US2791377A - Apparatus for counting particles - Google Patents

Description

May 7, 1957 Filed June 25, 1952 H. A. DELL ET AL APPARATUS FOR COUNTING PARTICLES 5 Sheets-Sheet 1 q mg J SAMPLE 9 f P/c/r-l/P $CA/VNER 4 AMPLIFIER sou/m 5 g 1 sw/rcH b7 5 u/wr F HT 4- ,i! m AHPLIf/ER I s ar WOBBLE spor-wosazs 4/ v \15 FROM P/C/rup 45/ :6

INVENTORS Emlyn Jones AGENT May 7, 17957 H. A. DELL ETAL APPARATUS FOR COUNTING PARTICLES Filed June 25, 1952 SWITCH CONTROL V BUFFER FILTER SWITCH CONTROL sumo 3 5 Sheets-Sheet 2 FORWARD COUNTER BAG/ WARD INVENTORS Emlyn Jones Hugh Alexa? Dell BY W 17 AGENT May 7, 1957 H. A. DELL ETAL APPARATUS FOR coum'mc PARTICLES 5 Sheets-Sheet 3 Filed June 25, 1952 team SWITCH ca/mzouza. cowl-Pa GENERATO i/ f INVENTORS H Emlyn Joges I g exo er e BY AGENT May 7, 1957 H. A. DELL ET AL 2,791,377

APPARATUS FOR COUNTING PARTICLES Filed June 25, 1952 5 Sheets-She'et 4 4 J D/FF 9 v v J Q' PULSE come/b.5022 coy/77E? SCANNER $5411 7 i. .sw/reh' &

Il INVENTORS Emlyn Jones A Hugh Alexond Dell BY;%(WW

AGENT May 7, 1957 H, A, DELL ETAL 2,791,377

APPARATUS FOR COUNTING PARTICLES Filed June 25,- 1952 5 Sheets-Sheet 5 BLEL'ELL 27 1 J3 JB\ 5 58 J0 Q26:

1 MEL'I'MIYICAL 4 gum 5 I JYbTEM 6 :9 5

R ffilli. ECELL ,1 1, ear.

lNVE'NTORS Emlyn Jones Hugh Alexander Dell AGENT United States Patent APPARATUS FOR COUNTING PARTICLES 5 Hugh Alexander Dell and Emlyn Jones, Harley, England, assignors, by mesne assignments, to North American Philips Company, Inc, New York, N. l., a corporation of Delaware The invention relates to apparatus for counting particles and is particularly but not exclusively concerned with the assessment of the dust content of an air sample.

Assessment of the contamination of air or other gas by dust has hitherto been effected by preparinga sample taken under controlled conditions, such sample being for example a transparent plate on which the dust particles have settled and been fixed, or an enlarged photograph of such a plate, visually examining the sample under a microscope and counting the particles in a representative area. This is a long and laborious task and the results obtained from the same sample by difierent observers may vary widely particularly when the particles vary greatly in size.

For certain purposes it is only necessary to have. a total count of the number of particles in a given sample without size discrimination, and the object of the invention is to replace visual counting by automatic electrical counting apparatus giving such a total count.

If the particles in a given sample vary in size then there remains the difficulty, if some form of scanning of the sample is used, of ensuring that a large particle scanned more than once is only seen as a single particle so that a spurious count is avoided.

A further object or the invention is to ditficulty. v

With these and other objects in view and according to the invention, particle counting apparatus comprises means such as a cathode ray tube for scanning a sample of the remove this particles to be counted, pick-up means such as a photoy electric ccll co-operating with the scanning means for producing an electrical signal which is a measure of the presence and distribution of the particles, means, for avoiding multiple mounting of a large particle scanned more than once and counting means responsive to the derived signal or signals for giving an indication of the total number of particles scanned.

When the sample is in the form of a transparent plate bearing the particles or a photographic representation of the same to the same or a different scale the scanning means may comprise a cathode ray tube, the beam of which is caused, by suitable saw-tooth time bases, to trace out a raster of rectangular form and of such size as to illuminate the plate or such part of it as it is required to examine. By suitable optical means the, light from the cathode ray tube face passing through the sample-is caused to fall on a pick-up device such as a photo-cell Patented May 7, 1957 is actuated when one only of said beams encounters a particle.

