The LIDAR Thomson Scattering diagnostic on the JET tokamak makes routine measurements of the plasma electron temperature and density profiles. During JET operations the windows used for scattered light collection gradually become coated with deposits due to the plasma erosion of the wall. The consequent loss in transmission can affect the electron density and also the electron temperature measurement. This paper describes both the method adopted to monitor the change in transmission due to the gradual build up of deposits and the application of laser window cleaning to counteract it.