Patent GB-739902-A (GB)
Title: Improved method of and apparatus for counting and sizing discrete particles
IPC: G01N15/14
Filed: 1952-02-13
Abstract: 739,902. Counting particles by radiation beams. YOUNG, J. Z., ROBERTS, F., and CAUSLEY, D. J. Feb. 5, 1953 [Feb. 13, 1952], No. 3874/52. Class 40 (3). In a device for counting the number of particles in a collection the collection is scanned by means of a pair of parallel beams separated bv a distance less than the dimension of the smallest particle to be counted, signals produced photo-electrically when the beams intercept a particle being counted. As shown, the two beams are produced from a scanning raster of horizontal lines produced on the end of a cathode-ray tube by means of a bi-refringent crystal 4. The two parallel beams thus produced scan the specimen 3 and are then directed respectively to photo-cells 7, 8 by means of a polarizing cube 13 which makes use of the fact that the beams are polarized in planes at right angles. The photo-cells produce pulses simultaneously when they both intercept a particle but only one cell produces a pulse when one of the beams only intercepts the top or bottom of a particle. The separate pulses produced by the cells are added in a circuit 25 and then separated again by a circuit 26 so that the pulse of longer duration always passes to a differentiating circuit 11 and the shorter to a differentiating circuit 10. Each pulse consists of a positive leading part and a negative trailing part and clipper 14 removes the negative part and feeds the positive to a clamp circuit 17. Clipper 15 separates the positive and negative parts the positive part feeding a " memory condenser " 21, a clamp circuit 20 and clamp 17, the negative part being also fed to clamp circuit 20. In operation the positive pulse presets gate circuit 20 so that the subsequent negative one renders it conductive. Memory condenser 21 discharges through this clamp 20 developing a pulse across a resistor 24 which is registered. If clipper 14 produces a pulse between the two pulses produced by clipper 15 the clamp 17 opens discharging condenser 21, this discharge not activating the register. Thus where the two beams produce one pulse only, the counter registers. Since this occurs at the top and bottom of the particle a scaler 27 divides the counter reading by two. Fig. 6 shows an alternative arrangement for operating a counter only when a single pulse is produced at the top of each particle. Each photo-cell is arranged to produce a negative pulse. One is passed to a valve V1, the anode load of which consists of an L.C. circuit which produces a positive pulse leading a negative one. The positive pulse is fed to the control grid of a valve V2 and a diode V3 to charge a memory condenser C1 by means of a resistor R1 in the cathode circuit of valve V2. The condenser charges to the peak valve and this value is applied to the grid of a valve V4 normally held blocked by the potential drop across a cathode resistor R5 in series with a valve V5. The negative pulse derived at the anode of valve V1 however blocks valve V5 and V4 conducts passing a pulse to a counter. Where the other photo-cell has also produced a pulse however valve V7 opens a diode V6 and condenser C1 discharges through the diode and a resistor R5. Thus no pulse is registered. Fig. 7 shows the application of this circuit to classify particles into size groups. The pulses produced from the cells are amplified in amplifiers 71 and 72 and fed to the circuit 70 which is as described with reference to Fig. 6 and produces a pulse when the top edge of the particle is scanned. This pulse is (1) applied to the horizontal time base circuit 75 to stop the horizontal traverse of the scanning beam which then traverses the particle downwards; (2) opens a gate circuit 76; (3) triggers a multivibrator circuit 78, and (4) is applied to a first counter not shown. The output of amplifier 71 is phase inverted at 73 and added in an adder 74 with that of amplifier 72. Operation of the gate circuit connects the adder to the coincidence circuit 77. The vertical scan of the particle by the two beams produces a wave-form as shown in Fig. 9F from the adder. The pulse PA is used to reverse the direction of the vertical time base and closes gate circuit 76. Negative pulse PB restarts the horizontal time base. If the pulse PA and a pulse generated after a predetermined time lag by multivibrator 78 are in coincidence the coincidence circuit passes a pulse to a counter (not shown). A number of multivibrators with different time lags will give the number of particles in various groups. The output of one of the photo-cells may be fed to a cathode-ray tube the time base of which is synchronized with that of the scanner to give a visual display. In an alternative method of producing the twin scanning beams an image of the field may be produced on the screen of a television camera the screen of which is then scanned by a spot rapidly deflected from one line to another by square switching pulses. The output from the camera tube is then switched to separate pulse amplifiers. In scanning the surface structure of metals a metallurgical microscope is used, a semisilvered plate being arranged between the beam source and the specimen so as to reflect the beams to the photo-cells. The specimen may be mounted on a rotating drum and scanned in a manner employed for facsimile transmission. Specification 707,406 is referred to.