With such an arrangement three modes of utilising the signals provided by the pick-up means may be distinguished. In the first mode the signal derived from one beam is employed, for example after inversion, to cancel the signal derived from the other beam when both beams substantially simultaneously encounter a particle overlapping :two or more lines of scan. Counting of such a particle or a small particle occupying only one scanning line may be elfected either-- (a) By utilising the signal derived from one of the beams when it alone scans the particle so that counting is effected either when the particle is first encountered by one of the beams or when it is last encountered by the other beam, or -(b) By utilising the signals from both beams (when they are not present substantially simultaneously) so that a particle is counted when it is first encountered by one of .the beams and when it is last encountered by the other beams, the total count thus obtained being divided by two.

In the second mode the electrical signals derived from the two beams may be of similar electrical polarity (for example positive going pulses) and are mixed so that when each beam only encounters a particle, a first type (or waveform) of signal is obtained whereas when both beams substantially simultaneously encounter a particle a different type (or waveform) of signal results. Such a different type of signal may be utilised to prevent actuation of the counter. In this case each particle will be counted twice as in the second of the regimes of the I first mode so that the total count must be divided by two.

In the first mode the signal provided by one beam may be regarded as a counting signal and the signal provided by the other beam as a controlling signal whereby the transmission of the counting signal to the counter (when a particle is encountered) is prevented when the two signals are present substantially simultaneously but is permitted when the counting signal only is present. In this case counting may take place either when the particle is first encountered or when it is last encountered whichever is most desirable in a particular case.

Since large particles overlapping two or more scanning lines may present a leading edge which is not normal to the direction of line scan, it is possible to arrange that one of said beams, for example the leading beam in the direction of frame scan, which may be termed the guard beam, should encounter the leading edge of the particle in advance (in the direction of line scan) of the other beam (the trailing beam in the direction of frame scan) which may be called the scanning or counting beam so that the output from the pick-up due to the guard beam shall be operative to prevent actuation of the counter slightly before the scanning beam encounters the leading edge of the particle.

According to a further feature of the invention there i fore the sample scanning means provides effectively two so that the output of the pickup device in time is an electrical representation of the presence or absence of particles in the scanning lines.

Since the particles may vary in size it is possible that a large particle may overlap two or more lines ofscan and according to a further feature of the invention the sample scanning means provides effectively two scanning beams scanning adjacent lines, the PiQk-llp means and associated amplifier or amplifiers and counting means being so arranged that when both beams encounter a spot in an appropriate direction and to the desired extent and at a suitable repetition frequency so that efiectively two scanning beams are .provided. In another form two flying spot light beams having distinguishably diiferent optical properties and scanning adjacent lines may be used.

particle the counting means is rendered inoperative-and In accordance with a further feature of the invention therefore, the sample scanning means comprises a flying only of said scanning beams. The flying spot light beam 1 may be provided in the known manner by a flying spot scanner of the cathode ray tube type or by a light source and mirror drum or drums or equivalent opticalelcments and the beam may be divided by any suitable optical element or elements, the beam differentiating means, comprising colour filters or polarising means, the photo-electric devices being correspondingly colour or polarisation sensitive. I Other features of the invention will be apparent from Alternatively the sample may be for example in the form of a photographic print from which light is reflected into the pick-up device and may be either a positive or negative image, that is to say the particles may appear as black marks on a white ground or as white marks on a black ground.

Apparatus for scanning such a sample and operating according to the first mode above mentioned is shown in one embodiment in Figure 2 in block schematic form. The scanning unit 3 comprises a flying spot scanner'of the cathode ray tube type, having the usual saw-tooth line and frame time bases to cause the electron beam to produce a rectangular raster on the tube face. Associated with the cathode ray tube is a pick-up device 4 such as a photo-cell, the particle sample being interposed between the cathode ray tube face and the device 4 so that as the 7 sample is scanned the photo-cell provides an electrical the following description of several embodiments which are given by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a representation of a portion of a particle sample;

Figure 2 is a block schematic diagram of particle counter;

Figure 3 is a circuit diagram of the switch unit of Figure 2;

of a first form Figure 4 is a block schematic diagram of a second form I v of particle counter; a 1

Figure 5 is a block schematic of a third form of particle counter;

Figure 6 is a circuit diagram of the pulse selector switch and switch control and count pulse generator units of Figure 5;

