GB861284A - Improvements in particle counter - Google Patents

Abstract

861,284. Particle counters. DU MONT LABORATORIES Inc., A. B. March 17, 1958 [March 15, 1957], No. 8522/58. Class 40(3) In a particle counter for counting the number of particles in a given area a signal produced in sensing means when a scanning beam encounters a particle is applied to a circuit which delays the signal for a period equal to the time between successive sweeps of the scanning means and the sensing signal and delayed signal are applied to an anticoincidence circuit which provides an output to counting means only when the sensing signal is not overlapped in time by the delayed signal. Light from the flying spot scanner 10, Fig. 2, is directed by the lens assembly 31 and mirror 32 to scan the transparent container 33 containing the particles to be counted. The mask 34 limits the counting area and the emerging light passes via the condensing lenses 35 and mirror 36 to the photocell 14. The output of the cell is applied via a cathode follower 15, Fig. 1, to a correction circuit 16 which ensures that the average signal valve remains substantially constant without appreciably degrading the signal information and a Schmitt trigger circuit 17 connected to the output of the correction circuit produces, when triggered, a signal of constant amplitude corresponding in duration to the width of the particle intercept. This signal is used to modulate the output of an R.F. oscillator 19 and the modulated signal is applied via an R.F. amplifier 20 to the delay line 21. The output signal of the line, delayed by the line scan period, is demodulated and applied to the record input of an anticoincidence circuit 24 which has a first input connected to the output of the Schmitt trigger circuit. The anticoincidence circuit provides an output when a pulse is present at the undelayed input only and this output is applied to the counter 25 and register 26. During the line fly back period a pulse is applied to the third anticoincidence circuit input terminal to prevent the passage of pulses during this period. To equalise the time delay and the line scan period, the pulse output of a blocking oscillator, Fig. 7 (not shown), having a natural time period larger than the delay time also modulates the R.F. carrier applied to the delay circuit, after demodulation this pulse is used to trigger the blocking oscillator thus equalising its time period and the delay time. A part of the blocking oscillator output is used to trigger the line scan. Specification 778, 288 is referred to.