US 3917947 A
This apparatus inspects products for presence of foreign fragments while the product is in motion and includes a timing screw driven by a synchronous motor, an X-ray source for intermittently pulsing an X-ray beam which is in phase with the motor in response to actuation by the product disposed at an inspection zone, an image intensifier, monitor, photomultiplier, conventional circuitry for defining inspection zone on the product which also sub-divides the zone into sections whereby each section is scanned and compared to a pre-set value and then rejected if the pre-set value is exceeded. Sensitivity can be varied in each section by raising or lowering the threshhold level of the pre-set value. A number of masks are used to facilitate the inspection process. X-ray mask is interposed in front of the X-ray source to flatten out the X-ray beam; outline mask disposed between the product and the image intensifier shields background radiation; product mask disposed on the output side of the image intensifier compensates for variations in product consistency and container irregularities; and the edge mask at the output side of the monitor blocks out background light from the monitor.
Description (OCR text may contain errors)
United States Patent [191 Fenton FOREIGN PARTICLE DETECTOR  Inventor: Marvin J. Fenton, Stamford, Conn.
 Assignee: Borden, Inc., Columbus, Ohio  Filed:
May 15, 1974  Appl. No.: 470,761
Related US. Application Data  Continuation of Ser. No. 327,060, Jan. 26, 1973,
Primary ExaminerArchie R. Borchelt Attorney, Agent, or Firm-George P. Maskas; George A. Kap; Daniel D. Mast [451 Nov. 4, 1975 ABSTRACT This apparatus inspects products for presence of foreign fragments while the product is in motion and includes a timing screw driven by a synchronous motor, an X-ray source for intermittently pulsing an X-ray beam which is in phase with the motor in response to actuation by the product disposed at an inspection zone, an image intensifier, monitor, photomultiplier, conventional circuitry for defining inspection zone on the product which also sub-divides the zone into sections whereby each section is scanned and compared to a pre-set value and then rejected if the pre-set value is exceeded. Sensitivity can be varied in each section by raising or lowering the threshhold level of the preset value. A number of masks are used to facilitate the inspection process. X-ray mask is interposed in front of the X-ray source to flatten out the X-ray beam; outline mask disposed between the product and the image intensifier shields background radiation; product mask disposed on the output side of the image intensifier compensates for variations in product consistency and container irregularities; and the edge mask at the output side of the monitor blocks out background light from the monitor.
23 Claims, 2 Drawing Figures X-RAY TRIGGER 3| SOURCE PHOTOCELL was rmms 6 28 ensa-15R LOGIC 3a\ RocEssmc MONITOR mPunsR CAMERA a GATES PHOTO REJECT t r MULTIPLER mTEsRAToR LEVEL ONE SHIFT E E T oerecron SHOT REGISTER sou-mom 44 55 58 60 i 62 4a ZONE counrms SELECTOR PHOTOCELL ZONE SENSITIVITYS U.S. Patent Nov. 4, 1975 Sheet 1 of 2 3,917,947
EoREreN PARTICLE DETECTOR This is a continuation of application Ser. No. 327,060, filed Jan. 26, 1973, now abandoned.
Detection of foreign particles in processed foods has been a critical problem in quality control programs. Most of the companies in the food processing, food manufacturing or food distribution business have, at one time or another, been subjectedto costly and protracted litigation because the product they make, sell or distribute contained a foreign particle which caused injury to the plaintiff. Awards in such cases have been stunningly high with the result that this and other considerations have prompted a development of detectors or inspection devices which could detect and reject a jar of food product containing a foreign particle.
The basic problem with such devices is one of detecting millimeter and larger size foreign fragments within filled containers at realistic throughput rates. Detection of foreign fragments with X-ray equipment results from localized attenuation caused by the fragment which, with presently available equipment, requires a difference of attenuation of at least about percent to be detectable using an X-ray radiation imaging system.
A device has now been developed for automatically inspecting filled containers for the presence of foreign fragments such as glass, metal or stone. Variation in image contrast is used to generate an electrical signal which controls a rejection mechanism. Inspection is performed on containers which are continuously moving on a conveyor and the rejected containers are automatically sorted from the main stream of containers. The inspection process has no harmful effect on the food in the containers, the container itself or the operating personnel. Product flavor and appearance are also unaffected by test conditions and the exposure levels are too low to achieve any sterilizing effect. The maximum external radiation at peak tube voltage and peak testing rate is 0.5 mR per hour. This level occurs close to the tube shield at the edge of the opening for the test beam. This location is impossible to reach with more than a small portion of the human body. An installed physical guard on the conveyor lines prevents the very low level of radiation from reaching any part of the operators or observers body. Inside the conveyor line guard, the stray radiation level is so low as to fall under the category of Exempt Protective Installation promulgated by US. Department of Commerce, Handbook No. 93, p. 1, item 2-l.
