The Theory of X-Ray Inspection

Table of Contents
Introduction
Basic Components
Safety
Operating Principles
'Product Effects' and Limitations
Enhanced Detection Facilities
Pack Fill Inspection
Conclusion

Introduction
Cintex Corporation Introduction Cintex Corporation's InSight X-ray system is a compact unit capable of detecting dense contamination in small packaged foodstuffs. Foreign material such as stainless steel, ferrous and non-ferrous metals, glass, mineral stone, PVC, dense rubbers, etc, may be detected within a closed package or loose on the conveyor belt. A 2.5 and 3-in pipeline version for liquids and slurries is also planned.

The standard InSight system is offered in a 200mm or 300mm wide format, with product heights to a maximum of 150mm. (There is a general restriction on product height that will become apparent later in this text.) CINTEX can also offer a 'banked' system that allows for wider conveyor widths utilizing multiple sensors and x-ray sets.

X-ray beams are formed as a conical (fan) shape, such that coverage is reduced (in width) at the top of a tall package. Therefore a width to match the pack should be chosen from the accompanying table.

The 300mm system is offered in two power versions. The standard system, in common with the 200mm wide unit, has a 100 watt x-ray set, providing 40-50kv at 2-2.5ma, depending on mode selected. The high power 300mm model has the capability of 60kv at 5ma to a maximum total power of 200 watts.

When selecting a model for a particular application, it is necessary to understand a little about the components of the system, the dynamics of the product and the way in which the food is inspected and should be presented to the system.

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Basic Components
The InSight system comprises three basic elements: an X-ray generator, a detector or sensor, and a data processing computer. The imaging section comprises the X-ray generator and the linear sensor.

X-rays are generated by a glass tube, which is enclosed in a vat of cooling oil. A high voltage in the order of 30 60kv is applied to the tube at a current of around 2-5ma. This causes electrons to travel across the tube where they strike a tungsten target. The target then emits a stream of X-rays that are mechanically focused into a conical beam. This beam is then further reduced by the system to a 1mm approx. wide 'fan beam'.

Once the fan beam is generated, it passes through the product (which we will describe in more detail later), where it enters the linear diode scanner (sensor). The sensor converts the X-ray signal via a phosphor scintillator layer into light, where it is turned into an electrical signal by the diodes. The signal from the detector is then passed on to the system computer for processing and detection.

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Safety
Important safety features are incorporated in the design. If the high voltage or current drive is removed from the tube, the system ceases production of X-rays immediately, with no residual radiation. The moment an interlock is opened or a safety stop is pressed, the production of X-rays ceases. No gamma sources or radioactive materials are contained within the system. By law the system must have a high standard of interlocking, with multiple cross-monitored contacts. Regulations also state that the system must not emit radiation from any part of its body, to which CINTEX strictly adhere. Each machine leaves the factory with an independent certificate of compliance directly relevant to that machine.

Radiation is all around us and is a part of everyday life. It can come from rocks, the soil, and the sun itself. Taking a flight in an aircraft considerably increases body dose during the flight as there is less atmosphere between a person and the sun to filter out the radiation. A similar increase in body dose will be received when spending a day on the beach. Working in close proximity to an InSight X-ray machine will not result in any increased dose than that experienced in the normal course of being alive.

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Operating Principles
The basic principle of detection is by density difference. When the X-rays penetrate the product, they lose some of their energy such that less signal reaches the detector in areas where dense material is present. The X-ray set power must be sufficient to allow the product to be penetrated, but not be so high as to over-penetrate the contamination. The system has adjustments to allow the correct dose to be selected however care must be taken to ensure that the system is specified with sufficient power to penetrate the product at the time of ordering. The system has a facility for determining the optimum operating levels, and produces a graphical display of the product and its profile.

The figure shows a trace of product density across the product. The parts at 100% are the white areas of the conveyor, where there is no product present. (A dense area is dark, while less dense areas are the brighter parts of the image.)

As the X-rays penetrate or leave the edges of the pack, the trace is seen to fall into the 'baseline' area of the product in the 'white zone'. This zone indicates an optimal area in which the products background density should generally fall. The more even and consistent the product, the flatter this portion of the trace will be. The large pulse, which crosses the horizontal line, represents a typical contaminant. It can be seen that this is denser than the rest of the product and passes below the threshold line, enabling automatic rejection using simple thresholding techniques.

There are other methods that can also be applied to enable detection. At this point, we will discuss further how the product signal is actually produced.

As described, the X-ray signal is reduced as it passed through the product, and the reduction is dependent upon the density. The X-ray beam is collimated into a thin beam approx. 1mm wide, which is passed through the product and is received as a thin line of radiation on a phosphor layer within the detector sensor unit. The phosphor layer is mounted under the conveyor belt, and immediately over the sensitive surface of an array of photodiodes. As the phosphor glows, in proportion to the x-rays falling on it, the diodes pick up the light and convert it to an electrical signal. This signal is scanned by the system electronics such that a 'line' of data, representing each diode in turn, is passed to the system computer.

