Metal detection and x-ray inspection offer differing capabilities. In some ways it makes choosing between them simple. Although both inspection solutions can detect metal contaminants, x-ray inspection offers greater capabilities for foreign body detection and quality assurance.

At high line speeds x-ray machines can also detect bone, glass, stone and high-density plastics and rubber compounds while simultaneously conducting quality checks such as mass measurement, fill level verification, identification of missing or broken products and more.

One must also consider the product type when choosing an inspection system. For example, products with high salt or moisture content can produce a ‘product effect’ that causes false readings on a metal detector leading to more false rejects. Higher reject numbers mean more labor is needed for rework, which adds strain to already tight resources.

To overcome this challenge, manufacturers often reduce a metal detector’s sensitivity to enable the majority of products to pass through without false triggering. However, lowering the sensitivity of a metal detector increases the risk of contaminants going undetected and ultimately ending up in the final product. An x-ray machine is not affected by temperature, moisture or salt content so it provides more accurate metal detection levels for these types of products.

As with any investment, cost is an important factor. The overall cost of an inspection system includes not only purchase price but also the ongoing expenditure incurred during the lifetime of the machine. An in-depth evaluation should be undertaken when selecting an inspection solution to fully understand the total cost of ownership.

Using the information in this article, you will be able to lay out a grid of features to see which ones match your application. Then it’s not so much a question of which technology is better, but which is most appropriate for your particular application and budget.

Metal Detection

Industrial metal detectors have been in existence since the 1960s and are used by food manufacturers at Critical Control Points (CCPs) in many production processes where a Hazard Analysis and Critical Control Points (HACCP) audit has identified the risk of metal contamination.

Modern metal detectors can identify all metals including ferrous, non-ferrous, and both magnetic and non-magnetic stainless steels. Systems can be installed to inspect incoming raw materials prior to processing, at numerous other points in the manufacturing process, or at the end of the production or packing line.

Metal detection is used to inspect a variety of different applications, including loose, unpackaged products, pumped products such as liquids, pastes and slurries, bulk powders and free-flowing solids under gravity-fall conditions, as well as packed products. The technology is also used to inspect tall, rigid containers such as bottles, jars and composite containers, although inspection must take place before metal caps or closures are applied.

How Does a Metal Detector Work?

Metal detectors consist of three coils wound onto a coil former. Each coil is equidistant from the other two. The center coil, known as the ‘transmitter’, is energized with a high-frequency electric current that generates a magnetic field. The two coils on either side of the center coil act as receivers. Since these two coils are identical in size and the same distance from the transmitter, an identical voltage is induced in each. When the coils are connected in opposition, these voltages cancel out, resulting in ‘zero output’.

Products are passed through the coil former for inspection, more commonly known as the metal detector aperture. When a metal object passes through the aperture, it disturbs the magnetic fields of both the receiver coils by slightly different amounts. This is subsequently detected by a change in the balance condition and interpreted as a metal contaminant. Metal exposed to such an alternating magnetic field will create an eddy current (a small amount of current induced into the metal due to its ability to conduct electricity) which alters the total magnetic field and leads to a further difference in voltage induced into the receiver coils. The difference in the voltages is what triggers a detection event.

Although the voltage change is very small, it is enough to be detected and interpreted by sophisticated electronic circuitry and advanced software algorithms. Software generates an electronic signal which can be used to raise an alarm and activate an automated product rejection mechanism to remove the contaminated product from the production process. Alternatively, the signal can be used to stop the production process by de-activating the conveyor or other packaging or processing machine.

What is Product Effect?

It is not just food packaged in metal that has the ability to generate magnetic fields and conduct electricity. Unpackaged food can do this too, although to a lesser extent than metal itself. Products with high salt or moisture content can produce a phenomenon known as ‘product effect’. Product effect is used to describe instances where a product’s own characteristics affect the metal detector in the same way as a metal contaminant, causing unwanted false readings.

In turn, metalized film packaging also creates a product effect for metal detectors. This is because the relatively high conductivity of the thin layer of aluminum film allows for the formation of eddy currents that generate a large enough magnetic field detectable by the metal detector.

Many factors affect the characteristics of a product and variations can be difficult to control on a production line.

To compensate for such variations, metal detector sensitivity is often reduced to enable the majority of products to pass through the detector without false triggering.

