X-ray Inspection Guidance

A basic understanding of x-ray physics and system components helps in making informed decisions about x-ray equipment and applications.  

2.1 Brief History and Nature of X-rays

X-rays were discovered in 1895 by Wilhelm Conrad Röntgen while investigating cathode rays in a glass tube; he observed a new type of radiation capable of exposing photographic plates and penetrating matter. X-rays are a form of high energy electromagnetic radiation, like light but with much shorter wavelengths and higher frequencies. Because of this higher energy, x-rays can pass through materials opaque to visible light, with the degree of penetration depending on material composition and thickness.  

When x-rays interact with matter, they are attenuated primarily by the electrons in atoms; materials with more electrons per unit volume (higher atomic number and density) absorb more x-rays. This difference in absorption between product and contaminant is the basis for x-ray inspection.  

2.2 Modern Uses of X-rays

X-rays are used in medicine for diagnostic imaging, in construction and manufacturing for non destructive testing, in security for baggage and cargo screening, and in food and pharmaceutical industries for product inspection. Medical systems use short exposures to create images while keeping doses within safe limits; industrial systems use stationary objects or flowing products with exposures tailored to the task.  

Security systems often use relatively low resolution imaging and rely on operator interpretation, whereas food inspection systems combine automated image analysis with high throughput and robust construction for harsh environments.  

2.3 Principles of Modern X-ray Machines

Food and pharma x-ray systems generate x-rays electrically inside a sealed tube comprising a cathode filament, an anode (often copper) and a tungsten target within a glass envelope. Heating the filament releases electrons, which are accelerated towards the target by a high voltage; when they strike the target and decelerate, x-ray photons are produced.  

The x-ray tube is immersed in oil within a metal tank to provide electrical insulation and cooling, and the useful beam exits through a small window. Collimators at the exit shape and narrow the beam, since short wavelength radiation cannot be focused by lenses. Heat generated in the tube is dissipated via the oil and external fins or, in higher power systems, via pumped cooling loops.  

2.4 Components of an X-ray System

An x-ray inspection system consists of three main elements: the x-ray generator, the detector and the control system. The generator produces a fan shaped beam that passes through the product and conveyor; the detector beneath or beside the product converts transmitted x-rays into electrical signals; and the control system reconstructs an image from these signals, applies detection algorithms and issues accept/reject decisions.  

Detectors normally use a scintillating material that converts x-ray energy into visible light, coupled to an array of photodiodes that convert light intensity into electrical signals. Diodes are arranged at a defined pitch across the belt width, with common pitches such as 0.4 mm, 0.8 mm or 1.5 mm, and each diode effectively samples a narrow “pixel” in the image.  

As product moves through the beam, successive lines of data are captured; by synchronizing scan rate with conveyor speed, a correctly proportioned two dimensional grey scale image is built up. Each pixel carries a value representing local absorption: darker regions correspond to higher density or thickness, and these variations are analyzed by software tools to detect anomalies.  

2.5 Absorption Difference and Detectability

X-ray inspection relies on differences in absorption between contaminants and surrounding product. Absorption depends on material thickness, density and atomic composition, and is quantified by the linear attenuation coefficient. Most food products are largely composed of low atomic number elements such as hydrogen, carbon and oxygen, and have densities near that of water.  

Contaminants such as metals, glass, mineral stones and calcified bone are typically denser and may contain higher atomic number elements, causing them to absorb more x-rays. For convenience, density (or specific gravity) is often used as a proxy for detectability: materials with much higher density than the product are generally detectable if particle size is sufficient.  

A wide range of metal contaminants, including ferrous, non ferrous and stainless steels, have specific gravities around 7–8 and are therefore readily detectable, while aluminum, at lower density, is somewhat harder to detect. Many plastics and organic materials have densities similar to water and may be difficult or impossible to detect unless they contain denser additives.  

One important advantage of x-ray is its ability to inspect products in aluminum foil or metalized film; because packaging layers are thin, they absorb relatively little x-ray energy and can often be effectively “ignored” by the system while still detecting denser contaminants inside.  

2.6 Image Creation and Inspection Tools

Once an image is obtained, detection algorithms identify suspicious features. Simple threshold analysis looks for pixels darker than an adaptive limit, suitable for homogeneous products where any high density spot is likely to be a contaminant. More advanced radial or neighborhood based analysis compares each pixel with its neighbors to identify small local density changes, supporting detection of smaller or lower contrast contaminants.  

Specialist tools are used for complex packages such as cans, jars and multi compartment packs, focusing on regions where contaminants are most likely to occur (e.g. the base or edges) and compensating for packaging features. Multiple tools can run in parallel to detect different sizes and shapes, providing robust coverage.