Figure 7 shows typical waveforms of signals applied to the circuit of Figure 6;

Figure 8 is a block schematic diagram of a fourth form of particle counter;

Figure'9 is a circuit diagram of the differentiator and pulse selector, and coincidence switch units of Figure 8;

Figure 10 shows typical waveforms of signals applied to the circuit of Figure 9;

Figure 11 is a diagrammatic representation of oneform of optical scanning system;

Figure 12 is a diagrammatic representation of a second form of optical scanning system;

Figure 13'is a diagrammatic representation of a scansignal which is a representation of the presence or absence of particles in the scanning lines. These scanning lines are indicated in Figure 1 by the horizontal lines a, b, c, d. With conventional single spot scanning and in the absence of size discriminating means the large particle will be scanned three times as the scanning beam traverses lines a, b and 0, thus giving rise to two spurious signals.

In order to avoid this and to provide such size discrimination, a modification of the known spot wobble technique used in television may be employed as indicated in Figure 2. A unit 5 providing spot wobbling potentials is connected to the scanning unit 3 and is so arranged that the cathode ray beam is deflected in the direction of frame scan by a distance equal to one line width. In addition and for a purpose which will be hereinafter described the cathode ray beam is also deflected in the direction of line scan by a predetermined amount suflicient to ensure that the beam in its deflected position first scans the side of a particle sloping in the direction of line scan as shown at 6 in Figure l. The repetition frequency at which the cathode ray beam is deflected is preferably many times the line frequency and in a practical example when the line frequency is 1000 c./s. the spot wobble repetition frequency may be for example 1 mc./s. The alternating potential giving rise to this deflection is preferably of square waveform.

For the purpose of the following description the cathode ray beam in its normal undeflected position will be called the scanning beam and in its deflected position the guard beam and the scanning will be assumed to be from left to right for line scan and top to bottom for frame ning system utilising a modified scanning electron microscope.

In the following description of several practical embodiments of the invention and with reference to the operation of certain thermionic valves the phrase open on the control (or suppressor) grid means that the potential of the control (or suppressor) grid is such as to permit passage of the electron stream. By the phrase cutoff on the control (or suppressor) grid is meant that the How of electron is substantially reduced or entirely prevented.

scan with reference to Figure 1.

Referring again to Figure 2 the spot wobble unit 5 is connected to two input terminals of an electronic switch unit 7. The pick-up device is also connected to another input terminal of the switch unit and two output terminals of the unit are connected respectively to two inputs of an amplifier 8. These two inputs represent respectively'the signals due to the scanning beam and guard beam and the amplifier is so arranged that when both signals are present one cancels the other thus effectively quenching the amplifier. The amplifier output is connected to a counter 9 of any suitableform.

A suitable circuit for the switch unit 7 is shown in a I, Figure 3 and comprises two thermionic valves 10, 11,

It is also to be understood that the size of the scanning I spot at the sample under investigation is preferably not larger than the smallest particle it is desired to count and may be smaller, and the distance between the centres of the scanning lines comprising the raster is notless than the diameter of the scanning spot.

Referring now to the drawing, Figure 1 shows for the purpose of simplifying the following description a portion of a sample plate containing only a small particle 1 and a large particle 2. The actual sample may be a transmeans.

parent plate having the particles adhering to it or it may be a photographic reproduction in the form of a lantern slide of an actual sample to the same or 'a different scale.

each having two control electrodes. These valves may be of the pentode type. The output of the photocell 4 is'connected to'the control grid 12 of valve 10 and to the control grid 13 of valve 11 and the spot wobbling square wave potential to the suppressor grid 14 of valve 10 The same square wave potential inverted is supplied to the suppressor grid 15 of the valve 11. The output of valve 10 is supplied to the input of the amplifier and the output of the valve 11 to the amplifier quenching In operation and assuming the scanning beam to be seaming line a in Figure 1 the output of the photocell will, for example increase as the scanning spot traverses the particle but this increase will be discontinuous due to the repetitive deflection of the spot to the guard position. Valve 10 will, therefore, pass a series of pulses since grids 12 and 14 of the valve will go positive together but valve 11 will deliver no output since there is no output from the photocell in the guar position, although theinverted wave applied to the grid 15 of this valve tends to open the valve at the appropriate instant. The output from valve 10 is preferably integrated and supplied to the input of the amplifier as a single pulse, which after amplification and squaring is supplied to the counter.