Sensitivity of the inspection device depends on many variables including glass container characteristics and normal variations in product density. Container characteristics include consistency of wall thickness, angle between the wall and the base, consistency of base thickness, presence of ribs or ridges or areas of increased or decreased thickness for decoration or for other purposes, contamination of glass itself, and the like.
The probability of detection is also influenced by particle position in the jar relative to the X-ray field of view. A particle at the edge of the field of view must be larger than a particle at the center of the field of view to be detected. The field of view subjected to inspection by the X-ray imaging process is limited by an outline mask which excludes from inspection wall of the jar, base and the closure. Configuration of the outline mask is matched to a particular jar.
The invention is described below in connection with the accompanying drawings in which:
FIG. 1 depicts structural outline of the inspection device as it relates to inspection of a sealed glass container moving at conveyor speed when the inspection is performed in a period of a few one-hundredths of a second;
FIG. 2 is an electrical block diagram showing the major logic functions of the system which senses, by a combination of electronic and optical means, the localized attenuation caused by a foreign particle within the container and subsequently rejects the container.
The preferred embodiment of the invention disclosed in FIGS. 1 and 2 is adapted to electronically and optically inspect a filled glass container for the presence of foreign particles. Glass jars 12 are shown disposed on and moved by conveyor 14 which includes link chain 16. Jars 12 should be spaced about one inch apart to facilitate triggering of the photo cell when a jar enters the inspection area and to facilitate ejection of the jars containing foreign matter at an appropriate point. As the inspection area is approached, the jars are impelled by spring biased guide rail 18 against timing screw 20 and cradled in the screw flights. The timing screw can be obtained from Earnst Manufacturing. The timing screw is driven by motor 22 through belt 24. A motor such as permanent magnet 3600 RPM synchronous motor manufactured by McLean under model No. 47GBH1B9, which has a 15:1 gear reducer, may be used. The motor is hooked into the power line which also supplies power to the electronic portion of the inspection system. By virtue of being driven by the synchronous motor which is provided with a 15:1 gear reducer, the bottles are mechanically in phase with the power line. Since the jars are in phase with the power line, the inspection area is located as close as possible to the terminal point of the timing screw so that when a jar enters the inspection area, an in-phase pulse of X- rays shall be emitted to initiate an inspection cycle.
Although it is not a prerequisite to efficient operation of the inspection device, the speed of the belt should be adjusted to the linear movement imparted by the timing screw in order to avoid motion non-uniformities and the problems associated therewith. In the preferred embodiment, the belt speed is feet per minute and due to a 15:1 gear reduction, the inspection rate is 240 jars per minute. At this rate, each slot in the timing screw is filled. Although at lower rates one or more of the slots will be empty, the inspection cycle will take place in exactly the same time and rate independent of the input rate. It should be apparent that the rate can be increased well beyond 240 jars per minute.
The use of the timing screw with the synchronous motor is a simple and practical solution to a problem of registering a moving test object with a timed burst of X-rays without having to intermittently arrest the motion of the conveyor or the test object.
It is the timing screw, which is in phase with the power line, that delivers the jar to the inspection area at the instant that a pulse of X-rays is released. As the jar leaves the timing screw, it enters the inspection area in which it is interposed between X-ray source 26 and image intensifier 28. The X-ray source used in the preferred embodiment is Americana X-ray head model WX900 and image intensifier is the Machlett dynascope 9TZ including DF2 power supply. In this position, the jar interrupts light beam from the trigger lamp 30 to trigger photocell 32 which causes an electrical 3 pulse to be generated from the power line and the inspection cycle to begin. Since the jar is mechanically in phase with the power line which also produces the trigger pulse, the jar will be in registry for a shot of X-rays when the initial negative cycle of AC line causes the X-ray pulse to be generated. Duration of the X-ray pulse is one-half of a complete line cycle of l/120th of a second. The succeeding positive half cycle of the line initiates image scanout by the closed circuit TV system 34 which continues for a complete line cycle or I /60th of a second. The TV camera is the Visual Educon Dage 800 camera with separate mesh vidicon and Concord TVL l4 close-up lense. As should be apparent, the complete inspection cycle in terms of duration is 1/40 ofa second (l/l2O l/60).
