An apparatus was designed and applied to measure the oil-film thickness in a production engine using the principle of laser-induced fluorescence. The apparatus incorporated fiber optics technology in its design with an aim to improve the ease of installation, portability, durability, spatial resolution and signal-to-noise ratio of previous designs using conventional optics, which hitherto have been used almost exclusively in studying oil-film characteristics in detail.Bench tests and engine tests were conducted to study the optimum combination of system components and to evaluate the performance of the design. These tests indicate that the goals of the design have been achieved. The increased spatial resolution enabled more precise identification of important lubricant features around the piston rings. Since the level of fluorescence decreases with temperature, the improved signal-to-noise ratio is expected to extend the applicability of the technique to engines operating at higher temperatures. The accuracy of the technique is presently equivalent to that of the Talysurf method for dry surfaces. The current system is sufficiently robust for further extensive measurements of lubricant-film characteristics in engines.