Daylighting applications of micro-textured optical surfaces

Abstract

Daylighting is the use of natural light to replace artificial light. In traditional rooms sunlight will only illuminate the area closest to the window due to the high solar angle. The rear of the room appears gloomy and occupants will use electric lighting even though there is sufficient daylight to illuminate the interior.

The first section of this thesis reports on the application of micro-prisms to glazing. Such systems could improve the penetration of the light and reduce the energy bill. Fig 1: (Left): A traditional window. (Right) A window with the top third coated in microprisms.

The aim of the work is to develop suitable structures than can be easily and cheaply mass produced using an industrial UV embossing process. Whenever possible the requirements of this process dictate the physical characteristics of the microstructures.

The development process includes all the stages from design to full-scale testing of the prototypes in an office. Several different mechanical methods are used to produce prismatic arrays that conform to an initial design calculation. Each sample is evaluated in terms of its physical characteristics, its optical properties and finally its ability to improve illumination within a room. The latter aspect is determined, not only by measurement, but also the subjective assessment of occupants.

The second micro-textured surface to be examined is the microlens. Three systems are investigated: - A controlled diffuser incorporating cylindrical lenses to improve the distribution of the daylight. - An afocal pair of lenses to improve the penetration of daylight through beam-steering. - An angular filter to exclude direct sunlight while admitting diffuse light.

Most of the research is concerned with the third system. On sunny days windows can cause sufficient glare that occupants will pull the venetian blinds. Not only will this exclude the direct sunlight but also the diffuse daylight, cause darkening of the room and leading to the use of artificial light. The angular filter or 'solar shade' uses microlenses to image the direct sunlight which can then be blocked by circular obturations. The diffuse sunlight is not focused and therefore transmitted so the room is not darkened.

The research is based on experimentation with small-scale systems and computer modelling to optimise the system. The results show potential improvements over new 'smart' windows although mechanical tolerances are high.