Patent GB-744470-A (GB)
Title: Improvements in or relating to the electronic counting of objects
Inventor: URRY DENIS EDWARD
IPC: G06M11/04
Filed: 1952-10-02
Abstract: 744,470. Counting particles by radiation beams. CINEMA-TELEVISION, Ltd. Oct. 2, 1953 [Oct. 2, 1952], No. 24692/52. Class 40 (3). Apparatus for counting those objects which are of a particular size in a collection of objects comprises means for scanning by a radiation beam and in contiguous lines a field containing the objects and for deriving a train of signals corresponding to the passage of the scanning radiation over the first scanned boundary of each object and a second train of signals corresponding to the last scanned boundary, means for delaying the first signal train by a time substantially equal to the time of passage of the scanning radiation over a particle of the median size to be counted and means for applying the delayed first train and the second train to a device which produces an output only if these two signals substantially coincide. To take account of the fact that these signals may be produced by a beam traversing a chord of a larger particle a pair of beams spaced in a direction perpendicular to the scanning direction may be used. This arrangement is shown in Fig. 3 in which the usual rectangular raster is produced on the screen of a cathoderay tube by a single beam which, however, by auxiliary deflecting apparatus 18, 18<SP>1</SP> is caused to alternate rapidly between two parallel lines 21, 22 (Fig. 4), during its passage along a line. This arrangement simulates a parallel pair of scanning beams, and a spacing is chosen between 0.5 and 0.9 times the diameter of the objects to be counted; 0.7 is said to give the greatest sensitivity. Light from the screen of the tube is focused in the plane of the collection 8 and after passing through this collection is passed by a lens 10 to a photo-cell 11. The output of the photo-cell is applied to an electronic switch 18 operated in synchronism with the means 17 for producing the double beam to direct the output of the cell alternately to devices 19, 19<SP>1</SP>, each of which thus receives the signals corresponding to one of the twin scanning beams (H.J., Fig. 5). Each of devices 19, 19<SP>1</SP> differentiates the input signals (K.L., Fig. 5) and separates the spikes corresponding to the leading edges of the particles from those due to the trailing edges. All leading edge signals are fed to coincidence devices 20, all trailing edge signals to 20<SP>1</SP>. Only if leading signals are received from both devices 19 and 19<SP>1</SP> simultaneously, will device 20 give an output (M, Fig. 5), while 20<SP>1</SP> gives an output (N, Fig. 5) only if it receives trailing signals simultaneously. The output signals from coincidence circuit 20 corresponding to the leading edges are then fed through a delay network 14 which delays it by an amount corresponding to 0.7 times the diameter of the particle (O, Fig. 5), to be measured and then applies it with the output of device 20<SP>1</SP> to a further coincidence circuit 15 which yields an output only when these signals substantially coincide (P, Fig. 5). Thus in Fig. 4, only particle g produces a pulse which is finally counted at 6 since the pulses produced by both the leading and trailing edges of both beams are substantially coincident and the separation between the leading and trailing edge pulse is within a certain tolerance of the required particle width. This tolerance is adjustable by varying the width of the output pulses of devices 20, 20<SP>1</SP> (Fig. 5). Specification 733,562 is referred to.