There is no short answer to this question!! Assuming that the skylight top structure has an opening that is matched by the ceiling opening and the connecting duct (for example 500mm diameter all the way through for a round or 500x500mm dimensions all the way through for a square), the following factors then need to be taken into account:
- Light surface area
- Type of plastics or glass used in the roof top structure
- Type of duct used to connect the roof top structure to the ceiling frame and panel
Light Surface Area – the bigger the surface area of the opening, the more light that can potentially move down towards the ceiling. Using basic maths calculations, the light surface area is calculated for some standard skylight sizes below:
Dimension of skylight:
Light surface area – square meters:
250mm diameter round
300mm diameter round
343mm diameter round
400mm diameter round
457mm diameter round
500mm diameter round
535mm diameter round
600mm diameter round
Types of plastics or glass used in the roof top structure – traditionally, square skylights have been manufactured using opalite domes which have a milky appearance. These are used to reduce the likelihood of a heat transfer issue at the roof level. Because round skylights in a similar size have a smaller light surface area (see table above), manufacturers have tended to use clear acrylics or polycarbonates to compensate for the lower light surface area. In tubular daylighting devices, the clear plastics used in roof top structure are often enhanced with special lenses that help capture low level light to maximise the light transfer down to the shaft.
Type of duct – the type of duct used in the skylight will have an impact on the amount of light achieved at the ceiling level. In WA, many of our conventional skylights use flexible duct to transfer the light from top to bottom. If the duct is pulled as tight as possible, the reflectivity of the duct can be up to 86% (refer to manufacturer SG Eco Industries testing). If highly reflective rigid duct is used (such as in tubular daylighting devices), reflectivity is 97-98%, so almost all of the light captured at the roof level is transferred to the ceiling. Because of the higher level of reflectivity of the rigid duct, it is often possible to select a smaller diameter skylight as this compensates for the lower light surface area.
When comparing similar sizes of round and square skylights it often comes down to personal preference for the shape that matches your room or other ceiling fittings, particularly if you are considering a skylight that uses flexible duct.