It’s well known that a criterion of estimating the varying natural lighting is exposure equal to the product of light intensity by its duration. Here we have made studies into the exposure in the room depending on the orientation of a light aperture and its location in space. The exposure has been considered by the example of three identical office rooms with the same light apertures oriented north, west and south and having three positions – vertical, inclined and horizontal. To calculate the annual exposure we made use of the well-known software package VELUX Daylight Visualizer 2. For convenience of analyzing the exposure there was introduced the concept of the natural exposure coefficient (NEC) which is a ratio between the exposure in the room and a simultaneous value of the outer exposure. Our studies have shown that exposure is an effective criterion to assess the indoor natural lighting in time. The existing system of estimating energy consumption in lighting buildings with the help of a simultaneous lighting is rough and does not take into account such factors as orientation of light apertures by the sides of the horizon and their location in space. The use of exposure let us improve the method of calculating energy consumption in lighting premises taking into account the light aperture location in space and their as orientation by the sides of the horizon. The numerical experiment performed has given a predicted result, namely, the most power-consuming
room is the north-oriented one with the vertical light aperture and the least power-consuming room is the one with the horizontal light aperture. The room with the inclined light aperture has average energy consumption.

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