As the scanning and guard beams approach the second, large particle, the photocell will deliver an interrupted output due to the guard beam since this encounters the particle before the scanning beam and this photocell output will only appear in the output circuit of valve 11, valve 10 remaining cut-01f. When the scanning beam encounters the particle there will also be an output from valve 10 but by this time the output of the valve ill will have reached a valve such that the output from valve 10 is quenched so that the counter is not energised. Therefore, on its first scan of the large particle (along line a Figure 1) the counter is inoperative and this applies also when line b is scanned since the guard beam encounters the particle in line 0. On the next line, line d, however, the guard beam does not encounter the particle and the output from the photocell as the scanning beam scans line 1: causes operation of the counter in the same manner as with the small particle.

While the large particle is being scanned by both beams (for example lines a and b) the output of the photocell due to the guard beam may terminate before the output of the cell due to the scanning beam because of the shape or disposition of the particle. This is undesirable since it may give rise to a spurious count. It can be avoided by the provision of time delay means for example, in such a way that the amplifier remains quenched for a predetermined time after the termination of the control potential due to the guard beam.

In an alternative arrangement not illustrated, the ou puts from valves 19 and 11 may be difierentiated or otherwise converted in the known manner so that the output due to the scanning beam when it encounters the particle appears as a single pulse of short duration corresponding to the leading edge ofthe particle which pulse is supplied to the input of the amplifier as before. The single pulse due to the guard beam may be used, for example, to trigger a flip-flop circuit which returns to its stable state after a predetermined interval. The operation of the flipflop circuit may be used to quench the amplifier and the time delay may be such as to ensure that the scanning beam pulse has had time to appear. In this case the time delay may be much shorter than in the previously described arrangement.

in the above described arrangements the guard beam scans the line preceding that scanned by the scanning beam (in the direction of frame scan) and in the case of a large particle the count only takes place after the particle has been fully scanned. The inverse arrangement may alsobe employed in which the guard beam scans the line succeeding the line scanned by the scanning beam (in the direction of frame scan). In this arrangement the particle is counted when it is first encountered by the scanning beam, subsequent encounters, in the case of a large particle, producing no actuation of the counter.

A second embodiment of the invention operating according to the second regime of the first mode is shown in block schematic form in Figure 4. In this arrangement the signal due to the main beam is taken to switch SM and the signal due to the guard beam is taken to a similar switch SG which switches are normally in the position shown in the drawing. Each switch is controlled by asignal fed into its associated switch control unit (i. e.

SM at H and S6 at I). Outputs from these two switches are fed to a counter C capable of addition and substraction. The arrangement will be more clearly understood from a consideration of the way in which it operates. Considering the scanning of a large particle and assuming in this case that the scanning beam is in advance of the guard beam in the direction of frame scan, then, when the main beam encounters the particle, a signal is routed to the counter via the switch SM and the counter counts +1. Similarly whilst the signal lasts it is fed into the other switch control unit associated with switch SG at ;I and thus operates switch S6 t9 its other position. On the first scan no signal due to the guard beam is ofiered and when the signal due to main beam ceases, switch SG reverts to the position shown in the drawing. On the next scanning line and while assuming that themain beam encounters the particle first, the arrangement works as on the first encounter so that +1 is added by the counter (making +2 in all) and switch SG is moved to its other position. The signal due to the guard beam now appears before the signal due to the main beam ceases and this guard beam signal is fed by the switch S G to the subtracting input of the counter which thus adds 1 (making +1 in all). This operation is repeated as long as both beams substantially simultaneously encounter the large particle until the scanning line is reached when only the guard beam gives a signal. This signal is routed to the switch control associated with switch SM at H and alsc to the counter which adds +1 (making +2 in all for the large particle).

In a similar way a small particle is counted twice so that the total count of the complete sample must be divided by two to arrive at the total number of particles.

it is to be noted that it does not matter whether the signal due to the main or guard beams appear first. Whichever it is adds +1 to the count and moves the others switch over so that when the other signal appears it will add 1 and give the correct count.

