Method and Apparatus for Inspecting Liquids in Transparent Containers

This invention relates to a method and apparatus for inspecting liquid filled transparent containers of any size for particulate contamination, especially such method and apparatus for detecting the presence of particulate matter in ampoules, bottles, flasks, syringes or vials.

The art of inspecting liquid filled containers, especially vials and ampoules, for the presence of particulate matter is a relatively old and crowded one as exemplified by U.S. Pat. Nos. 2,132,447 (Stout); 2,426,355 (Kellogg); 2,531,529 (Price); 3,029,349 (Schell); 3,217,877 (Honjyo et al); 3,292,785 (Calhoun); 3,415,997 (Vinzelberg et al); 3,496,369 (Makino et al); 3,528,544 (Noguchi et al); 3,598,907 (Drinkuth); 3,627,423 (Knapp et al); 3,777,169 (Walter et al); 3,830,969 (Hofstein); and 3,914,058 (Knapp et al). In each of the foregoing U.S. Patents, and also in the present case, the inspection process includes the steps of rotating a container to be inspected, suddenly stopping the rotation of the container, and then inspecting the illuminated rotating fluid for moving particles. This simplifies detection of particles since a moving illuminated particle can be easily detected and is readily distinguishable from the stationary reflection which results from container imperfections.

Many prior art systems have been developed which inspect liquid filled transparent containers with photo transducers for both large particles (e.g. glass fragments) and fine particulate matter (below 50 microns). The rotating liquid eddy or vortex produces a change in the illumination signal, which is indistinguishable from a moving particle signal. The prior art systems therefore had to wait for the vortex to disappear before the photo transducer could make an accurate reading of particulate movement. However, when small particles are present, there is a considerable possibility that they will rise to the meniscus and be flung to the wall of the container by centrifugal force, where they could cling and thereby avoid detection. The prior art systems, as for example, U.S. Pat. No. 2,531,529; were able to eliminate the effect of the vortex signal by blocking it until it was no longer appreciable. However, none of the prior art systems recognized that the interference due to the vortical meniscus decay at the liquid-container interface could be corrected thereby allowing a faster and more accurate inspection of the rotating meniscus.

Prior art systems have generally used only one or two photo transducers each of which are directed at the entire rotating solution, or as in U.S. Pat. No. 2,132,447 a plurality of photo transducers aligned in the vertical direction to allow different portions of the container to be given preferential treatment. If specular reflections from either the surface of the liquid or optical flaws in the container are of sufficient intensity, a photo transducer could saturate at its peak current. If this occurred, it would be impossible to detect any change in illumination due to particle movement at the liquid volume which was monitored by that photo transducer. Since the movement of the particles is a strongly horizontal one, if one of the vertically aligned photo transducers was so saturated, any particle in the liquid zone of that particular photo transducer would be missed.

Prior art systems have generally monitored the rotating liquid along one or two viewing axes with a single image plane being formed at each axis. If a light or heat sensitive composition was involved the intensity of the light source would have to be minimized with a resulting reduction in the range of sharp focus for the particles of interest. When the range of sharp focus of the image plane was less than the diameter of the article being inspected, it was necessary to increase the inspection time to insure that a particle of interest would be detected as it passed through the range of sharp focus.

An advantage of the invention is to provide an improved system which can inspect the entire liquid in a transparent container including the rotating meniscus for particulate contamination in a faster and more reliable manner than previous methods.

A further advantage of the invention is to provide an improved system which can inspect a liquid in a transparent container for particulate contamination with the same particle size sensitivity independent of the container size and specular reflections from the liquid surface or from marks and defects in the container.

Another advantage of the invention is to provide an improved system which can inspect light or heat sensitive compositions in a faster, more reliable manner than previous methods.

In the method and apparatus of the instant invention, the container to be inspected is illuminated by a source of radiant energy. Any source of radiant energy such as single or multiple optical glass fiber bundles which are used in U.S. Pat. No. 3,627,423; or ultraviolet or infrared light could be used. The preferred mode of illumination is as described in the above noted U.S. Pat. No. 3,627,423 to Knapp et al, which is herein incorporated by reference. A means to hold the light source at a constant intensity is essential whenever a calibrated size response is needed, as in any parenteral drug inspection in which relative size of the particulate matter is of importance.

