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Photometric Terminology

Light Transmittance (Tv) is defined as the amount of light transmitted to the indoor environment divided by the intensity of incident daylight radiation.

 

Visible Light Reflectance (Rv) is the amount of visible light reflected from the glass surfaces the two values of visible light reflectance are determined:

  • The visible reflectance as perceived from outside the building, Routv.
  • The visible reflectance as perceived from inside the building, Rinv.

 

Solar Energy Reflectance (Re) is the proportion of solar radiation at near normal incidence that is reflected by the glass back into the atmosphere.

 

Solar Energy Absorptance (Ae) is the proportion of solar radiation at near normal incidence that is absorbed by the glass.

 

Direct Solar Energy Transmittance (Te) is the proportion of solar radiation at near normal incidence that is transmitted directly through the glass.

 

Solar Factor (SF) is the total solar energy transmission also referred to as the g Value in Europe, or as the Solar Heat Gain Coefficient (SHGC) in the USA. The Solar Factor is the proportion of solar radiation at near normal incidence that is transferred through the glazing by all means. It is composed of the direct transmittance, also known as the short wave component and the part of the solar absorptance that is released inwards by long wave radiation and convection, known as the long wave component.

 

Shading Coefficient (SC) is derived by comparing the total solar energy transmittance of the glazing with a clear float glass having a total solar energy transmittance of 0.87.

This corresponds to float glass of thickness 3-4mm. The shading coefficient may be divided into two components: the short wave shading coefficient which is the solar transmittance divided by 0.87, and the long wave shading coefficient, which is the inward flowing fraction of the absorbed solar energy, (qi) divided by 0.87.

 

U Value (U)

The U value, or thermal transmittance, is defined as the steady state density of heat transfer rate per temperature difference between the environmental temperatures on each side in the absence of solar radiation in W/m2K. The U value is defined for the transparent centre of glass part of the glazing. (To convert W/m2K to Btu/ft2/h/oF ÷ 5.6783)

 

Relative Heat Gain (RHG) is the amount of heat gained through the glass, taking into consideration the U value and the shading coefficient. Using ASHRAE standard environmental conditions is calculated as follows.

  • Metric             RHG = (Summer U- value x 7.8oC) + (Shading coefficient x 630 W/m2)
  • Imperial           RHG = (Summer U- value x 14oF) + (Shading coefficient x 200 Btu/ft2)

 

Ultra Violet Transmission (Tuv) is the proportion of solar radiation at wavelengths between 300nm and 380nm that is transmitted through the glass.

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