A third embodiment of the invention operating according to the second mode above mentioned, will'now be described with reference to Figure 5 in which, to avoid elaboration, the scanning beam dividing and pick-up means are indicated as unit 16. They may be similar to those described with reference to Figure 2 or as described hereinafter.

From the pick-up means two electrical signals are produced, one due to the scanning of a particle by the scanning beam and the other due to the guard beam as in the preceding embodiment.

In this embodiment however, each signal is of the same polarity and is differentiated in the known manner as indicated at 17 and 18 so that if for example the output of the photocell due to the scanning beam as it traverses a particle is a positive-going pulse of substantially rectangular form it is converted into a short positive going pulse (or spike) following by .a negative going pulse (or spike). (See Figure 7a.)' The two differentiated signals are added together and the resulting mixed pulses are supplied to the input of a switch unit 11 having outputs and to a pulse selector 20, the output of which goes to one input of a switch controller and count pulse generator unit 21. Two other inputs to the unit 21 are connected to the switch unit 19 and a further output from the unit 21 is taken to a counter 9 of any suitable form.

T he operation of this arrangement will be more readily followed from a consideration of practical forms of the units 19, 2t) and 21 which will now be described with reference to Figure 6. In this *figure the switch unit 1% comprises two thermionic valves V1, V2 which are of the pentode type, the mixed signal above mentioned being supplied to the control grids of both valves. in the starting condition both valves are cut ofi on their control grids, the valve V1 is also cut oif on its suppressor grid and V2 is open on its suppressor grid due to starting condition. is so connected that the connection of these suppressor-grids to the switch control andpulse generator unit'21r' A r This unit'comprises a triodeivalve' V3 and a pentode valve V4 which together form abi-stable multivibrator, the anode of each valve being connected to the control grid of the other in the known mannerf Thefsuppressor grid of valve V1 is connectedto the anode of valve V3 and the suppressor grid of valve; V2 is connected to the anode of valve V4. The anode of valve V1 is connected to the control grid of valve V4 with av time delay network -R1Cr and the anode of valveV2 is, connected to the control grid of valve V3 withya'similar delay network 'R C2. In the starting conditionthe valve V3 is conduct- ..ing and valve V4 is cut off on its control grid but is open on its suppressor grid. Ifnow a differentiated signalof the form shown in Figure 7(a) representing the scanning of a small particle by one of the beams is applied to the control grids of ,valves V1 and V2, the valve V2 conducts to its anode (-since it is open on its suppressor grid) whilst valve ,Vr remains cut off on its suppressor grid. A negative -pulse is produced at the anode ofzvalve V2 and this is icommunicated to the control grid of valve V3 which is now cut oif, the rise of voltageon'its anode being communicated to the control grid of'valve'V4 so causing this valve to conduct and the multivibrator has flipped to its other stable -stat,e.*Due to this action the valve V1 is now openedonits suppressor grid due to the rise of potential at the anode or V3 and the valve V2is now cutoff on its suppressor gridas well as on its control grid. When the negative pulse (or spike) representing the trailing edge of the particle appears at the control grids of valves V1 and V2; since both valves are cut ofi. 1

This pulse is however supplied through the pulse selector unit 20 which comprises diode V5 to the suppressor grid of valve V4 causing this valve to be cut off on its .suppressor grid. 7 e V A greater proportion of the emission current of this valve then goes to the screen grid producing a negative pulse which is communicated to the counter to register the presence of the particle.

This action of the suppressor grid in substantially cutting otf the anode current of the valve V4 causes the rnultivibrator' V3, 'V4 to flop to its otherjstable state in which Va conducts and V4 iscut ofl? on its control grid but open on its suppressor grid. This is accompanied by the resetting of the valves V1 and V2 to their original It is to be noted that the diode V5 positive puls-es'are prevented from reaching the suppressor grid of valve V4 and only negative going pulse-s have any etfect on this electrode.

When a large particle is encountered by both scanning and guard beams a waveform of the general type shown .in Figure 7(b) is produced and the action of the units .19, and 21 with such a signal will now be described.

When the first positive going pulse representing the leading edge of the particle, as seen by one of the beams,

appears on the control grids of valves V1 and V2, the

-circuits operate to the new stable state as already described above in which condition the'multivibrator V3,

-Vs-has flipped and valves V and V2 are both out otf.