The liquid filled container is rotated at a speed sufficient to rotate the liquid and any particles therein, but below the condition that will cause cavitation and bubbling of the liquid. The rotation of the container is then suddenly stopped. The rays emanating from the radiation source strike the internal surface of the container and are directed towards the particles moving in the solution, thereby illuminating the particles. The illumination signal generated by the moving particles is distinguished from the constant illumination signal generated by stationary scratches, flaws, stains or other optical defects in the walls of the container or by particles, dust, lettering or art work that may be on the outside of the container. The rotation of the container is preferably about a vertical axis giving any particles therein a strongly horizontal motion. It is, of course, recognized that a non-vertical axis could be used, and if such were the case the motion of any particles therein would be strongly directed in planes which were perpendicular to the axis of rotation.

The article delivery means can consist of any one of the numerous feed means known in the art, e.g. the rotating turret of U.S. Pat. No. 3,528,544. The liquid filled container can be rotated upon reaching the inspection station. It has been found, however, that the meniscus is periodically displaced to each side due to the relatively fast intermittent motion of the container on the article feed means, and a more reliable inspection is achieved if the rotation of the container is initiated on the article delivery means during the transport of the article toward the inspection station. By rotating the liquid while it is moving on the article feed means, the meniscus is able to more quickly approach the preferred inspection levels.

The motion imparted to any particulate contaminants which are in the container is a strongly horizontal one as a consequence of the spinning action. Utilizing this motion the container image as transmitted by an image forming means, is dissected, using a plurality of fiber optic bundles, into a number of vertical columns which extend from the bottom of the container through the top of the liquid meniscus. The columns are further subdivided into a plurality of unit rectangular areas with each unit area being monitored by a different segment of a fiber optic bundle. The illumination value detected by each fiber optic bundle segment is electronically translated into a voltage signal using a constant sensitivity photo transducer. The signals from the photo transducers are processed through a differentiator so that only changes in illumination are recorded. Therefore constant illumination signals from container imperfections, lettering or art work, or a constant saturation signal, where specular reflections are encountered, are not recognized. Any constant sensitivity photo transducer, i.e., which generates an output signal which is substantially directly proportional to the illumination value viewed, within its operating range, could be used. By so utilizing a number of vertical columns the image of particulate matter which moves past a column with a saturated sensing means will be detected as it passes through a column with an unsaturated sensing means and a particle which passes through a unit volume obscured due to lettering or art work on the container will be detected as its image passes through the area associated with an unobscured unit volume. It is, of course, recognized that non-vertical columns or any other appropriate detection areas could be used so long as the detection areas were aligned to intersect the path of particle motion.

The sensing device consisting of the fiber optic bundles and the array of photo transducers further divides the image of the rotating liquid into block areas which permit the central meniscus volume and the meniscus edge images to be separately monitored from the remainder of the rotating liquid. The lower portion of the rotating liquid image, below the maximum meniscus displacement, may be immediately monitored for particulate contamination at the lower unit rectangular areas of the vertical columns, since any changes in illumination occurring in the submeniscus volume image would only be from moving particulate matter. The volume of liquid which contains the center of the meniscus may be monitored shortly thereafter, when it has substantially reached its rest position and the interfering illumination signal which it generates is negligible. This time lag is necessary for small particle identification since the interfering illumination signal which is first generated by the center of the meniscus would make identification of small moving particles virtually impossible, and the meniscus decay correction step as discussed below, if extended to include the entire meniscus could result in an error signal as great as a small moving particle signal.

The circumferential edge of the meniscus which is in contact with the wall of the container is slower to approach its rest position because of the liquid's tendency to adhere to the wall of the container. However, in applicants' improved system, the image of the edge of the rotating meniscus can also be monitored for particulate contamination by compensating for the interfering illumination signal which is generated by the decaying meniscus as it approaches its rest position.

The peripheral edge of the meniscus will decay with essential uniformity to its rest position, therefore the illumination signal which is generated by the meniscus image decay will be substantially uniform along the peripheral edge. Accordingly, if the illumination output signal originating in one of two volumetric portions of the meniscus image edge, in which the illumination or voltage signals generated by the meniscus image decay would be substantially similar, is subtracted from the illumination output signal originating in the other portion, the interfering output signal due to the meniscus image edge decay is substantially eliminated, and any signal which is recorded will be due to particulate movement. The meniscus image edge could be monitored in this manner for particulate contamination as soon as the center of the meniscus has substantially reached its rest position as discussed above.

