should be movable/ controllable and simple louvers and blinds should be removable.

• Few systems can actually increase the amount of daylight entering through the window opening and only under certain conditions.

Systems that can adapt to / be controlled according to changing sky conditions and sun position perform better.

• Most systems are very expensive (and far from what is normally considered to be sustainable design solutions).

The solution is seldom to choose a cheaper system but rather to re-design the facade and only use expensive systems in parts of the facade where (and if) the costs can be justified by the achievement of improved daylighting conditions.

Energy savings from use of daylighting systems depend on the system being designed as part of an integrated strategy that includes daylight-responsive lighting controls. Careful integration of the daylighting system with the rest of a building’s design should begin early in the design process to produce a high-quality work environment and provide building owners with a highly valued space.

The future will bring many new technological daylighting solutions to the market, but most of the limitations will remain, and only when manufacturers and building designers appreciate this, successful use of daylighting systems can be achieved.