CTU in Prague in cooperation with ICA SAS in Bratislava carried out laboratory measurements of the light transmission efficiency of direct hollow tubular light guide sample under the artificial sky. Tubular light guide of diameter 530 mm and length 1170mm was made from aluminium with polymer film on the inner surface. Two illuminance meters with spectral and directional corrected heads were used for measuring entering and tube exiting light. Sensor of illuminance meter measuring the quantity of exiting light was moved with constant step on the horizontal plane located just below edge of the tube. The second sensor measuring outdoor horizontal illuminance was set in stable position close the upper tube edge. The paper information about experimental measurements and discusses the obtaining results.