The maximum error signal which results from the meniscus correction step is of small magnitude and can be easily distinguished from the particulate movement signals. If an even greater sensitivity is desired, however, the inspection of the image edge can be delayed until shortly after the meniscus center inspection. By waiting this slight additional time, the maximum error signal which can result from the meniscus correction step will be even smaller. Accordingly, particles which generate smaller signals can be monitored.

In the inspection of parenteral solutions for particulate contamination, the preferred embodiment delays the start of the inspection of the meniscus image edge until shortly after the start of the inspection of the central meniscus volume. The signal from each photo transducer is amplified as required, differentiated and summed. The differentiated signals indicate whether any change in illumination occurs in the image of each of the unit areas instrumented. Change of unit area illumination after the container is at rest is due to particle movement. The accept/reject decision is based upon the comparison of a standard reference signal to a composite of the magnitude of the differentiated signals, the integral of all such signals, any signal detected over a certain predetermined value or any combination of the above.

FIG. 1 is a side elevational view of the apparatus showing the array of detectors and the print-out chart.

FIG. 2 is an enlarged schematic view showing the passage of light through the container and liquid, with the array of detectors located in the shadow zone.

FIG. 3 is a partial sectional taken along the line 3--3 of FIG. 1 showing the array of photo transducers.

FIG. 4 is a partial sectional taken along the line 4--4 of FIG. 1 showing the optic fiber bundles.

FIG. 5 is a front elevational view of an ampoule image showing the maximum meniscus displacement and the meniscus level at different times within the block areas.

FIG. 6 is a front elevational view of an ampoule image showing the relationship of the assigned unit areas, monitored by each photo transducer, within the block areas.

FIG. 7 is a typical graph resulting from a single inspection of an acceptable ampoule which has a very low contamination of small particles.

FIG. 8 is an illustrative graph of an unacceptable ampoule having a slight contamination of small particles with a few larger particles.

FIG. 9 is a typical graph of an unacceptable ampoule having a high contamination of both small and large particles.

FIG. 10 is a typical graph showing the relationship of the illumination received by a constant sensitivity photo transducer to the signal transmitted by the photo transducer.

FIG. 11 is a block diagram showing a representative electrical circuit for processing the signals from each of the unit areas.

FIG. 12 is a preferred embodiment which uses two detector arrays with plural stepped optic bundles for improved depth of focus.

FIG. 13 is a partial sectional taken along the line 13--13 of FIG. 12.

FIG. 14 is a sectional taken along the line 14--14 of FIG. 12.

Referring to FIG. 1 and 2, cables 2,4 consist of a fiber optic bundle 8 within a protective shield 6. The ends of the cables 2,4 are open with one end of each connected to a light source 10. The light from the source is transmitted through the fiber optic bundle 8 to the opposite ends of the cables 2,4 which ends are mounted in fixed position behind the baffles 12,14. Any light source having an intensity sufficient to illuminate the liquid and particles, and compatible with the viewing means, may be used. A 150 watt 21 volt incandescent lamp, for example, General Electric type EKE, has been found suitable. The light source 10 is maintained at a constant intensity by circuit 11. Circuit 11 consists of a fiber optic light intensity sampler 13 which continually monitors the intensity of illumination of the light source 10, a photo transducer 15 for generating a voltage signal 17 proportional to the light intensity monitored by sampler 13, and a servo amplifier 19 for controlling the current 21 to the lamp 10, based upon a comparison of voltage signal 17 to a stored reference level signal 23.

Turntable 16 is mounted on shaft 18, driven by motor 20 and mounted in a fixed position on support 22. Shaft 18 is movable vertically through the lever 24 pivoted, at 26, to shaft 18 and pivoted, at 28, to fixed support 30. Sealed container 50, containing solution 52, is placed on turntable 16 and the turntable is lifted by depressing lever 24. As turntable 16 and container 50 reach the position shown in phantom lines in FIG. 2, microswitch 54 is closed and, through timer 56, actuates motor 20 for a preset time interval to rotate turntable 16, container 50 and solution 52. The speed and time of rotation is sufficient to cause solution 52 and any particles therein to rotate but is below the speed of cavitation, bubbling and entrapment of air in the solution. Instead of the manual lever system it would, of course, be an obvious expedient and within the scope of this invention to raise the bottles on the turntable automatically.

