A method for identifying the presence or absence of an aggregation of particles distributed on a translucent surface includes the steps of providing a light sensing device for sensing light incident thereon and producing an output value which represents the intensity of the incident light, and identifying a first output value which is produced by the light sensing device when there is no light incident thereon. The method also identifies a second output value which is produced by the light sensing means when, with no particles present on the translucent surface, light is directed through the translucent surface onto the light sensing device. With a distribution of particles deposited on the translucent surface, light is directed through the translucent surface and onto the light sensing device, causing the light sensing device to produce a third output value. A final output value is then determined as a function of said first, second and third output values.