'When the second positive pulse representing the leading edge of the particle, as seen by the ot her beam, appears on the control grids of valves V1 and V2 the valve V1 conducts since it is open on its suppressor grid, but

valve V2 remains cut otf on its suppress-or grid. The drop of potential at the anode'of valve Vrcauses the valve Vito be cut olf on its control grid so that the multivibrator resets or flops to itsoriginal stablestate in I which Vs conducts. When the multivibrator is actuated inQthis way no negative pulse'appears on the screen grid ,of. .valve V4. The resetting ofthe multivibrator causes Vi to be' cut otf on its suppressor grid and V2 to be no action takes place passed to the control grid of valve Vs.

opened on its suppressor grid, both valves remaining cut ofi on their control grids. j r

When the first negative going pulse appears, it has no action on valves V1 and V2 but it is passed by unit 20 to the suppressor grid of valve V4. Since this valve is cut off on its control grid there is no accompanying decrease of screen potential as in the case of the small particle so that no pulse is emitted to the counter.

When the second negative pulse is received there is again no action and both units 19 and 21 are in their original stable state. It will therefore be seen that a count is only effected when a positive going pulse is followed by a negative going pulse produced either by the scanning or the guard beam. Thus each particle is counted twice and the total count must be divided by two to arrive at the number of particles actually scanned. When a mixed signal comprising two adjacent positive pulses is received then no count is made and the switch and switch control units are left in their original operative condition.

For carrying out the third mode of operation above mentioned, the apparatus shown in block schematic form in Figure 8 may be employed. In this arrangement the scanning and double signal producing means are indicated at 16 the signal due to the scanning beam being differentiated as in the preceding embodiment, and only pulses of one polarity permitted to pass to the input of a coincidence switch 22. The differentiating and pulse selecting means are shown at 23 in Figure 8. The signal due to the guard beam, which may be of substantially rectangular form, is also supplied to the coincidence switch and the arrangement is such that when the signal due to the scanning beam is alone present the counter 9 is actuated by a pulse delivered by the coincidence switch. When both signals are present substantially simultaneously the switch is prevented from being operated so that no signal is passed to the counter.

A circuit for providing this mode of operation is shown in Figure 9. The signal due to the main beam, after differentiation/is applied to the anode of diode Vs, the cathode of which is connected to the control grid of triode valve V7 which acts as a pulse inverter. Positive-going pulses of the differentiated signal are applied to the control grid of valve V7 but negative going pulses are prevented from appearing on this grid. Such positive pulses provide at the anode of V7 negative going pulses which are applied to a bi-stable multivibrator of the same general type shown in Figure 6. This multivibrator comprises a triode valve Va and a pentode valve V9, the anode of each valve being connected to the control grid of the other in the usual manner. The differentiated incoming signal is also applied to the suppressor grid of valve V9 and its control grid is connected to a negative H. T. line through a resistor 24, across which is shunted triode valve V10. The anode of V10 is therefore coupled to the control grid of V and has its cathode connected to the negative H. T. line. The signal due to the guard beam is applied to the control grid of V10. 7 I

The method of operation of this embodiment is as follows:

Initially the valve V8 is conducting and' the valve V9 is cut olf on its control grid and valve V10 is non-conducting due to the connection of its control grid to a negative bias line designated HT2 in Figure 9.

Considering first the operation with a small particle, the positive-going pulse representing the leading edge of the particle traverses valve V6 and causes V1 to conduct so that a negative pulse from the anode of this valve is Valve Va is thereby cut off and the multivibrator flips to its other stable state in which V2 is conducting being open both on its control grid and its suppressor grid. When the negative pulse representing the trailing edge of the particle appears itcannot efiect the control grid of V7 due to rectifier Vs but it appears on the suppressor grid of arenas? *9 valve V9 limiting the current to the anode, which causes the multivibrator to flop to its initial stable state. Substantially the whole of the emission current of the valve momentarily goes to the screen grid, the potential of which exhibits a negative pulse which passes to the counter.

