Daylight varies during daytime and it is an important health aspect of indoor visual environmental quality. Measurements of daylighting are very complicated in natural conditions, therefore processing and evaluation of measured date requires special care. The instantaneous illuminances in one room and outside in Bratislava with 5 - second time step were measured by two illuminance meters with data loggers. The first illuminance meter registered indoor illuminance and was placed on the level of the working plane inside the room in its centre. The second illuminance meter with an unscreened sensor was placed on the roof and measured exterior global horizontal illuminance. Daily illuminance courses derived from 5 sec instantaneous data and 1 min and 1 hour averages were inspected and evaluated. This paper presents results of evaluations of real measurements and the discussion focus on the influence of evaluation interval on the accuracy of daily illuminance courses