Technical Guidance

Light Transmission

Rooflights can provide up to three times more light than vertical glazing elements, and can provide a more even and usable distribution of natural light into a building, particularly in large structures where light is required deep into the building or in enclosed areas that cannot be lit through an external wall.
The light that enters a building can be direct light such as that which passes through clear or transparent materials, or diffused light that is created by surface textures designed to create diffusion by surface refraction, or by materials that are translucent and naturally diffusing. Glass Reinforced Polyester (GRP) rooflights are manufactured using clear polyester resins reinforced with transparent glass fibres. As the light passes through the combination of materials, the light is scattered by refraction to create a diffusing medium.

Direct light

Light that enters a window or rooflight with minimal disturbance through transparent type materials, such as glass and polycarbonate, provides a strong and direct light path that can create high localised levels of illumination and provide clear vision to the outside of the building. For some applications, direct daylighting can offer positive benefits particularly in window applications, however in periods of strong sunlight it can create high contrasts between light and dark, undesirable shadows and glare.

Diffused light

When light is scattered as it passes through translucent diffusing materials such as GRP, or transparent materials that are modified to produce diffused light transmission, it does not necessarily reduce the levels of light transmission. It makes more efficient use of the available light by spreading it over a far greater area. This significantly reduces the apparent difference between light and shade and minimises the creation of shadows, thereby creating a more consistent and evenly lit space.

For information on daylight design for buildings, looking at recommended light levels, daylight factor considerations and energy savings,  you can download the section from our technical manual here.

The next section of the technical guide discusses thermal transmission (U-value); a key specification consideration when looking at sustainable building design.