If researchers measure illuminance with illuminance meters, they should apply meters appropriate quality, with known accuracy and type of uncertainties. Therefore the test measurements should be performed before carrying out experiments. Illuminance measurements applied in research require using calibrated photometric heads because sensors measure various illuminance levels due to degradation in time. Accuracy of every measurement is influenced by errors of instrument construction and of measuring procedure. During preparation of experiment and its carrying out should be these errors reduced at the most. This paper discusses influence of illuminance quality, importance of the photometric heads calibration on the Daylight Factor measurements.