Radiation is often misunderstood, and concerns about worker safety and consumer perception can slow adoption of x-ray inspection. This chapter explains radiation basics, puts doses in context and outlines how food x-ray systems can be operated safely.  

3.1 Radiation Basics and Background Dose

Radiation includes various forms of energy emitted from natural and artificial sources, including light, radio waves, microwaves and ionizing radiation such as x-rays and gamma rays. Ionizing radiation has enough energy to remove electrons from atoms and can damage biological tissue at sufficiently high doses.  

Humans are continuously exposed to background radiation from cosmic rays, terrestrial sources such as radon gas, naturally occurring radionuclides in food and drink, and medical exposures. Natural sources typically account for around 80% of annual exposure, with average global background doses of about 2400 microsieverts per year.  

3.2 Radiation Dose Units and Everyday Comparisons

Radiation dose is expressed in sieverts (Sv), with millisieverts (mSv) and microsieverts (µSv) used for common levels; dose rate is given in µSv per hour. Comparing doses from everyday activities helps contextualize the very low exposures associated with food x-ray inspection.  

For example, eating one banana per day contributes a small annual dose due to naturally occurring potassium 40; frequent air travel increases exposure from cosmic radiation; and common medical procedures such as chest x-rays deliver doses significantly higher than those encountered near properly shielded inspection systems. Regulatory limits for public exposure are typically around 1 mSv per year from artificial sources, far above the leakage permitted from compliant x-ray machines.  

3.3 Food Irradiation vs. Inspection

Food irradiation is a separate process in which food is deliberately exposed to high doses of ionizing radiation to reduce microorganisms, extend shelf life or control pests. These doses, measured in kilograys, are many orders of magnitude higher than those used in inspection, and even at levels up to 10 kGy, studies have found no adverse effects on food safety or nutrition.  

In contrast, the dose received by products in x-ray inspection systems is typically in the hundreds of micrograys or less—insufficient to alter food or make it radioactive. Thus, x-ray inspection does not “irradiate” food in the regulatory sense and has no measurable effect on product safety, quality or organic status.  

3.4 Working Safely with X-ray Systems

Inspection systems use electrically generated x-rays that can be switched on and off, unlike sealed radioactive sources. Properly designed cabinet x-ray machines enclose the generator, beam path and detector within shielded housings that prevent significant radiation from escaping during operation.  

Protection principles include minimizing exposure time, maximizing distance from sources and providing effective shielding using dense materials such as steel and lead where necessary. Regulations specify maximum permitted leakage rates at defined distances from the cabinet, often 1 µSv/h or lower in many jurisdictions. Systems must comply with relevant standards, be CE marked where applicable and undergo commissioning surveys to confirm leakage within limits.  

Provided safety interlocks, guarding and procedures are followed, x-ray inspection systems present no special restrictions for operators, including pregnant staff. 

Metal detection and x-ray inspection are complementary technologies with different strengths, limitations and cost profiles. Choosing between them—or deciding to use both—should be based on hazard analysis, product and packaging characteristics, and program objectives.  

4.1 Capabilities and Product Effects

Metal detectors detect metal contaminants by electromagnetic induction and are effective for ferrous, non-ferrous and many stainless steels. They are compact, relatively low-cost and suitable for a variety of formats including bulk, gravity-feed and pipeline applications. However, “product effect” in wet or salty products and the presence of metalized packaging can limit performance.  

X-ray systems detect dense contaminants regardless of magnetic or conductive properties, including metals, glass, stone, calcified bone and some dense plastics and rubbers. Their performance depends on density differences and product thickness rather than conductivity, and they can operate effectively with many challenging products and packages where metal detectors struggle. X-ray performance is influenced by product density, thickness and homogeneity, with larger or denser products generally yielding lower sensitivity.  

4.2 Packaging Effects

Packaging strongly influences technology suitability. Products in plastic, paper or other non-metallic materials can usually be inspected by either technology; selection then depends on target contaminants, budget and desire for additional quality checks.  

Metalized film can sometimes be handled by metal detectors using low frequency or multi-simultaneous frequency techniques, though sensitivity may be reduced and must be validated. X-ray systems are largely unaffected by thin metalized films. Aluminum foil packaging is essentially opaque to metal detection using balanced coils and requires specialized ferrous-in-foil detectors limited to magnetic metals, whereas x-ray can inspect such packs effectively.  

For gravity-fed powders and granules, metal detection is often preferred because x-ray solutions for these flows are more limited and may not deliver satisfactory performance. Non-metal contaminants in any packaging generally require x-ray.  

4.3 Choosing One or Both Technologies

A HACCP-driven hazard analysis should identify likely contaminants, their sources, and where they can be controlled. If only metal is a realistic hazard and packaging and product properties are favorable, metal detection may provide the most cost-effective solution. If non-metallic contaminants are also of concern, or packaging or product effect restricts metal detection, x-ray is required.  

