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