Effect of VLT in Daylighting

What is VLT of glass?

VLT stands for Visible light transmission. It is the amount of visible light that can pass through an optical or sun lens. It can also be called visible light transmittance or VLT%. This is measured as a percentage which indicates the darkness of a lens within a sunglasses frame. The lower the VLT, the darker the sun lens will be.

Fig. VLT represented as VT in the given image

Daylighting

Daylight is merely the visible part of the radiant energy that enters through windows. Furthermore, the bulk of the daylight energy that enters a space is converted into thermal energy after just a few reflections. Many office buildings in moderate climates now have air conditioning largely due to the high internal gains. In warmer climates cooling may be needed for large parts of the year.

Most of the software used today are based on radiance but have different interface. While there are a number of options available for daylight simulation, there are certain criteria that you should ask yourself before choosing the simulation software for daylighting.

Here, DF- daylight factor is considered to compare different VLT options.
Faylight factor (DF) is the ratio of the light level inside a structure to the light level outside the structure. It is defined as:

                                             DF = (Ei / Eo) x 100%

Ei = illuminance due to daylight at a point on the indoors working plane
Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky. 

Analysis

This analysis is done on a show box model room of size 5m X 7m X 2.7m with windows of sill level at 450mm and lintel level at 2100mm. An online dynamic daylight analysis tool is used by Andrewmarsh.

Climate selected is Delhi, India (composite climate). Model is oriented east west direction.
VLT ranges from 40% to 90%. Wall, floor and ceiling reflectance taken are 0.6, 0.4 & 0.7 resepctively.

VLT- 40%

VLT- 50%

VLT- 60%

VLT- 70%

VLT- 80%

VLT- 90%

Above analysis shows that Daylight Factor increases to double the percentage when the VLT of glass goes from 40% to 90%. But 90% VLT is not recommended as a 90 percent visible light transmittance glass will also have high SHGC which invites high solar heat gains as well. That’s where whole building performance come into play where we need to balance these two in energy consumption terms.

Fig. Graph showing relationship between VLT and Daylight factor.