When a large particle is encountered by both beams and assuming that the guard beam encounters the particle before the scanning beam, then the control grid of valve V will go positive :(assuming a positive-going pulse) so that V10 fully conducts and reduces the potential of the control ,grid of V9. When the positive pulse of the ditferentiated signal due to the scanning beam is applied the valve V7 and :the corresponding negative going pulse is applied .to valve V8 them-ultivibrator cannot flop since the control grid of valve V9 is being forceably held at a very lowpotential. Thus when the negative-going pulse .of the scanning beam signal is applied to the suppressor :grid of valve V9 nosignal appears on the screen grid and thus no count is registered. 7

In Figure 10 are shown (in pairs) the waveforms or the signals due to the scanning beam and the guard beam respectively for various types of particles. The scanning and counting of a small particle takes place when the waveforms shown at (a) are applied to the coincidence switch. The waveforms at (b) are produced when a long substantially parallel sided particle, sloping in the direction of line scan, is scanned by both beams, and at (c) when such a particle slopes in the opposite direction. At (d) and (e) are shown the waveforms produced when a particle having tapering sides is scanned by both beams, in the first case the sides are convergent in the direction of frame scan and in the second case divergent in the same direction.

.At (f) the guard beam alone produces a signal and in each of these cases (b to f) no signal reached the counter since the guard beam has either prevented the multivibrator being set or has returned it, to the condition in which it is non-responsive to the counting signal represented by the trailing negative-going pulse of the differentiated signal due to the scanning beam.

in the preceding description specific reference has only been made to flying spot scanning means of the cathode ray tube type with spot-wobbling means for producing and distinguishing the signals due to the two beams. It will be clear that the same result may be obtained by utilising a flying splot light beam produced by a flying spot scanner of the cathode ray tube type or by a light source and mirror drum or drums, or equivalent optical elements, the beam being divided by any suitable optical element or elements. The two beams thus produced can be given distinguishably different optical properties by being difierently polarised or of different colour and two photocells are then employed each responsive to one of the beams only.

One example of such an arrangement is shown diagrammatically in Figure 11 in which a light source 25 in conjunction with a condenser lens or lens system 26 illuminates a double-hole diaphragm 27. Colour filters 23 and 29 which may be respectively red and blue are placed over each hole and an objective lens or lens system 30 projects the images of the two spots of coloured light into a scanning unit 31 which may be for example of the Well-known mirror drum type. The emergent raster-producing beams are caused to scan the sample 32, the transmitted (or reflected) light being picked up by one or other of the photocells 33, 34. Each cell may be inherently responsive to only one of the colours and/or colour filters 28a, 29a may be employed to ensure that one cell only sees the red beam and the other only the blue beam. If the particles are of such colour or colours that normal colour filters cannot be used with success, interference type filters may be employed to distinguish the two beams.

In another alternative the raster-producing light beams,

however, initially produced, may be distinguished by being differently polarised that is 'to say they may be plane polarised at an angle to one another or differently circularly polarised, the photocells each being made sensitive to one of the beams only. Such means are wellknown and do not need further description.

In Figure '12 is illustrated diagrammatically another arrangement which comprises a flying spot scanner 35 of the cathode ray tube type providing a raster produced by a single electron beam in the normal manner.

The double scanning beams for scanning the sample are produced by a split lens system 36 one (or both) halves of the system being adjustable to enable the mutual separation of the beams to be accurately determined.

As previously indicated the beams maybe distinguished by being differently coloured or of difie'ren-t polarisation, colour filters or polarising elements generally indicated at 37, 38 being disposed in the path of the beams between the lens system 36 and the sample, corresponding filters or polarising elements 370, 38a being disposed in front of each photocell pick-up 39, 40. The two separate electrical signals may then be employed as hereinbefore described to provide a total count of the number of particles in the sample.

In yet another alternative form diagrammatically illustrated in Figure 13 the sample suitabiy prepared is scanned directly by an electron beam. The drawing shows diagrammatically a scanning electron microscope of the known type, having a cathode 4t, and an anode 42. provided with aperture 43. The anode may therefore be considered as the source of a beam of electrons which is focussed on the image plane 44 by an electron lens or lens system 45'. The electron beam is deflected by a deflecting system 46 to which the spot-wobbling potentials are supplied so as to produce a double beam raster at the image plane 4-4. An electron field lens 47 at this plane causes the electrons emergent from the plane to be concentrated on the aperture of an electron reducing lens 48. A reduced image of the raster at the image plane 44 is thus formed at the sample plane 49. The sample 50 is prepared as a thin film of collodion or the like which the electrons can penetrate and in which the film is opaque to electrons in areas corresponding to the particles. As the sample is scanned by the electron beams the potential of a collector anode 51 will vary depending on the presence or absence of the electron beams and this varying potential can be employed as hereinbefore described to obtain a count of the total number of particles. With such an arrangement it is possible to obtain a count of particles which are smaller than those which can be resolved by an optical microscope and this may have important uses in, for example the biological field.