Positioned between the X-ray source 26 and jar 12 is an X-ray mask 31 which functions to flatten out the gross non-uniformities of the X-ray beam itself. The mask can be made with an aluminium backing l/l6 inch in thickness with a layer of solder or lead thereon facing the X-ray source. This layer is generally convex and about /8 inch thick. The shape of the X-ray mask is dictated by the fact that the X-ray beam is strongest at the center and weakens towards the fringes. This mask produces a field which is within 30 percent of being uniform.
If a foreign particle is present in a jar, the X-rays passing through the jar will impinge upon the front surface of the image intensifier and will form an image which is attenuated in the region of the foreign particle. Since diameter of the entrance pupil in the image intensifier is 9 inches at most, this permits products of up to 8 inches in height to be inspected in one pass. Larger articles can be inspected by passing them through the X-ray field a number of times until all of it has been inspected. The X-ray shadow in the image intensifier is amplified electrically about 5000 times and appears as a visible minified image on intensifiers output screen 29, which is approximately 1 inch in diameter.
An outline mask 36 is positioned between the jar and the front face of image intensifier 28. The outline mask can be made of lead or any other material which can effectively absorb and thus block X-rays. The mask has the outline of a jar which corresponds to the jars being inspected. The outline mask is necessary to block X- rays outside of the bottle outline from entering the image intensifier. in absence of the outline mask, the powerful unattenuated X-rays would diffuse into the bottle outline and blanket out any foreign particles disposed at the edge of the jar.
Another mask is also positioned between the output screen 29 of image intensifier 28 and TV camera 34. This is a product mask 38 which is simply a film negative of a control jar taken from the output side of the image intensifier. This mask compensates for difference in density of the various products and bottle configuration such as shape of bottle, ribs or ridges on the bottle, etc. A different mask is made and used for different products. The non-uniformities due to the X- rays in the image on the output side of the product mask are further reduced by the product mask to about 3 percent.
After passing through the product mask 38, the light is detected by the closed circuit camera 34 and converted to video signals which are inverted, gated and applied to the TV monitor 40. TV monitor 40 is the Sony View Finder Monitor model AVF 3200. Since the image is electronically inverted, it appears on the moni- 4 tor as dark except where additional attenuation is obtained from the foreign particle. Therefore, when a bottle is being inspected, a bright spot is seen when a foreign particle is present in addition to a partial outline of the bottle.
The light from monitor 40 is passed through edge mask 42. This mask functions to block out extraneous light outside the jar outline and can be made of any suitable material which is not transparent. After passing through edge mask 42, the light is converted into electrical signals by photomultiplier 44, amplified and detected by a level detector. The photomultiplier tube is the RCA 931 A tube. When the light output exceeds a pre-set level, a reject pulse occurs, a count is stored in the reject counter and in a shift register. The shift register meters off a fixed distance on the conveyor by sensing links in the link chain 16 by means of the counting photo cell 46 and operates reject solenoid 48 at a fixed distance from the inspection area. When the bottle reaches reject station 50, it is ejected by reject solenoid 48. In the preferred embodiment, ejection takes place 38 links past the inspection area. Since each link is fiveeighths inch, this distance is about 2 feet. If only one reject pulse is registered, only one actuation of the reject solenoid will occur. If a series of reject pulses are inserted, a series of reject pulses will occur, all delayed by the pre-set distance.
The major logic functions of the system are illustrated by the block diagram in FIG. 2.
