A sample testing apparatus is disclosed for use in optical transmission analysis of fluid samples such as oils or engine oils. The apparatus comprises a transmission cell comprising first and second fixed walls (1,2) and a movable window (3) that is moved with respect to the first and second walls in and out of a test region (6), defining an optical path through a fluid sample with a portion extending through the or each gap (L1,L2) between the walls (1,2) and at least a portion of the movable window (3). The window may be stepped or continuously varying in thickness for linear movement. There may be more than one window mounted on a carrier of differing thicknesses, including rotational movement such as using as an impeller.