Sky dome

The Sky dome is the Welsh School of Architecture’s artificial sky and heliodon facility.

Designed and constructed through funding from HEFCW, the dome was opened in 1999. This major facility allows researchers and designers to simulate using scale models, the lighting conditions within and around their buildings.

This facility accurately simulates external natural lighting conditions; those arising from the sun, sky, and clouds, and the reflections from the ground and nearby structures. It can do so for all weather conditions, seasons, and locations. At eight metres in diameter this facility is one of the largest available.

The hemispherical dome contains 640 individual luminaires, mounted within an open geodesic framework. The luminaires contain low energy compact fluorescent lamps and can be selectively dimmed to model the luminous distributions of different types of skies: overcast, clear or mixed. A cloudy sky, for instance, has a zenith three times brighter than the horizon. A clear sky has the greatest brightness near the sun position, but is also bright around the horizon.

The dome also contains a heliodon, which sets the relative angles between the model and the artificial sun, as appropriate for the time, date and location being studied. The artificial sun travels vertically in the slot visible in the images above; this sets the relative solar altitude. To set relative azimuth, the model is placed on a rotating turntable. Under the computer controlled system, an accelerated day can be run in only a few minutes.

Modelling natural lighting

Electric lighting accounts for a large proportion of the total energy use of a modern building. The inclusion of daylight in a design can successfully reduce the energy demand of buildings, as well as providing a more pleasant environment for the occupants. However, care must be taken in a daylighting design to eliminate excessive solar heat gains, glare or visual discomfort. Those prediction tools available to the designer are often rule-of-thumb and may be limited in their application to modern materials, devices or concerns. The more capable, but complex, numerical prediction systems still lie in the realm of the specialist.

Much useful daylight is diffuse, coming from the sky dome rather than directly from the sun. Standardised model testing is centred on the CIE cloudy sky; a total overcast sky suitable for Northern Europe. Designing for clear sky conditions is more problematic, and cannot be easily achieved by simple prediction methods. Testing under non-standard conditions can also be important, particularly in the development of novel lighting or control strategies.

Consideration of direct beam sunlight is also important to a lighting assessment and the facility includes a heliodon. The sky dome and the heliodon will be usable at the same time, allowing the evaluation of novel technologies such as “smart” glazing.

As well as in the design of buildings and lighting systems, the Sky dome facility provides many opportunities for research:

  • the validation and development of computer based lighting prediction methods
  • the testing and development of lighting energy management systems
  • the design and assessment of shadows and sunlight availability, for instance in landscape or the application of photovoltaic facades.

Physical scale models

While there have been major advances in capability in numerical computation of daylighting, for instance via the Radiance system, physical scale modelling is still an attractive and viable alternative. Physical modelling provides:

  • fast turnaround – results can be immediate, and although a model needs to be constructed, they can often be very simple and still provide good results, e.g. for proof of concept investigations
  • there are still many materials and devices that cannot be included in the simpler calculation methods
  • manipulation of real objects (the models) still engages the designers and clients more readily than screen or paper representations.