When the leading edge of a jar interrupts the light from trigger lamp 30, a pulse is generated by trigger photocell 32 which is sent to the timing logic 52 which initiates the inspection cycle. Since the energy for the X-ray pulse is directly derived from the power line, the inspection cycle is also phased to the power line. The power line, therefore, is also used in the timing logic. When the line starts to go negative subsequent to the trigger, it is sensed by the timing logic and a high power pulse is generated for the X-ray source. Although this pulse has a duration of one-half cycle of the line, that is l/ 1 20th of a second, because of the nonlinear relationship between voltage and usable X-ray energy, only a portion of this l/th of a second is actually used for the detection process. Therefore, the effective exposure time is only a few milliseconds. From the timing logic 52, a pulse is sensed by the zone and multi-vibrators unit 64 where, in conjunction with a gate in the processing amplifier and gates unit 54, the monitor is blanked only for the portion covering the edge of the jar. This is done by triggering a multivibrator in unit 64 which closes a gate in unit 54 for a split second that it takes to blank the edge of the jar. The multivibrators are set separately in each zone and are adjusted to completely blank out the monitor in the leading edge region of the jar. After initial half cycle of the line when a burst of X-ray energy is released, scanout takes place during the succeeding full line cycle.
As previously described, the shadow image of the jar produced by the X-ray source is converted to a visible minified image by image intensifier 28, changed to a scan video signal by TV camera 34, passed through processing amplifier and gates unit 54 to TV monitor 40. Signal from camera 34 through sync processor 74 triggers multivibrators in unit 64 to generate zones which are scanned in the manner described. Unit 54 is part of camera 34. The output light from the monitor is detected by photomultiplier detector 44, applied to gated integrator 56, the level detector 58 and reject one shot 60, which generates a pulse when a reject is present. This pulse is entered into the shift register 62 and after a pre-set number of pulses are counted, which corresponds to a given conveyor distance, the reject solenoid 48 is actuated and the jar containing a foreign particle is ejected The system is completely insensitive to signals except during the unblanking and zone periods. The unblanking and sensitivity section is formed by quite a straight forward process. First, there is a general rectangular zone which is sufficiently large to encompass the entire jar. The zone is defined by zone and blanking multivibrators in unit 64 in conjunction with processing amplifier and gates in unit 54. This zone defines the inspection area within the outline of a jar. In the vertical direction, four additional zones are used. These may be set from top to bottom for any regions of the jar desired.
Since a jar is normally not completely cylindrical or uniform in wall thickness throughout vertical dimensions, the sensitivities may be increased in the areas of uniformity and somewhat decreased in the areas where, the masking cannot remove all the non-uniformities. As the camera continues to scan the image in the horizontal direction, a sync pulse is applied to zone and blanking multivibrators in unit 64 which triggers a blanking multivibrator in the same unit. In each of the four zones, in addition to multivibrators which blank out the leading edge of the jar, a second multivibrator deter mines the length of the inspection period. With the jar completely cylindrical, all of these second set of multivibrators are set the same. In regions where the jar has different cross sections, the multivibrators, that is the inspection time, is correspondingly altered.
The integrator 56, which is used to accumulate the electrical signals generated from the light output from the monitor 40, is likewise dead whenever the monitor signals are blanked. This provides double insurance against any noise spikes entering the system and cans ing false rejects.
With a large, completely cylindrical jar, approximately 200 horizontal scan lines would be observed from the top to bottom of the jar. Each of the four zones, therefore, would contain about 50 lines. At the end of each of these zones, the integrator which accumulates light is reset. Therefore, the light from the particle would need be larger than the accumulated light from 50 of normal scanlines. In addition, however, within each four zones are lO-line multivibrators 66, which dump or reset the integrator at the end of each horizontal lines. After every scan of the 10 lines, in tegrator 56 is cleared by means of gate unit 68. Therefore, the light output from the particle need be only larger than the light output from 10 horizontal lines containing no foreign matter.
The monitor is quite dark except for some random noise and the particle produces a bright image within the outline mask. This light, then, generates the defect signal. The left outline or the leading edge of the bottle is completely blanked electrically. In those regions of the bottle where the diameter changes cross sections throughout a single zone, the light from the right hand side of the bottle is blanked by the outline mask. The larger the particle or the more dense it is, the larger will be the output signal. Zone sensitivities 70 are individually set in zone selector 72 for the particle size desired within a given zone. Selection of the particular sensitivity or threshhold level of light in any desired zone adds another dimension to operation of the inspection device described herein.
The circuitry utilized in the inspection device described above is conventional as evidenced by the Webb et al. US. Pat. No. 3,580,997 and especially by Tatham et al. US. Pat. No. 3,390,769. These patents are hereby wholly incorporated by reference to complete description of the circuitry as they relate to the inspection device described herein. In fact, the bulk of the circuitry can be purchased in a single unit from Electro Data Concepts under designation 3047. This unit includes timing logic 52, processing amplifier and gate unit 54, sync processor 74, zone and blanking multivibrators unit 64, l0-line multivibrators 66, gate unit 68, integrator 56, level detector 58, reject one shot 60, shift register 62, zone selector 72 and zone sensitivities 70.