It will therefore be understood that the invention may be employed for counting particles of any kind having a random distribution on a surface and may be used, for example, for counting bacteria or finely comminuted materials of any kind provided they are suitably presented for the particular form of scanning used.

What we claim is:

1. Particle counting apparatus comprising scanning means for providing effectively two scanning beams simultaneously scanning adjacent paths of a sample of the particles to be counted, pick-up means cooperating with said scanning means for producing an electrical signal which is a measure of the presence and distribution of the particles, counting means coupled to said pick-up means and responsive 'to the derived signal for giving an indication of the total number of particles scanned, and means rendering said counting means inoperative when both beams encounter a particle substantially simultaneously but actuating said counting means when only one of said beams encounters a particle whereby multiple counting of large particles is avoided.

2. Apparatus as set forth in claim 1 wherein said scanning means for providing effectively two scanning beams includes means for providing a main scanning beam and means for providing a guard beam, and means for maintaining a predetermined relative positioning between said beams to cause said guard beam to scan along .a line adjacent to the line scanned by the scanning beam and in advance of it by a predetermined distance in a direction parallel to the line scanned by said main scanning beam.

3. Apparatus as set forth in claim 1 wherein said scanning means comprises a flying spot scanner of the cathode-ray tube type and further including spot wobbling means coupled to said scanner for repetitively deflecting the electron beam in a predetermined direction at a repetition frequency greater than the line frequency for producing effectively two scanning beams.

4; Apparatus as set forth in claim 1 wherein said scanning means provides a flying spot beam and further including beam dividing and ditferentiating means for producing two scanning beams having distinguishably different optical properties, said pick-up means comprising two photo-electric devices each responsive to only one of said scanning beams.

tube type.

6. Apparatus as set forth in claim 4 wherein said scanning means includes a light source and a mirror drum.

7. Apparatus as set forth in claim 4 wherein said beam dividing means comprises a double-hole diaphragm and a lens system.

8. Apparatus as set forth in claim 4 wherein said beam dividing means comprises a split lens system.

9. Apparatus as set forth in claim 4 wherein said beam differentiating means comprises color filters, said photoelectric devices being correspondingly color sensitive.

10. Apparatus as set forth in claim 4 wherein said beam differentiating means comprises polorizing means, said photo-sensitive devices being correspondingly polarization sensitive.

11. Apparatus as set forth in claim 3 further including an amplifier coupled at one end to said counting means and switching means interposed between the other end of said amplifier and said pick-up means,

and wherein one of said two scanning beams is a main scanning beam and the other beam is a guard beam and wherein said spot wobbling means comprises an oscillator generating an alternating potential of substantially square waveform, .said alternating potential being applied to said switch means, the arrangement being such that a signal generated by the pick-up means due to the main scanning beam is passed to one input of said amplifier and that due to the guard beam to another input of said amplifier whereby when both signals are present substantially simultaneously one cancels the other so that there is no output signal from said amplifier to said counting means.

12. Apparatus as set forth in claim 2 further including means for electrically differentiating and mixing electrical signals derived from said main scanning beam and said guard beam, a switch control and count pulse generator means coupled to the input of said counting means, a switch unit interposed between said differentiating and mixing means and said control and generator means, and pulse selector means coupled between said differentiating and mixing means and said control and generator means.

13. Apparatus as set forth in claim 2 further including means for differentiating the signal derived from said main scanning beam and a coincidence switch unit interposed between said differentiating means and said counting means, the signal from said guard beam being applied as a pulse of substantially rectangular form to an input of said switch unit.

14. Apparatus as set forth in claim 1 wherein said scanning means comprises an electron microscope having a source of an electron beam adapted to scan said sample and wherein the sample is opaque to electrons only in areas corresponding to the particles, said pick-up means being a collector anode positioned on the opposite side of the sample with respect to said source of the electron beam.

Photo Cell Counts Blood Cells Faster, Popular Science, May 1949, page 170.

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