The Mars Soiling Sensor has outputs for Transmission Loss and Soiling Ratio.
Transmission Loss, TL, is a dimensionless factor that represents the portion of incident light blocked by the dust on the sensor's window. For example, if TL = 0.1, it means that 10% of the incident light is blocked by dust.
Soiling Ratio, SR, is a dimensionless factor that represents the ratio of the incident light received through the sensor window to that which is incident on the window. For example, if SR = 0.9, it means that 90% of the light goes through the window, and 10% is lost.
Note that SR = 1 - TL.