Light Properties
Light properties are fundamental to understanding the amount of daylight that passes through the glass, the reflection on the glass panel, and the colour of the glass. Some of them are directly related to comfort within the building, and others are more aesthetic.
The VLT is the percentage of daylight passing through the glass compared to the external environment. The higher the VLT, the light passes through the glass. This is especially important to consider when conducting daylight analysis on a building. What affects the VLT?
Glass substrate: Mid Iron glass allows for less light to get in Presence of coatings reduces the amount of energy transmission through the glass, like Low-E or Solar control coatings. Any fret in the glass. The higher the coverage percentage, the lower the final VLT to be achieved.
External Reflection Of Glass In Buildings
The light reflection, seen from the interior or exterior of the glass panel, is the percentage of the reflected light incident on the glass. The higher the value, the greater the specular effect on the glass. It is also important to consider these parameters in the design of the glazing panel:
- The internal light reflectance creates issues looking through the glass with higher percentages.
- The external light reflectance can be limited in some countries to disregard the risk of birds’ impact on the glass. Typically, values less than 15% are sufficient. Regarding bird protection, there are also specific coatings on the market that are able to achieve such requirements.
Colour Rendering Index
The Colour Rendering Index, or CRI, is a measure of the ability of a light source to reproduce the colours of various objects being lit by the source. It is a method devised by the International Commission on Illumination (CIE). The best possible rendition of colours is specified by a CRI of one hundred, while the very poorest rendition is specified by a CRI of zero. Numbers from 80 to 100 are excellent CRIs.
Tips And Advice
To maximize VLT, usually, low-E coatings, compared to Solar control coatings, are preferable. Please note that external shading shall be provided in this case to reduce the amount of solar energy entering the building in the summer.
There are often local requirements related to the amount of daylight that shall be provided within the building, either as an average per year or daily.
Sometimes, maximizing the VLT in some types of glass can be detrimental: skylights will need to be properly designed to avoid glare during the use of the building.
External and internal reflection can be reduced using anti-reflective coatings. These should be considered for high-end shopfronts.