X-ray Inspection

X-ray equipment first appeared on production lines in the late 1980s and is used by food manufacturers globally to protect consumers, reduce the risk of product recalls and safeguard brand reputations. X-ray systems are capable of detecting a wider range of contaminants than metal detectors, including metal, glass, mineral stone, calcified bone, high-density plastics and rubber compounds.

How Does X-ray Inspection Work?

X-rays belong to the group of gamma rays which are one of several naturally-occurring sources of radiation and are an invisible form of electromagnetic radiation like radio waves. All types of electromagnetic radiation are part of a single continuum known as the Electromagnetic Spectrum. The spectrum is arranged according to frequency and wavelength and runs from radio waves at one end (which have a long wavelength) to gamma rays at the other (which have a short wavelength). The short wavelength of x-rays enables them to pass through materials that are opaque to visible light, but they do not pass through all materials with the same ease. The transparency of a material to x-rays is broadly related to its density - the denser the material, the fewer x-rays that pass through. Dense contaminants like glass, calcified bone and metal show up because they absorb more x-rays than the surrounding product.

An x-ray system is essentially a scanning device. When a product passes through the system, it captures a greyscale image of the product. The software within the x-ray system analyzes the greyscale image and compares it to a predetermined acceptance standard. On the basis of this comparison, it either accepts or rejects the image. In the case of a rejection, the software sends a signal to an automatic reject system which removes the product from the production line.

X-ray Inspection Equipment Design

There are three key components of an x-ray inspection system:

  • An x-ray generator (A)
  • A detector (B)
  • A computer (C)

After leaving the exit window of the x-ray generator, the x-ray beam travels in a straight line through a collimator, through the product, and onto the detector. A greyscale x-ray image is created and can be analyzed.

To accommodate larger products, the x-ray generator has to be moved further away to create an x-ray beam wide enough to inspect the whole product. However, increasing the distance between the generator and detector reduces the sensitivity of the x-ray system.

How Do X-ray Systems Detect Contaminants?

X-ray inspection is about differences in absorption. The amount of x-ray energy absorbed during an x-ray beam’s passage through a product is determined by product thickness, product density and its atomic mass number. The absorption is known as the ‘linear attenuation coefficient’. When a pack or product passes through the x-ray beam, the x-ray photons are absorbed by the pack or product, with only the residual energy reaching the detector. Measurement of the differences in x-ray beam absorption between product and contaminant is the basis of x-ray inspection.

Usually food products contain compounds made from elements with an atomic mass of 16 and under – mainly hydrogen, carbon and oxygen. The absorption of x-rays by food products containing low-mass elements is proportional to their density and thickness. In other words, the thicker or denser the product, the more x-rays it absorbs.

A potential contaminant becomes detectable by an x-ray system if it has a high atomic mass; this is a feature generally related to the contaminant’s density. Some contaminants, such as mineral stone or glass, may contain trace amounts of elements with very high atomic numbers. These elements have a multiplying effect on the contaminant’s x-ray absorption.

Food products generally contain low atomic mass elements and have low density, while contaminants usually contain high atomic mass number elements and have a higher density. For this reason, it is convenient to use density as the benchmark for contaminant detection. In general, contamination detection is only possible where contaminants are denser (i.e. they have higher specific gravity) than the food product in which they are embedded. This means that low-density contaminants such as insects, wood and polyethylene film cannot normally be detected effectively by x-ray technology.

Lower False Reject Rates and Minimize Product Effect with X-ray Inspection

X-ray systems are capable of inspecting food in a wide range of formats, from unpackaged raw, bulk-flow (loose) and pumped products, to those in a variety of different types of packaging, including glass jars, bottles and metal cans.

In contrast to metal detection, conductive foods (foods that are salty, acidic or have high moisture content) pose no challenges for x-ray inspection systems in any size or format. For example, seafood meats have conductive properties that can mimic a foreign object. This high ‘product effect’ causes excessive false rejects which increases time for rework and wasted product. X-ray inspection sensitivity is not affected by temperature, moisture or salt content. Additionally, an x-ray system’s detection accuracy is not affected by frozen or thawing foods.

X-ray inspection equipment not only detects metal but other foreign body contaminants, including bone, in a variety of meat, poultry and seafood products at higher line speeds with a lower false reject rate. Advanced x-ray technology is capable of detecting down to .4 mm metal contaminants in fish and poultry applications.