At the end of the container spin time, the container image transmitted by lens 33 is immediately dissected by the input end of a plurality of downwardly directed light conduits, e.g. fiber optic bundles 31, with each light conduit being operatively connected at its output end to one of the photo transducers 34 of the photo transducer array 32. In a single axis inspection, as shown in FIG. 2, the fiber optic viewing means is preferably directed downward at an angle of approximately 17.degree.-37.degree. to the horizontal with the most preferred angle being 27.degree.. Each of the fiber optic bundles are directed to a different unit volume of the rotating liquid, for purposes more apparent hereinafter. Each photo transducer is responsive to an illumination value within its respective unit volume which is within its operating range (see FIG. 10). All of the signals from the photo transducers are processed through an electronic circuit 36 and the resulting composite output signal is graphically printed at recorder 40. Recorder 40 can be used by an inspector to determine if the particle content of the container is acceptable. Of course, if a completely automatic system is used recorder 40 could be replaced by any standard signal processor which had a stored acceptable criteria for the container under inspection, as will be discussed hereinafter in regards to FIG. 11.

In the method and apparatus of the preferred embodiment of the instant invention, the surface of any particle in the solution is illuminated both by direct and reflected light. The light means directs at least two light beams at the liquid-filled container along light paths angularly disposed to the line of sight extending from the fiber optic bundle to the container so that the light paths intersect in the container and illuminate substantially all of the liquid and form an angular shadow zone wherein the fiber optic bundle viewing means is situated. This phenomenon is illustrated in FIG. 2 and described in the aforementioned U.S. Pat. No. 3,627,423.

FIG. 3 shows a partial cross sectional view of the photo transducer array 32. Each of the photo transducers 34 would be operatively connected to one of the fiber optic bundles 31.

FIG. 4 shows a partial cross sectional view of the fiber optic bundles 31. The bundles are shown to be rectangular at their viewing end although any shape which enabled the container image to be dissected along vertical scan lines could be used. The fiber optic bundles would, of course, be bunched at their opposite ends to conform to the shape of their respective photo transducers 34.

FIG. 5 shows the shape of the meniscus image at its maximum displacement, at rest, and at various times during the rotation of the liquid in the ampoule. Level "A" is representative of the shape of the meniscus image at its maximum depth and level "D" is representative of the shape of the meniscus image when the solution is at rest. Levels "B" and "C" are representative of the shape of the meniscus image at two intermediate times, with the time interval between levels "B" and "C" being approximately the same as the time interval between levels "B" and "A". As can readily be seen from FIG. 5, the center portion of the meniscus very rapidly approaches its rest position when the container spin is stopped, while the edge portions of the meniscus image move much slower due to the tendency of the fluid to cling to the walls of the container. As a result of this characteristic of the meniscus decay, the center portion of the meniscus can be monitored soon after the container spin is stopped. However, by using the new improved system of the instant invention, even the meniscus edge areas can be meaningfully monitored for particulate contamination, while the interfering output signal from the decaying meniscus is still relatively large, by compensating for the meniscus decay in a manner hereinbelow described in relation to FIG. 6. It has further been found that the meniscus will more quickly approach the preferred inspection levels depicted in FIG. 5 if the liquid filled container is rotated during transport to the inspection station.

FIG. 5 further shows the four block areas into which the ampoule image is divided by the array of detectors. The blocks are selected so that the container may be inspected even though there are error signals originating from the vortex and the resulting meniscus decay. Block I is the area of the container image which extends downward from a point just below the maximum meniscus displacement to the bottom of the container. Block II is the upper central portion of the rotating liquid volume image and extends from a point above the maximum meniscus creep on the wall of the container downward to the upper edge of Block I. Blocks III and IV are the image edge areas of the rotating meniscus volume and are positioned above Block I, and are adjacent to and on opposite sides of Block II.

The inspection of the container is started the moment the container spin is stopped. The continuous monitoring of Block I is started immediately since no interfering output signal originates in Block I. Block II is continuously monitored from the time the interfering output signal due to the decay at the mensicus center is negligible, in comparison to the signal which would be generated by the smallest moving particle of interest, e.g. level "B". Blocks III and IV will continue to each have a substantial interfering output signal due to the meniscus decay even after the signal from Block II has become negligible. The instant system electronically substracts the signal which is generated in Block III from the signal generated in Block IV. This can be accomplished for example, by electronically inverting one of the signals as shown in FIG. 11. Since the interfering output signal due to the meniscus decay is substantially the same in both blocks, the signal which remains after the subtraction step will be due primarily to particulate movement, although a slight error signal is possible even after the subtraction. The maximum error signal which is possible after the subtraction will decrease as the meniscus further decays, e.g. from level "B" to level "C". Therefore, if an inspection for very small particles is required, it may be necessary to delay the monitoring of Blocks III and IV until the maximum possible error signal after the subtraction step is less than the signal which would be generated by the smallest particle of interest.