In many lines, the optimal approach is to use both technologies at different CCPs: for example, metal detection or bulk x-ray early in the process to protect equipment and remove large contaminants, and x-ray or metal detection at the end of the line to provide final assurance. The choice should also consider ancillary capabilities such as quality checks, space constraints, testing requirements and lifetime costs.  

Selecting the appropriate x-ray system format is fundamental to achieving reliable inspection and easy integration. Key factors include product and packaging type, line layout, required sensitivity, environmental conditions and retailer or standard‑specific expectations.  

5.1 Vertical Beam System

Vertical-beam systems are the most common, especially for relatively flat products on conveyors. The generator is mounted above the belt, projecting a downward fan beam through the product onto a detector beneath; the resulting image is a plan view that clearly shows internal structure and component placement.

Systems can be configured for sealed packs, bulk flows and pipelines. For sealed packs, the x-ray conveyor accepts products from the upstream line, passes them through the beam and delivers them to downstream equipment, often with integrated reject and guarding. For bulk products such as nuts, cereals or snacks, shallow, even product layers across the belt give excellent sensitivity and are often used at early stages to remove contaminants before value is added.  

Pipeline systems inspect pumped liquids, slurries and pastes through a non-metallic section and divert contaminated product via a valve. These systems benefit from homogeneous product streams and small inspection depths, yielding strong performance, and are often positioned upstream of filling and packaging.

5.2 Horizontal Beam Systems for Tall Containers

Horizontal-beam systems are used where pack height exceeds width and inspecting through the smallest cross-section improves sensitivity. The generator and detector are placed on opposite sides of the conveyor, and the beam passes sideways through cans, jars, bottles or cartons.

For low-density packaging such as plastic bottles, cartons or composite cans, a single horizontal beam is often sufficient to inspect for contaminants and quality defects. For medium-density containers like metal cans, dual-view or split-beam arrangements capture images from different angles, improving coverage of challenging locations such as sidewalls and bases. For high-density packaging such as glass, dual-beam or combined vertical-and-horizontal systems provide enhanced detection of glass shards in complex regions like the crown, base and shoulder.

5.3 Transport and Reject System Design

Successful inspection requires stable and consistent product presentation through the beam. Conveyors should minimize vibration, maintain orientation and spacing, and avoid features that mask or mimic contaminants. Product guides, spacing devices and speed control may be required to maintain proper pitch.

Automatic reject systems should remove contaminated packs completely and reliably, independent of where the contaminant is located in the pack. Options include air-jets, pushers, sweep arms, drop flaps, retracting belts and diverters, selected based on product weight, fragility and line layout. Reject receptacles must be suitably sized, secure and positioned so that rejected product cannot re-enter the good stream.

Retailer and industry codes often require additional features such as lockable reject bins, bin-full detection, reject confirmation sensors, belt-stop interlocks and clear visual and audible alarms.

Design features influence reliability, sensitivity, maintainability, hygiene and safety. Understanding them helps in evaluating systems and suppliers.

6.1 Generator and Detector Design

Generator design determines available power, beam quality and stability. Correct tube selection balances penetration needs against dose and image contrast, considering product density and thickness. Features such as stable high-voltage supply, effective cooling and robust shielding contribute to consistent performance and safety.

Detector design focuses on scintillator material, diode pitch, signal-to-noise ratio and dynamic range. Finer pitches allow detection of smaller contaminants but increase data volume, while coarser pitches reduce resolution but may suffice for larger products or contaminants. Detector linearity, uniformity and long-term stability are essential for reliable image analysis.

6.2 Mechanical, Hygienic and Safety Design

Cabinet construction must withstand the production environment, including wash-down, temperature changes and vibration, while maintaining shielding integrity. Stainless steel is commonly used for strength, corrosion resistance and cleanability. Design should minimize dirt traps, allow full access for cleaning and support hygienic practices consistent with standards such as EHEDG or 3-A where applicable.

Safety features include interlocked doors and covers, emergency stops, fail-safe indicators, radiation warning signs and compliance with relevant radiation regulations. Machines must be designed so that x-ray generation ceases immediately if protective housings are opened or faults occur, and leakage must remain below regulatory limits.

6.3 Software, User Interface and Fail-safe Functions

Software governs image acquisition, detection algorithms, quality checks, configuration and data logging. User interfaces should be intuitive, supporting quick product set-up, clear status indication and minimal risk of operator error. Access control via passwords or user levels helps protect critical settings.

Fail-safe system design includes continuous monitoring of key parameters, automatic detection of hardware or software faults, and controlled responses such as stopping the line and raising alarms. Features like reject confirmation, bin-full detection, encoder monitoring and periodic internal checks help ensure that failures are detected promptly and do not result in unmonitored product passing unchecked.