Factors That Affect Sensitivity of Detection for X-ray Systems

Several factors affect sensitivity of contaminant detection. These include:

The type of product: Homogeneous packs are the easiest type of product to inspect via x-ray technology because of the product consistency. However, many food packs comprise areas of varying absorption caused by variable product amounts, gaps or air pockets which affects detection.

High density variations: Some foods, such as granola with clusters of fruit and trail mix, contain high variations in density. Finding physical contaminants in these products can prove challenging for traditional single energy detectors as the varying densities create ‘busy’ x-ray images. However, x-ray systems equipped with dual energy x-ray technology lend themselves to inspecting ‘busy’ x-ray images caused by products with high variations in density due to their ability to discriminate materials by their chemical composition.

The thickness/depth of the product: As the density and thickness of the product increases, more x-ray energy is required to penetrate or pass through it. Increasing the x-ray penetration power (kV) reduces the contrast created by the contaminant which, in turn, reduces sensitivity. As the product depth increases, more x-ray energy is required to penetrate it.

The speed line affects detection: As production line speed increases, the signal to noise ratio also increases due to less integration time on the detectors. This means for faster lines you need more x-ray energy or larger detector pitch to achieve a certain result.

Industry Standards and Codes of Practice

As well as helping manufacturers comply with HACCP and retailers’ own specifications, market-leading x-ray inspection systems can help food manufacturers achieve certification to a number of Global Food Safety Initiative (GFSI)-recognized schemes, including Safe Quality Food (SQF) 2000 Code and BRCGS. By incorporating features such as validation mode for unique login credentials and XML files for data storage that help to support the audit process, x-ray systems can help food manufacturers meet the rigorous requirements necessary to achieve SQF and BRCGS certification.

Furthermore, x-ray systems allow manufacturers to save and export valuable production data, as well as save rejected product images for off-line review. Images are tagged with the product name, date, time and reason the product was rejected. By tracking production in this manner, x-ray systems enhance product traceability and provide manufacturers with due diligence capabilities

X-ray Inspection Sees the Unseen

By utilizing density differences and analyzing the resulting greyscale x-ray images, x-ray inspection equipment has moved beyond product safety into other areas of quality control for various product and packaging types.

One X-ray System – Many Quality Control Functions

In addition to detecting foreign bodies, modern x-ray systems are multitasking defenders of product and brand quality. In a single pass at high line speeds, x-ray systems can perform several inspection tasks simultaneously, including:

  • Measuring product weight, width, area, and volume
  • Identifying missing or broken products
  • Monitoring fill levels
  • Measuring mass
  • Inspecting for food trapped in seals

Measurement of Product Length, Width, Area, and Volume

Measuring the length, width, area, and volume of a product, in conjunction with contaminant detection is the simplest form of product inspection. The process is known as ‘object finder’.

As previously explained, an x-ray image is a greyscale image. The darker the grey, the more product is in the path of the x-ray beam. The software uses greyscale values to calculate, for example, the area of the product. This type of image analysis takes quality control to a new level of sophistication. It identifies products that don’t look right, even if they’re the correct weight, in the correct position, and free of foreign bodies. It’s hugely useful for manufacturers of products that depend on visual appeal. For example, one of the three meat patties shown in figure 5 has a hole in it. It shows up as a light patch amid the uniform grey. One of the three patties shown in figure 6 is misshapen.

Identification of Missing or Broken Items

X-ray inspection will also detect faulty and missing products. Examples are:

  1. Detection of damaged products
  2. Detection of missing products
  3. Insert identification

1. Detection of Damaged Products

The detection of damaged products relies on the same principles as length and volume measurement. By setting minimum and maximum sizes for pack width, height, volume or surface area, x-ray analysis software can spot a deformed product.

2. Detection of Missing Products

X-ray systems can provide a look inside the final sealed packaging to check that all components are present. It can count products and components that cannot be seen or counted by cameras or human eyesight. For example, it can count needles and syringes in a box, count cheese cubes in a tray, or pralines in a gift box.

Spotting the missing sausage in figure 8 was easy. The software found five dark zones in the greyscale x-ray image when it was programmed to expect six.

3. Insert Identification

If x-ray inspection can identify objects that shouldn’t be in a pack, it can also find ones that should be there, such as desiccants for packaged foods.

Another example comes from the meat packing industry. Many meat-based products contain oxidizers (known as ‘scavengers’) to help keep the product fresh. Oxidizers can be quite dense, which could reduce the effectiveness of foreign body detection. Figure 10 shows how, in a packet of cooked ham, the x-ray system not only checks that the oxidizer is present, but also removes it from the x-ray image for optimum foreign body detection.

