The article evaluates the possible measurement of light transmittance of glazing windows embedded in construction structures of existing structures. In practice, it may be necessary to determine the light transmittance on an already embedded element with glazing. In the case of embedded structures, the position of the glazing in relation to exterior background conditions plays a major role. Laboratory measurements and calculations are used by default to determine the value of glazing transmittance, when it is possible to set and ensure the marginal conditions.