Particle analysis system with photochromic filter

Claim 1

A particle analysis system including in combination; means for forming a flow stream containing particles to be detected, a source of light for producing a light beam, means for directing said light beam through said flow stream at a first location in a first direction, said light beam passing through said flow stream producing light reflections and a light pattern, said light pattern, light reflection and light beam passing through said flow stream varying rapidly and for a short time period in response to passage of a particle through said first location, a background light filter aligned with said light beam for receiving said light beam passed through said flow stream and for receiving said light pattern and light reflections, said background light filter being operative to slowly increase in optical density at the points thereon where said light pattern light beam and light reflections strike said filter and attenuate said light pattern, light beam and light reflections as they pass through said filter in said first direction, said filter being substantially insensitive to said rapid, short time period variations in said light pattern, light reflections and light beam for passing said variations through said filter in said first direction substantially unattenuated, said filter including a photochromic transparent medium, photodetector means for receiving said light beam, light pattern and light reflections passed through said filter in said first direction, said photodetector means operative to develop signals which vary in accordance with variations in said light reflections, light pattern and the light beam received thereat, and detection means coupled to said photodetector means and operative in response to said signal variations to detect said particle.