The embodiment described and illustrated herein is the preferred embodiment. It should be understood that modifications therein can be made within the spirit and scope of the invention. For instance, this invention need not necessarily be employed in connection with any particular article. The substance inspected can be in glass, metal or plastic containers or can be packaged in plastic or paper bags. Definition of the term containers" shall mean anything which can be used to store or package articles whether it be paper or plastic bags or jars, bottles or cans made of substantially rigid material such as metal, glass and plastic. In some applications, the use of the monitor, which converts electrical signals from the camera into visual images, can be dispensed with and the signals from the camera processed in a known manner. Under some operating conditions, the inspection device can be successfully operated without the edge mask which can be made of any material which does not transmit light, such as cardboard. Although spacing of about 1 inch between test objects is recommended, this spacing can be reduced and even entirely eliminated so that the test objects are in contact during the inspection process. The circuits in the inspection device are solid state, integrated circuits which should be isolated from high power AC source, but here again, isolation may not be required at some sites. Generally speaking, minimum detectable size for glass containers is roughly one-half the wall thickness of the containers but this can be still further reduced and under certain operating conditions, it may be possible to detect foreign particles of a size approaching the distance between the scan lines. Since the inspection device is so adjusted that registry of a test object at the inspection station is within 1/ 16th of an inch, if registration is off by more than that value, the test object will be rejected as if it contained a foreign fragment. The 1/ 16th of an inch tolerance can be adjusted to any desired level. Although a constant potential X-ray head can be used, a selfrectified X-ray head utilizing AC power is preferred because it costs a great deal less and is particularly adapted to the type of inspection performed herein. It is known that the X-ray beam is most intense at its central point and for this reason, an X-ray mask is interposed to flatten out the X-ray beam. With this in mind, the exterior outline of the mask is therefore, convex although a filler can be used to make the exterior surface of the mask dimensionally flat and still perform the same function.
The distance between the end of the timing screw and the inspection station is so adjusted that when a test object enters the inspection station, it will coincide 7 with the triggering of the X-ray source. For this reason, the distance between the timing screw and the inspection zone should be kept to a minimum. The inspection station coincides with the position of a test object in which it is in registry with the test object outline in the outline mask.
1. Inspection apparatus comprising X-ray source connected to an electrical power source for delivering X-rays across a path in the form of discrete beams, means connected to and being in phase with said power source for continuously moving test objects along said path, means actuated by said test objects for actuating said power source to allow a beam of X-rays to be emitted as one of the test objects becomes registered at an inspection station, detecting means for receiving the X-rays, means for analyzing the X-rays received by said detecting means and comparing them with a pre-set value, and means for rejecting certain of the test objects in response to actuation by said analyzing and comparing means.
2. Apparatus of claim 1 wherein said means for continuously moving the test objects is a timing screw driven by a synchronous motor synchronized with said power source.
3. Apparatus of claim 2 wherein said X-ray source is a self-rectified X-ray head and said synchronous motor is a permanent magnet synchronous motor.
4. Apparatus of claim 3 including an X-ray mask interposed between said X-ray source and the test objects as they move past the inspection station, said X-ray mask being provided for the purpose of rendering the X-ray beams more uniform in intensity.
5. Apparatus of claim 4 wherein said analyzing and comparing means includes an image intensifier with an output screen which electrically amplifies the X-ray image of a test object and converts it to a visible minified image on the output screen and a TV camera where the image is converted to video signals which are electronically inverted so that the image is dark except where additional X-ray attenuation occurs as a result of the presence of a foreign particle in a test object.
6. Apparatus of claim 5 including a product mask disposed on the output side of said image intensifier for compensating variations in the test objects.
7. Apparatus of claim 6, wherein the test objects are glass jars containing an edible substance and wherein said product mask is a film negative of a control test object taken on the output side of said image intensifier and is used to compensate for variations in density of the edible substance and various irregularities on or in the glass jars.