FIG. 6 shows the container image block areas further divided into a plurality of vertical columns, e.g. A, B, . . . , F, with each vertical column extending the length of the rotating liquid volume. Since the motion which is imparted to each of the contaminant particles is a strongly horizontal one, it is essential that any individual vertical column be monitored along its entire length so as to prevent a particle from going undetected while rotating in an unmonitored zone. Each of the vertical columns are further subdivided into a plurality of unit rectangular areas, e.g. A.sub.1 . . . A.sub.7 ; F.sub.1 . . . F.sub.7. Each of the rectangular areas are monitored by one of the fiber optic bundles. The fiber optic bundles allow the unit areas to be clearly defined while preventing an unmonitored zone to occur between adjacent vertically spaced rectangular areas.

Since each of the photo transducers is of constant sensitivity, they will record a signal substantially proportional to the illumination viewed. This signal can be the result of either specular reflection from the container, a flaw or mark on the surface of the container, meniscus decay, liquid surface specular reflection, or particulate movement. These signals are further processed through a differentiator so only signal changes are recorded. A composite signal from each of the block areas is then compiled. That is, each container image block area will generate a composite of all of the differentiated signals coming from that respective block area. Container flaws or marks and specular reflections will generate a constant signal and will not be recorded. When specular reflections are sufficiently large the corresponding photo transducer will saturate at a constant saturation signal which will not be recorded, see FIG. 10. If a photo transducer is saturated, a moving particle image which may be present in the respective area will not go undetected, since it will be detected when it passes through an adjacent unsaturated unit rectangular area. The meniscus decay image which occurs in Block II will generate a signal but it will be negligible and easily distinguishable from a moving particle signal. The meniscus decay image which occurs in Block III will generate a composite differentiated signal which will be very close to the composite differentiated signal from the meniscus decay image which is generated in Block IV. Subtraction of the composite differentiated signal of Block III from the composite differentiated signal of Block IV will therefore compensate for the meniscus decay signal within the liquid volume monitored by these Blocks. The remaining composite signal after the subtraction of the signals from Blocks III and IV will indicate only the moving particulate matter present in the meniscus edge. The slight difference in the differentiated signals which may result from the meniscus decay seen in Blocks III and IV will be negligible and will accordingly be easily distinguishable from any signal resulting from particulate movement. Any number of detectors could be used and this number would, of course, be highly dependent on many factors including the container size, the light intensity and the degree of particle sensitivity desired.

FIGS. 7-9 are schematic views showing the results of a single inspection of a container. As can be seen from the figures, wherein signal magnitude is displayed against time, when the ampoule is first spun a considerable amount of interference is detected, but as the spinning liquid approaches its maximum, the interference signal decays to an insignificant noise level. Each signal which is observed after the start of the monitoring time is indicative of a change in illumination resulting from a moving particle, with the height of the peak and the total area of the signal under the peak being indicative of the type and size of the particle observed. For example, a large spherical dark particle will probably generate a short and wide signal, while a small highly reflective particle will probably generate a high, thin signal. Also a large thin particle will generate a signal according to how much of a surface it presents to the photo transducer. If the photo transducer only "sees" its edge then a small signal will be recorded. Since this is a continuous monitoring system these apparently false misrepresentative signals will not go undetected for the large thin particle will be detected when it eventually presents its larger surface to one of the photo transducers. Of course, signals need not be graphically recorded. Any standard signal processor with a stored acceptable criteria which had, for example, an integrator, comparator, peak detector and adder or any combination thereof could be used to accept or reject the container.

FIG. 10 shows a graph of the signal generated by a typical photo transducer vs. the actual illumination viewed in the zone monitored. As can be seen from the graph, the photo transducer will not generate a signal until the illumination is above a certain minimum level called the dark current or noise level. Similarly the photo transducer will saturate at a certain peak illumination and further readings will go undetected. Since a constant sensitivity photo transducer is used, the operating range between the dark current or noise level and the saturation limit results in a substantially straight line.

