The DF_ave formula can be used in reverse to provide a first estimate of total window size for a particular space with a particular type of glazing for a chosen DF_ave, giving information to enable a scale model to be constructed. This should also consider optimum glazing ratios to avoid excessive solar gains or heat loss.

A productive approach for designers is to observe and record the daylighting characteristics of existing buildings. Visual assessment of an interior, and measurement of light levels with an illuminance meter will provide a useful basis for daylighting design. The DFave can be roughly measured to aid comparison between calculation and reality, by using two illuminance meters simultaneously to measure indoor and external illuminance under daylight. The internal measurements should be averaged over the whole floor area. Even a simple box model combined with use of two lux meters and a compass can generate a lot of information.

Orientation to Avoid Solar Penetration

Sunpath diagrams are available for all regions. These provide information on the path of the sunpath for particular times of year and day. These sunpath diagrams are useful in estimating depth of solar penetration into a space and in designing appropriate overhangs and louvres.

Understanding Daylighting of Sports Halls

Simulation

To calculate and judge the effectiveness of any daylighting design strategy it is necessary to perform some form of modelling exercise. Simple hand calculations of average daylight factors to the fully rendered computer images of simulation programs, such as Radiance can be used. The more complex methods are not necessarily the most effective for all situations. Numerical analysis beyond daylight factors can be difficult. Instead, architects and engineers are advised to develop computer visualisation, shaded perspectives and/or models.

Computer simulation requires designers to be fluent in a particular program but it becoming increasingly simpler to undertake. Many engineers are familiar with

Cymap, which provides simple daylight factor modelling. Complex tools are now available in PC versions and can be downloaded free from the internet. However, the cost comes in the length of the learning process. This usually requires at least 3 months of training, usually by external tutors.

Programs such as Radiance, allow fully rendered interior and exterior images to be made. Scenes can be input using standard CAD packages and built-up within the simulator. Outputs range from visual representation of a daylit space; full simulation of a year, including sunlight; modelling of infinite forms of structure, glazing, shading and orientations; accurate values of Daylight Factors, illuminance, glare index, colours and the incorporation of artificial lighting schemes.

Modelling

The method traditionally used for daylighting assessment is the scale model. Importantly, light is independent of scale, so a model of any scale can reasonably accurately show the effect of light through windows. A model allows the designer to judge the effectiveness of any daylighting design, if used under a real sky or in a designed artificial sky. Models provide an objective assessment to be made, but are less able to provide detailed measurements. They are an excellent way for design practices to extend their skills without having to invest heavily in computer simulation.

It is usually sufficient to construct a model no larger than a desktop. Surfaces should have the same reflectance and colours as in the completed room. Sensors mounted inside a model can allow a designer to take readings and assess the daylight factors and the physical intensity of light received by, or emitted by, a surface. Window location, shading options and sunlight access - a frequently overlooked part of daylighting in sports hall design - can be assessed.

The example above indicates the need to prevent solar ingress from high level in the summer but the sources of glare extend into the winter with low level sun a particular problem.

The fact that we do not carry out social activity symmetrically around mid day means that the low level sun in the evening to the west is likely to give rise to more complaints than the early morning sun to the east.