The scanning process operates at real production line rates, with speeds up to 200 feet per minute (60mtrs approx.) being normal. The system can work at higher rates than this, due to the integrating technology employed in the CINTEX detector sensor, but detection levels will reduce at elevated speeds.

The scanning process can be thought of as being similar to a fax machine. (A fax machine also scans a line of data via photodiodes.) In this case the paper simulates a conveyor, an internal light source is akin to the x-ray source, and the print on the paper can be thought of as 'product density'. As the paper is fed through, the print is scanned in a similar manner to the product on the conveyor being scanned.

The InSight system incorporates a Pentium microprocessor and high speed data processing card that reads in the data from the diode sensors. Once the system has acquired the data from the detector, statistical techniques can be performed with the Pentium that allows the image to be analyzed, including performing simple thresholding, illustrated earlier.

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'Product Effects' and Limitations
products that may be effectively x-rayed. This is because the basic density of food materials and the contamination within them vary widely. Therefore not all products may be submitted to the maximum depth of 150mm, and not all contaminants will be optimally detected at that depth. In product terms, for example, 150mm of meat will have a density much higher than 150mm of bread. In contaminant terms, a steel fragment is far denser that an item such as a chicken bone.

Due to the variation in product types it is always necessary to test a customer's product before quoting absolute sensitivities, or a specific machine type.

The main advantage of an X-ray machine is that it can find materials other than just metals, and it can work with products and packaging materials that metal detectors find difficult. X-ray machines are not affected by the common limitations of metal detectors. Foil packaging, frozen products, high conductivity i.e. wet products do not cause any degradation in performance. An X-Ray machine should always be considered as the most effective option on any foil or metallized packaged product where a ferrous in foil detector would normally be used. The system does not suffer from 'phasing' effects that tend to limit metal detector sensitivity to stainless steels. However, X-ray technology does have its own set of restrictions.

The basic limitations apply to two product attributes: Total through density and product variation. The natural variation of the density of the product can be considered as a 'product effect'. The best way to describe this is as follows:

Imagine an ideal product, being a tray of water 10mm thick. In fact, most foods can be thought of as presenting a 'water equivalent density' (WED). For example, a block of ice cream 100mm thick is actually whipped full of air and low density fat, and only has a WED of around 50mm of water. Taking a reference density of 10mm of water as a guideline, we can apply some comparative measurements.

Water has a specific gravity (SG) of one. Therefore, you can view our 10mm sample of water as possessing an 'SG total' of 10 (because with 10mm of water each 1mm will have an SG of 1, giving an 'SG total' of 10 x 1 = 10). If we place a 1mm stainless ball into the tray it will displace 1mm of the water. This gives a through density of 1mm of steel and 9mm of water. As steel has an SG of around 12, this now gives us 9 SG units of water, and 12 SG units (from the 1mm ball of steel) of metal, making a shift from an SG total of 10 to 21.

Therefore the density of the product at the point of the steel ball shifts from 10 SG units to 21 SG units, which is VERY detectable. If we change the WED of the product to 100mm of water, the figures become different. With a 1mm steel ball we have 99 SG units of water, plus 12 SG units of steel, giving a total of 111. This is a total shift of 11% as opposed to a shift of 110% with a 10mm tray of water.

It is useful to compare the density of other dense materials in this equation. For example, glass has an SG of around 3. Therefore in 10mm of water a shift from 10 to 12 will occur (20%). Therefore it can be seen that better performance to non-metallic contaminants will be had on thinner products.

It can be seen that when a product has a high density, the overall sensitivity will be reduced. There comes a practical limit at which increased product height makes the system more expensive (in order to have X-ray power great enough to penetrate the product) but has a reduced sensitivity to the point where price/performance becomes unacceptable.

In general, good performance is obtained with the InSight system to a WED of around 50mm, on metal of 1-1.5mm, with other materials becoming detectable at 2.5 3mm approx.. Penetrating higher WED products requires a more expensive system of higher power and in this configuration sensitivity to non-metallic bodies will be reduced.

The other way in which density is a factor is in the natural variation of the product. A fine-grained product, such as instant potato powder, presents an even product density. A larger grained product, on the other hand, presents a varying profile, which requires a more sophisticated technique to detect a density difference within a product that itself has many density differences.

The two traces illustrate an ideal and difficult product. The upper trace would typically be a box of fine-grained powder. In this we can easily detect contamination, as the background density of the product is relatively constant. This causes the particle to give a clear 'darkest point' to the signal.

The lower trace is in a sphere, or 'ball shaped' product. In this package we have a large density variation. Although the trace does not show a 'noisy' product (for clarity) there could also be a large variation in the basic signal to compound the product density variation. At the edge of the 'sphere' we almost strike a tangent through the product, where very little product will be in the x-ray beam. In the center of the sphere we have a maximum density, through the whole product. If we use sufficient power to penetrate the center, we risk over penetrating the edges of the products, and any contamination within. It can be seen that although the two contaminants are identical, only one passes through the threshold, and therefore only one is detectable.

