A Guide to Photometric Glass Terms

When selecting glass types such as low E glass or solar control glass, it is important for architects, M&E Consultants and owner/occupiers to measure how the glass will perform in a window or façade. The values published are known as Photometric properties (light) however we also include thermal properties, energy properties plus acoustic properties of the glazing if available.

It is possible to construct double or triple glazed insulated glass units with different glass types & thickness, different cavity thickness & different gas fills which make up the extensive range of products from Carey Glass.

Carey Glass can calculate the Photometric properties of the unique combinations.

Glass properties which are listed will impact in the performance of a building over its lifetime. For example:

  •     Poor light transmission though the glazing will mean more lighting
  •     The lower the U-value the better thermal insulation (less heat loss) &
  •     The lower the solar factor will mean less solar heat gain, which will all have a impact on the air conditioning load.

Acoustics is another import factor, particularly in urban areas or buildings constructed near infrastructure such as airports and motorways; health care, educational & office space where noise control is paramount.

A typical example of glazing properties is shown below

Below is an explanation of the most common terms used in Photometric properties.

    LT – (light transmission) refers to the percentage of the visible spectrum transmitted through the glass.

    Light reflectance in - (internal light reflection) is the percentage of visible light reflected away from the inner surface of the glass, compared to the total visible light that reaches the glass at 90 degrees.

    Light reflectance out - (external light reflection) is the percentage of visible light reflected away from the outer surface of the glass, compared to the total visible light that reaches the glass at 90 degrees

    General Color rendering Index – (Ra)describes how much a objects color changes when it is observed through the glazing as determined according to EN410

    Direct Energy Transmittance - The fraction of the solar radiant heat at normal incidence that is transmitted directly through the glazing without change in wavelength.

    Energy Absorbance - The fraction of solar radiant heat at normal incidence that is absorbed in all the layers of the glazing. Over 60% Toughen Glass. Over 70% a thermal check should be carried out.

    SF (Total Energy Transmittance) - Solar Factor, commonly used in Europe sometimes known as ‘g’ value is the same as Solar Heat Gain Coefficient (SHGC) used in the USA. The Solar Factor is the ratio of the total solar energy (UV, Visible & near Infra Red) entering through the glass compared to the incident solar energy. This total energy is the sum of the solar energy entering by direct transmittance and the part of the energy absorbed by the glass and re radiated to the inside.

    SC – Shading co-efficient Comparison of the performance of the glass specified to 3mm clear float which has the highest solar factor.

    U value – the center pane insulation value of the glass,  given in W/m²K

    UV Transmittance - is the percentage of Ultra violet radiation transmitted through the glazing.

    Rw - The accessed noise reduction extent in dB with no noise transfer over the adjacent components.

    Rw +C and Rw+Ctr -  Noise reduction plus the correction factor for spectrum I and spectrum 2.

If you would like to find out more information on this or any of the Carey Glass products, please contact us today.

Source: Carey Glass International