Daylight is important for the human body primarily from the physiological and psychological aspects. Today is not inconsiderable economic aspect, too. In the absence of daylight leads to visual fatigue, which can lead to injury to the eyesight and various illnesses, as discussed by many experts. Rooms that are not in contact with the external environment are suitable for illumination light guides that can transmit light over long distances. The purchase of those is not the cheapest investment, therefore we should pay attention to optimize the design, to make installation bring the most efficient use. At the same time we should design the light guide the way that it wouldn´t form a thermal bridge and prevent the surface of condensation.