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). When the movable window (3) is moved into the test region (6) an optical path through a fluid sample in the cell is defined, the optical path through the sample comprising a portion extending through the or each gap (L1,L2) between a one of the first and second fixed walls (1,2) and the at least a portion of the first movable window (3). Also disclosed are methods of using the sample testing apparatus and methods of performing a measurement for use in optical transmission analysis of a fluid sample.