The Chart below offers performance data of double glazed units from Pilkington glass and Interpane glass with a selection of different external panes of high performance solar control super low e glass combined with an inner pane of 6mm thick glass. All of these options will decrease the overall light transmission percentage of the product to some extent in comparison to traditional clear glass, but with the added benefit of significantly reducing the total heat transmission percentage value.

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Basic Principles
Glass transmits solar radiation from the sun by three mechanisms, reflection, transmission and absorption, which for solar control purposes are defined in terms of the following parameters:

Reflectance:
The proportion of solar radiation at near normal incidence which is reflected by the glass back into the atmosphere.

Absorptance:
The proportion of solar radiation at near normal incidence which is absorbed by the glass.

Direct Transmittance:
The proportion of solar radiation at near normal incidence which is transmitted directly through the glass.

Total Transmittance:
The fraction 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 absorptance dissipated inwards by long wave radiation and convection, known as the long wave component. The proportions of the absorbed energy which are dissipated either inside or outside depend on the glazing configuration and the external exposure conditions (see above diagram).
All solar radiant heat properties are angle dependent.

Shading Coefficient:
The solar radiant heat admission properties of glasses can be compared by their shading coefficients. The shading coefficient is derived by comparing the properties of any glass with a clear float glass having a total solar heat transmittance of 0.87 (such a glass would be between 3 and 4mm). It comprises a short wavelength and long wavelength shading coefficient. The short wavelength shading coefficient (SWSC) is the direct solar heat transmittance divided by 0.87. The long wavelength shading coefficient (LWSC) is the fraction of the absorptance released inwards, again divided by 0.87.
Shading coefficients are calculated for radiation at near normal incidence. For other angles of incidence, the glass is compared with clear glass in the same situation. As a result, the shading coefficients are substantially constant at all angles of solar radiation.