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Enhanced Detection Facilities
It is possible to add more sophisticated detection methods to inspect 'difficult' products, but care must be taken not to provide a high level of false rejects. In the sphere example, there still remains a problem with 'dynamic range'. Anyone who has taken a photograph of someone against a bright scene will understand this concept the subject becomes dark as the background is too light and the film cannot cope with the wide dynamic range. The X-ray sensor will also have difficulty when a product of highly variable profile is presented resulting in an effect called 'edge burn' where parts of the product or package have 'whiteout'.

We have mentioned that there are other methods that can be used to adjust and adapt to the product profile. Image Processing or 'Morphology' is two terms used to describe numerical processing of the signal, which can provide better detectability on difficult products. It is still necessary that the basic signal from the detector be generated in such a way as to avoid dynamic range and over-penetration problems.

One way in which a varying product, such as a cheese wedge, can be inspected is to adopt an adaptive threshold that tracks the increasing (or decreasing) density of the basic product. Two of these methods can be seen in the following drawing.

The threshold can better match the product variation in an adaptive method. However, in a product with a high variation ('product effect') the threshold would 'chase' itself, and result in false rejects. In these products it may be better to use a kernel-based method or a form of 'edge' detection.

Edge detection relies on detecting a transition within the basic product, which does not necessarily fall below an absolute threshold, but has a great enough profile to be detected as being different from the rest of the product. We can use pulse size, position and width as parameters in this detection method.

The 'high end' method of detection is to employ a digital filter, or Kernel. In this method, a matrix, or 'mask' is passed as an invisible 'grid' over the product data. This 'grid' could be from 3x3 in size to perhaps 15x15 or greater. In this method the center of the grid is used to inspect the pixel data from the product, which is compared, with special 'weightings' against other pixel data adjacent to it on the same line, and on preceding and subsequent lines. In this way, a change in the product profile can be detected, and can be enhanced further by selection of appropriate 'weightings' within the grid, which can serve to accentuate the change in signal.

The Kernel method can be effective, but as with all methods, has its limitations.

Apart from the laws of physics (dynamic range, overall density shift, etc), there can be problems if the product is unpredictable. For example, a chicken fillet is a product that varies widely in density, and it is here that a kernel or edge based method would help detection. A fillet changes in thickness in virtually all directions. Therefore a method of detecting a 'pulse' or 'edge' from the product is more preferable. However, the method must be intelligent enough to understand the differences between a bone, or other contaminants, and perhaps an edge cause by other features of the product. For example, the edge of the product is an edge (!) as could be a fold in the meat from the goujon, an overlapping fillet, cuts in the meat, or a blood spot.

Once more, it is important to stress that the product must be tested on a machine under comparative conditions with real product. In this way a detection method and performance capability can be determined and presented to the customer for acceptance BEFORE a quotation is issued or a sale agreed.

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Pack Fill Inspection
An important selling feature of an X-ray machine is its capability to do more than 'just' detect contamination. The InSight system can also detect for missing or improperly packaged product AT THE SAME TIME as inspecting for foreign bodies. The InSight will also provide separate reject signals for each of these non-conformities so that contaminated and improperly packaged products can be kept separate.

As the product passes through InSight, it can examine the package, producing a histogram profile, or pixel count of the product. If the resulting profile or count does not match that previously learned by the system, the pack will be automatically removed from the line.

The InSight system can in some instances out-perform a checkweigher. Where a product contains a known number of items, such as a pack of 12 candy bars, the consumer will expect there to be 12 present. The fact that the pack made weight (which it could do due to product variance) will be of little consolation to the customer if an item is missing. Inspecting the pack for actual product presence, rather than weight alone, will ensure proper pack fill levels.

The product fill feature can be used to perform varied functions. Detection of a (required!) coin in a cake, and a large plastic toy in a breakfast cereal box are two of the more extreme (and real) applications.

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Conclusion
InSight can perform contamination detection within products that metal detectors find difficult. Metallized foils, partially or completely frozen products, high water content or 'wet' products are not a problem. There are no 'blind spots' to any grades of stainless steel, even within a metallized wet packaged product.

InSight can detect other dense materials such as stone, bone, glass, PVC, some rubbers, etc. Metal detectors can only detect metals. A very basic detection rule.

If the contaminant sinks, it may be detectable. If it floats on water (lower density than water) it cannot be detected. Examples of items that cannot be detected are wood, plastic films, string, Band-Aids, dirt, insects, cherry pits (they are wood!) and human hair.

InSight can also perform dual inspection, allowing both contamination AND pack contents to be monitored at the same time. This can allow improperly filled packs to be rejected where weight is not a true indication of fill level. Rejects are put into separate reject bins if required. The ability to add other quality checks to the basic contamination detection can help justify the increased cost over a metal detector system.

For more information contact: CINTEX of America Inc., 6919 - 51st St., Kenosha, WI 53144. Tel: (414) 657-7848; Fax: (414) 657-3056.