The laser-induced fluorescence technology is used to measure the horizontal tube falling film thickness of pure water and natural seawater with 2.66% salinity. This experiment focuses on the film thickness distribution characteristics outside 19 mm and 25.4 mm outside diameter Al-brass tubes in column flow with the tube spacing ranging from 20 mm to 40 mm and the Reynolds number varying from 184 to 368. The fluorescent in liquid is induced by the laser. And the images of gas liquid interface are captured by the digital camera. The quantitative descriptions of the liquid film thickness around the horizontal tube are obtained by the image processing technology. The results show that the film thickness increases with Reynolds number and decreases with tube spacing. In the axial direction, the thickness of the film increases with the distance from the section of liquid column center and maximized at the middle of two adjacent liquid columns. In the circumferential direction, the thickness of the film decreases until it reaches the minimal value near phi = 90 degrees and then increases. The comparison of film thickness between water and seawater is also presented. (C) 2015 Elsevier Ltd. All rights reserved.