The Openness Factor

The openness factor (OF), sometimes referred to as the openness co-efficient refers to the percentage of perforations (holes) in a fabric against the total surface area of the fabric. It is one important factor in determining how much light will transmit through a fabric along with the level of outward vision that is maintained. Openness factors will usually vary from as low as 3-4% upto as much as 16%.

A material with a lower openness factor will typically have a lower light transmission level and therefore less of a view through it, whereas a higher openness factor will give a better view through the material but also let more light through which could potentially lead to glare issues.

Glare occurs when large amounts of light are directed in one place, therefore causing the field of view to become bright and subsequently discomfort to eyes which can lead to fatigue, headaches and error in work to name but a few things.

Of course the size of the perforations in a material will play a big role in determining whether there is a possibility of glare. If two fabrics have the same openness factor but one just has a single hole (not the norm!), whereas the other has lots of microscopic holes, the fabric with just one single hole will no doubt lead to glare as it is directing all the light in one place, but the fabric with lots of little holes will diffuse and deflect the light, leading to an evenly distributed natural light source in the room and at the workstation.

There are many additional factors that will affect the transmission of light through a fabric along with outwards visibility. These include:

• Colour of the material
• Thickness of the material
• Construction of the material
• Coatings on the material (ie reflective top coatings)
• Conditions in the sky (ie cloud cover)

Openness factors will usually only be found for fabrics on external roller window and roof blinds as they run down directly in front of and cover the glazing. Retractable folding arm awnings use solid fabrics as they project out from a building and therefore a clear horizontal view is kept.

More information on glare control will be published soon including recommended fabric light transmission levels for the different facade windows on commercial office buildings.