Method and apparatus for detecting and sizing particles on surfaces

A particle detection apparatus comprising,

• means for supporting an object with a surface to be tested,
• a source of vaporizable liquid material,
• a wick in fluid communication with said source and disposed over a portion of said surface, said wick being heatable so as to cause liquid material drawn by said wick to vaporize,
• means for heating said wick, whereby a local zone of vapor supersaturation is provided over said portion of said surface, droplets of liquid being thereby capable of condensing on any particles in said portion,
• a light beam source for producing a beam,
• means in a path of said beam for directing said beam in a scanning motion toward said surface, said beam scanning a path adjacent to said local zone of vapor supersaturation,
• means associated with said support for moving said object relative to said wick and said scanning beam path, whereby any droplets which condense are moved into said beam path so as to scatter said light beam,
• a light detector positioned to receive light scattered by any said droplets, whereby particles around which said droplets have condensed are detected.

A method of detecting and classifying particles on a surface comprising,

• (a) providing a first supersaturated vapor over a test surface, whereby droplets condense on a first set of particles on said surface,
• (b) scanning said surface so as to detect said droplets,
• (c) repeating steps (a) and (b) a predetermined number of times with other supersaturated vapors, whereby droplets condense on other sets of particles on said surface and are subsequently detected, and
• (d) comparing said droplet detections from the repeated scans and classifying said particles on said surface according to the scan or scans in which their corresponding droplets are detected.