This paper discusses tube transmission efficiency of two straight hollow light guides. Two samples with diameter of 530 mm and length 1170 mm were investigated under the artificial sky in the laboratory at ICA SAS in Bratislava. The entering luminous flux was calculated from measured illuminance in the point located on the top of light guide. Below the bottom of the light guide was located a set of measuring points on the special construction in the shape of a cross. In these points, one by one, the elementary illuminances were measured and the luminous fluxes leaving the light guide were calculated. Paper presents methodology for laboratory light transmission measurements and discusses effects of two various coating materials on light transmission efficiency of hollow light guides.