1. A method of sample analysis comprising the steps 1. illuminating a particle containing sample with light which is modified by the sample; 2. producing a first signal representing a first predetermined wavelength band of the sample modified light at a region in said sample; 3. producing a second signal representing a second predetermined wavelength band of the sample modified light at said region, said first and second wavelength bands being selected to produce a differential contrast between said region and at least one other region in said particle sample; and, 4. algebraically combining with thresholding said first and second signals to classify said sample region in at least one of a predetermined number of categories.