1. A particle detection system for detecting particles generated within a semiconductor wafer process chamber, comprising: a radiation source for the emission of radiant energy of visible spectrum into said process chamber; an image sensor for detecting a two-dimensional image of scattered radiant energy within said process chamber and producing therefrom a video frame representing a matrix array of pixel intensities; and a processor connected to said image sensor for detecting differential pixel intensities between a first video frame representing a background image obtained by the image sensor before said process chamber begins a wafer processing and a second video frame representing a target image obtained by the image sensor after said wafer processing is started, and making a decision on said differential pixel intensities to produce an output signal representing presence or absence of said particles, wherein said processor is arranged to: divide the background pixel intensities into a first plurality of groups according to different values of the intensities, divide the target pixel intensities into a second plurality of groups corresponding to the first plurality of groups according to different values of the target pixel intensities, determine a differential pixel count between the number of pixels contained in each of said first plurality of groups and the number of pixels contained in each corresponding ground of the second plurality of groups, and multiply the respective intensity of each of the second plurality of groups with the differential pixel count of the group to thereby produce said differential pixel intensities.