Patent ID: 7084401

Claim:
Particle detecting apparatus, comprising first and second radiation emitting means for respectively emitting first and second radiation along substantially the same predetermined path into a scattering volume when respectively rendered operative, radiation sensing means for receiving and sensing said first radiation forward-scattered from the scattering volume by the presence of particles therein and for receiving and sensing said second radiation forward-scattered from the scattering volume by the presence of particles therein, processing means responsive to the received and sensed first radiation to produce a first signal in dependence thereon and responsive to the received and sensed second radiation to produce a second signal in dependence thereon, output means for comparing the two signals whereby to produce a warning output when the comparison indicates that the particles are of a predetermined type but not when the comparison indicates otherwise, and control means operative when the first radiation emitting means is rendered operative to maintain the second radiation emitting means inoperative until the first signal has exceeded a predetermined value and for then rendering the second radiation emitting means operative.