A sample testing apparatus for use in optical transmission analysis of a fluid sample of oil, e.g. engine oil, the sample testing apparatus comprising; a transmission cell comprising first and second walls fixed in a spaced relationship relative to one another to define a space therebetween for receiving a fluid sample in use, at least the first wall being associated with a window (1, 2), wherein electromagnetic radiation may be introduced through the window into the transmission cell in use for detection after passing through the sample; the apparatus further comprising a first movable window (3, 3', 41), window, the first movable window being movable with respect to the first and second fixed walls, wherein the first movable window is configured to initiate movement in response to one or more actions by a user, wherein the apparatus is operable to cause the first movable window to move with respect to the first and second fixed walls to move at least a portion of the first movable window into and out of a test region of the transmission cell, the test region being a region between the first and second fixed walls in the optical path of electromagnetic radiation introduced through the window associated with the first fixed wall into the transmission cell for passing through a fluid sample located in the space between the first and second fixed walls prior to detection in use; wherein, when the at least a portion of the first movable window is located in the test region, a gap is defined between the at least a portion of the first movable window and one or both of the first and second fixed walls in the test region of the cell, such that an optical path is defined through a fluid sample in the cell for electromagnetic radiation introduced through the window associated with the first wall, the optical path through the sample comprising a portion extending through the or each gap between a one of the first and second fixed walls and the at least a portion of the first movable window, and wherein when the at least a portion of the first movable window is located in the test region, the optical path defined through the fluid sample is up to 1000 micrometres; characterised in that the apparatus comprises a set of one or more wipers arranged for wiping the sample contacting surface of the or each window associated with the first and second fixed walls of the apparatus during relative movement between the first movable window and the first and second fixed walls. a transmission cell comprising first and second walls fixed in a spaced relationship relative to one another to define a space therebetween for receiving a fluid sample in use, at least the first wall being associated with a window (1, 2), wherein electromagnetic radiation may be introduced through the window into the transmission cell in use for detection after passing through the sample; the apparatus further comprising a first movable window (3, 3', 41), window, the first movable window being movable with respect to the first and second fixed walls, wherein the first movable window is configured to initiate movement in response to one or more actions by a user, wherein the apparatus is operable to cause the first movable window to move with respect to the first and second fixed walls to move at least a portion of the first movable window into and out of a test region of the transmission cell, the test region being a region between the first and second fixed walls in the optical path of electromagnetic radiation introduced through the window associated with the first fixed wall into the transmission cell for passing through a fluid sample located in the space between the first and second fixed walls prior to detection in use; wherein, when the at least a portion of the first movable window is located in the test region, a gap is defined between the at least a portion of the first movable window and one or both of the first and second fixed walls in the test region of the cell, such that an optical path is defined through a fluid sample in the cell for electromagnetic radiation introduced through the window associated with the first wall, the optical path through the sample comprising a portion extending through the or each gap between a one of the first and second fixed walls and the at least a portion of the first movable window, and wherein when the at least a portion of the first movable window is located in the test region, the optical path defined through the fluid sample is up to 1000 micrometres; characterised in that the apparatus comprises a set of one or more wipers arranged for wiping the sample contacting surface of the or each window associated with the first and second fixed walls of the apparatus during relative movement between the first movable window and the first and second fixed walls.