8. Apparatus of claim 7 wherein said analyzing and comparing means further includes a monitor which re ceives the image from said camera intensifier after the image has passed through said product mask, and a level detector; said apparatus further including an edge mask on the output side of said monitor for blocking out extraneous light from said monitor, a photomultiplier detector which converts the image from said monitor into electrical signals, means for amplifying the electrical signals from said photomultiplier detector and means for integrating the amplified electrical signals from said photomultiplier detector and rejecting the glass jar for which the integrated value exceeds certain pre-set value.
9. Apparatus of claim 8 wherein said analyzing and comparing means further includes means for defining a main inspection zone, means for sub-dividing said main zone into secondary zones for processing inspection in each of the secondary zones, means for separately integrating each of the secondary zones, means for comparing the integrated value with a pre-set value, and means for rejecting a test object if the pre-set value is exceeded.
10. Apparatus of claim 9 including means for separately varying sensitivity in each of the secondary zones.
1 1. Apparatus of claim 1 including a timing logic circuit for initiating inspection of a test object by triggering said X-ray source in phase as a test object comes into registry for inspection at inspection station.
12. Apparatus for inspecting containers filled with a product comprising an X-ray source for delivering at an inspection station X-rays across a path in the form of discrete pulses; an X-ray mask disposed in front of said X-ray source for imparting uniformity to the X-ray pulse; conveyor means for continuously moving the containers through the inspection station so that said 7 X-ray mask is interposed between said X-ray source and the containers as the containers move through the inspection station; an outline mask which blocks out X-rays passing beyond the outline of the containers as the containers move through the inspection station and allows passage of the X-rays falling within the outline of the containers; an image intensifier with an output screen which electrically amplifies the image formed by X-rays and converts it to a visible image on the output screen, said image intensifier being disposed on the output side of the said outline mask; a product mask disposed on the output side of the said image intensifier for compensating container and product variations; a TV camera which receives the image through said product mask and converts the image to video signals which are electrically inverted so that the image is dark except where additional X-ray attenuation occurs as a result of the presence of a foreign particle in the container; and means for comparing light derived from the containers and rejecting the container when a pre-set light value is exceeded.
13. Apparatus of claim 12 wherein said X-ray source is a self-rectified X-ray head which operates on AC power, said apparatus further including a monitor which receives electrical signals of the image from said TV camera and converts them to visual images, an edge mask on the output side of said monitor for blocking out extraneous light from said monitor, a photomultiplier detector for converting the image from said monitor into electrical signals, means for amplifying the electrical signals from said photomultiplier detector and means for integrating the amplified electrical signals from said photomultiplier detector and rejecting the container for which the integrated value exceeds a certain pre-set value.
14. Apparatus of claim 13 wherein said X-ray mask consists of an aluminum backing and a layer of material selected from auto body solder, lead and mixtures thereof, said material having a generally convex exterior outline.
15. Apparatus of claim 13 wherein said outline mask is made of lead and said product mask is a film negative of a control container taken on the output side of said image intensifier.
16. Apparatus of claim 15 including means for defining a main inspection zone, means for subdividing said Zone into secondary zones for processing inspection in 9 each of the secondary zones, means for separately integrating each of the secondary zones, means for comparing the integrated value with a pre-set value, and means for rejecting a test object if the pre-set value is exceeded.
17. Apparatus of claim 16 including means for varying sensitivity in each of the secondary zones.
18. Apparatus of claim 17 wherein said conveyor means includes a timing screw driven by a synchronous motor synchronized with an AC power source, said apparatus including a timing logic circuit connected to the AC power source for triggering said X-ray source in phase with said synchronous motor.
19. Apparatus of claim 18 wherein said motor is a permanent magnet synchronous motor and said edge mask is made from cardboard.
20. Automatic inspection apparatus comprising an X-ray generator for delivering X-rays on signal across a path; means for continuously moving test objects along said path; means for actuating said signal by the test objects to allow a pulse of X-rays to pass through a test object; means for detecting the X-ray image field passing through the test objects; means for converting the X-ray image field to a visual image field; means for converting the visual image field to electronic values; means for processing, analyzing and comparing the electronic values with pre-set values; and means for automatically rejecting certain of the test objects in response to said comparing means.
21. Apparatus of claim 20 including an X-ray mask interposed between the test objects and said X-ray generator for rendering the X-rays more uniform in intenmy.
22. Apparatus of claim 20 wherein said means for converting X-ray image field to visual image field is an electron optical intensifier.
23. Apparatus of claim 20 wherein said means for converting visual image field to electronic values is a