FIG. 11 shows an electronic circuit (e.g. 36 in FIG. 1) which could be used to practice the preferred embodiment of the invention. In FIG. 11 the illumination value within each unit rectangular area, as detected by each fiber optic bundle, is monitored by a single constant sensitivity photo transducer which generates a signal when the illumination is greater than the lower detection limit as shown in FIG. 10. This signal, if any, is then differentiated and amplified. The amplified signals from each unit area are then summed along the vertical column portion within the respective block and the composite summed vertical signals are then summed throughout their respective blocks. Each of the block summing amplifiers are started at the appropriate time, as discussed above, through a control signal. Once the block summing amplifier is operating, it continually amplifies signals which can be processed and compared to a standard acceptable criteria upon which an accept/reject decision can be made. The composite meniscus edge signal from Blocks III and IV is formed by continuous electronic subtraction of the two signals. This can be accomplished by inverting one of the signals (e.g. positive to negative) before the final summation step. The signal processor can be any known device for that purpose, such as, a comparator for any maximum peak height over a certain level, an integrator for all signals, a peak detector and adder for all signals, or any combination of the above. It is, of course, realized that this electronic circuit could be modified in numerous ways without changing the spirit or scope of the invention. For example, the amplifier could be omitted, the signals could be first summed horizontally, or the differentiation could take place after one of the summing operations.

FIG. 12 shows a stationary container 50 with the liquid rotation in the direction of the arrows. The image of the rotating liquid, as transmitted by lens 33, is inspected by a plurality of stepped fiber optic bundles 35 along two viewing axes in a manner to be hereinafter explained. Any number of steps in a given fiber bundle may be used; for illustrative purposes three steps x, y, z have been shown in FIGS. 12-14. Each verticle step of the fiber optic bundle 35 is bunched to conform to the shape of a respective photo transducer 34 of the array 32, e.g. steps x, y, z are all bunched in one photo transducer.

FIG. 13 shows a partial cross sectional view of the stepped fiber optic bundles 35 similar to FIG. 4.

FIG. 14 shows a cross sectional view of the stepped fiber optic bundles 35 similar to the unstepped fiber optic bundles shown at the left of FIG. 1.

The stepped fiber optic bundles 35 have particular utility when the liquid contents under inspection are either light or temperature sensitive. When this occurs, the intensity of the light source must be kept to a minimum to prevent deterioration of any active ingredient in the liquid. The lower light intensity requires a decrease in the lens "f" stop which inherently decreases the range of sharp focus of the system e.g. at a given light intensity, an f8 lens has a smaller range of sharp focus for the particles of interest than an f16 lens. In the past when this situation occurred, additional amplification of the signal was required which decreased system capability to detect small particles. It was also necessary to increase the inspection time to insure that any contaminating particle moving in the liquid would have sufficient time to be detected as it passed through the reduced zone of sharp focus.

With the improved system, it is now possible to inspect the entire liquid contents much faster and with a higher degree of reliability than was heretofore known. The stepped viewing means can also be used to inspect large containers in which the range of sharp focus of the lens 33 is considerably less than the diameter of the container under inspection. An increase in the light intensity could be used to increase the range of sharp focus of the lens 33 (provided the contents were not light or temperature sensitive), but the cost could be prohibitive since the range of sharp focus of the lens for the particles of interest is proportional to the square of the light intensity, e.g. to triple the range would require a ninefold increase in light intensity. Any number of image planes about which a suitable zone of sharp focus exists could be used depending on the container size and the desired light intensity.

Other equivalent means could, of course, be used to increase the number of sharply focused image planes. For example, (1) a plurality of lens and mirror combinations could be used to focus the image of a plurality of viewing planes on a single image plane using the hereinabove described unstepped viewing means or (2) a series of partially and totally reflective mirrors (e.g. image detector splitters) in combination with a plurality of unstepped viewing means could be used to focus the image of a plurality of viewing planes on a plurality of image planes.

FIGS. 12-14 further illustrate a preferred variation in which two viewing axes are used. The viewing axes are preferably 60.degree.-120.degree. apart with an angle of 90.degree. being most preferred. As in FIGS. 1 and 2, the viewing means should be located in the shadow zone if the preferred previously discussed lighting means is used. The plural viewing axes permit a faster more reliable inspection since it is easier to detect a particle which is moving essentially parallel to a viewing plane than it is to detect a particle moving essentially perpendicular to a viewing plane. Plural viewing axes could, of course, also be used with any of the viewing means modifications previously discussed. In a dual axis inspection system, as shown in FIG. 12, the fiber optic viewing means is preferably directed downward at an angle of approximately 5.degree.-25.degree. to the horizontal with the most preferred angle being 15.degree..

The invention is not limited to the embodiments described above, but all changes and modifications thereof, not constituting departures from the spirit and scope of the invention are intended to be covered by the following claims.