Light transmission factor

One of the basic tasks of each window is to let the right amount of natural light into the room. Interior lighting is largely influenced by the technical parameters of the glazed unit. The light transmission factor shows how much of the visible solar radiation passes through it.

Role of light transmission factor

Among the most important parameters describing the properties of glazed units is the light transmission factor L_t. This factor shows how much of the visible solar radiation, falling on the glass at an angle close to 90°, reaches the room. It is calculated as the ratio of light passing through the glass to the amount of visible solar radiation falling on it. Accordingly, the light transmission factor is expressed as a percentage. A high value of the parameter indicates that the use of a glazed unit is associated with a well-lit interior. As a result, less electricity is needed to illuminate the room. According to the PN-EN 410, the light transmission factor is determined as τ_V.

What does light transmission depend on?

One of the factors influencing the amount of light entering the interior is the number of glass panes in the glazed unit. The more panes in the glazed unit, the harder it is to achieve a high L_t factor value. In addition, the degree of light transmission depends on the composition of the materials used to produce the glass. The thickness of the individual panes that make up the glazed unit is also of great importance. Moreover, the L_t value is influenced by the presence of low emissivity coatings, which are used to limit the heat transfer from the room to the outside. They reduce the amount of visible light entering the interior. This does not mean, however, that the use of low emissivity glass is associated with a poorly lit apartment. Modern low emissivity coatings, such as ClimaGuard® Premium 2, have a beneficial effect on the thermal insulation of the window, while providing the right amount of light in the room. The value of the light transmission factor can be changed by using appropriate panes to control sunlight.