Measuring Mass and Monitoring Fill Levels

Maintaining the correct mass and fill levels of a product is a constant challenge in food and pharmaceutical manufacturing. Measuring overfills and under fills has an effect on manufacturing costs as well as consumer satisfaction.

X-ray inspection can analyze:

  • The overall mass of a product
  • The individual masses within various zones or compartments of a product
  • The overall fill level of a product
  • The individual fill levels within various zones of compartments of a product

Overall Mass Measurement

As explained above, an x-ray image shows up as varying tones of grey. The software uses the combination of the greyscale values and 2D image to calculate how much product or contaminant is in the pack. This volumetric check is also used for mass measurement.

Some x-ray systems have an auto-learn function whereby an acceptable weight pack (close to the nominal weight) is passed through the x-ray system, typically 10 times. The gross weight of the pack is then entered into the system. (The user must have previously weighed this pack on a set of calibrated static scales offering a suitable weight range and accuracy.) That way the analytical software can calculate the weight of subsequent packs by comparison to the weight of its learned reference pack. The x-ray inspection system can now compare all future products against its ideal reference product. If the calculated mass falls within a programmed tolerance, the package is good. If it deviates, the package will be rejected.

Figure 11 shows a pouch containing frozen chicken strips that has been identified as an underweight pouch.

Salmon ready meal with low fill

With x-ray inspection, every salmon ready meal, for example, can be checked, even when the line runs at 100 feet per minute (Figure 12).

The x-ray inspection software examines each greyscale x-ray image. If the mass meets the preset standard, the meal passes the test. If it fails, it’s rejected from the line and the manufacturer can adjust the equipment to maintain the standard.

The relationship between mass and total product x-ray absorption is not a straight line. Using a single product auto-learn feature is quite accurate when production pack weights are near the target weight. More sophisticated systems use a three product auto-learn process: the low rejection point, the target weight, and the high rejection point. This method allows calculation of the mass from variations in x-ray absorption within a narrower range. It provides greater accuracy than that offered by the normal production weight range.

Accuracy of mass measurement is good on homogeneous packs (e.g. a block of butter), but not as good on loose packed products (e.g. sausages in a bag, or products for which ingredients can vary between batches). X-ray mass measurement is particularly effective for high-speed applications where traditional in-line weighing systems may not offer the same level of accuracy. It lets manufacturers comply with minimum weight, EU average weight, or US zoned weight regulations.

In every case, the x-ray system produces relevant statistics on rejects. Mass measurement does not offer a global solution to weights and measures compliance. Some countries expect R51 type approval, which only applies to gravitational weighing systems.

Zoned Mass Measurements 

For products that are in defined compartments, a two compartment ready meal, mass measurement can provide results for each individual zone/compartment. It lets manufacturers check the overall mass of a product and the masses within each compartment.

Figure 13 shows a twin compartment ready-meal (TV dinner). The x-ray software is simultaneously checking the overall mass of the pack and that of each compartment. In this case the overall weight is right, but there is low fill in the rice compartment, so the pack is rejected.

Overall Fill-Level Inspection 

Fill-level inspection is different to mass measurement. It’s a 2D process based on a simple inspection process: you set maximum and minimum fill levels and reject any product that falls outside them. It doesn’t matter what the product is made of, or the mass of it. It simply has to reach a certain height within the pack or container. Fill level becomes a simple 2D image check instead of the 3D volumetric check required for mass measurement.

Manufacturers who need an inspection machine to detect contaminants in their food and beverage products must decide what type of system will best get the job done based on a variety of factors. This is a straightforward decision once you take into consideration the type of application, likely contaminants, product packaging, production speed, floor space and budget.

Metal detectors are a good choice when metal is the likely contaminant in your product, but if you want more from your inspection system, an x-ray machine provides additional functionalities beyond detecting just metal contaminants. X-ray also detects bone, stone, glass and high-density plastic and rubber compounds. In a single pass at high line speeds, a multi-tasking x-ray system can also provide simultaneous quality checks such as mass measurement, identification of missing or broken products, fill level monitoring, compromised seal detection and more. The detection sensitivity of an x-ray inspection system is not affected by a product’s temperature, moisture or salinity. Additionally, product packaging isn’t an obstacle for x-ray technology, as it can detect contaminants through metalized film.

Images courtesy of Eagle Product Inspection