Achievable sensitivity in practice depends on many factors: product density and thickness, contaminant type and orientation, packaging, x-ray energy, detector resolution and image-processing methods.

Generally, smaller contaminants require higher contrast relative to background noise to be reliably detected. Dense contaminants such as steel, stone and glass in low‑density products are easiest; low-density contaminants in dense, heterogeneous products are the most challenging.

Sensitivity decreases as product height or thickness increases because x-rays are more attenuated and contrast between contaminant and product is reduced. Highly variable products or those with internal structures that resemble potential contaminants can also make detection more difficult and increase false positives if algorithms are not tuned carefully.

Selecting appropriate x-ray energy and filtration, optimizing product presentation and using advanced algorithms can significantly improve sensitivity. In some cases, tailored inspection regions or exclusion zones are used to focus on areas of interest and ignore irrelevant features.

Detector diode size (or pitch) is a key parameter affecting resolution, sensitivity, noise and cost.

Smaller diodes provide finer spatial resolution, improving detection of small contaminants, particularly wires, shards or small fragments. However, smaller pixels collect fewer photons, which can reduce signal-to-noise ratio unless compensated by higher exposure or improved electronics. They also increase the amount of data that must be processed, potentially affecting throughput.

Larger diodes offer better photon statistics per pixel and simpler data handling but may not resolve very small contaminants or closely spaced features. Selecting diode pitch involves balancing required contaminant size, product dimensions, line speed and system cost. Many applications perform well with moderate pitches such as 0.8 mm, while high-risk or high-value products may justify fine-pitch 0.4 mm detectors.

Beyond foreign‑body detection, x-ray systems can perform numerous in-line quality and integrity checks that add value to inspection programs.

9.1 Mass and Fill-Level Measurement

By analyzing overall absorption, x-ray systems can estimate gross product mass within a pack. Deviations from target mass can be used to detect underfills or overfills and to monitor process performance. Zoning of the image allows separate mass checks in different regions, such as compartments in ready meals or segments in multi-unit packs.

9.2 Component Count and Completeness

X-ray imaging can identify and count distinct components within a pack-such as bottles in a case, chocolates in a tray, sticks in a carton or tablets in a blister-to confirm completeness. It can also detect missing, broken or misaligned items, ensuring that multipacks and promotional combinations meet specification.

9.3 Package Integrity and Seal Inspection

Systems can detect damaged packaging, crushed corners, missing caps or closures and mis-applied components. Specialized high-contrast configurations can focus on seal regions to identify product trapped in seals that could compromise pack integrity or shelf life. For containers with metal caps, auxiliary sensors such as vacuum probes can verify closure integrity by measuring cap deflection associated with internal vacuum.

9.4 Presence of Inserts and Non-Food Applications

X-ray systems can verify the presence and position of premiums, de-oxidizers, labels or other inserts placed inside packs. They are also used in non-food applications such as verifying components and detecting broken needles in garments, or checking assembly completeness in medical devices and inhalers.

Purchasing an x-ray system is a strategic investment that should be evaluated beyond initial cost or headline sensitivity. A “value package” includes equipment performance, service and support, ease of use, integration, documentation, training and upgrade paths.

A structured approach involves:

• Clarifying objectives and requirements
• Conducting hazard analysis and CCP identification
• Performing product trials
• Evaluating suppliers’ technical and service capabilities
• Reviewing lifetime support offerings
• Considering future needs such as additional product lines or quality checks

Impulse decisions based solely on price or a single test report can lead to mis-sized systems, integration issues or under-utilized capabilities. Taking time to align system choice with process and program objectives reduces risk and maximizes long-term value.

An x-ray inspection program serves multiple purposes: protecting consumers from harm, safeguarding brand reputation, meeting regulatory and retailer requirements, and supporting efficient operations.

X-ray systems detect a broad range of contaminants, including those not detectable by metal detectors, thereby reducing the risk of serious incidents involving glass, stones or dense plastics. They also help prevent costly recalls, withdrawals and damage to brand trust.

In addition, x-ray data and quality checks support process control, yield management and continuous improvement, offering operational benefits beyond safety compliance.

An effective program integrates prevention, detection, correction and improvement within a documented framework aligned with HACCP and relevant standards.

Core elements include:

• Defined policies and objectives
• Hazard analysis and CCP identification
• Appropriate equipment selection and placement
• Validated operating sensitivity
• Documented procedures for testing, operation, maintenance and incident management
• Training for all relevant staff

The program should be tailored to business size and complexity while meeting external requirements from regulators, standards bodies and customers.

Documentation—procedures, records, test results, logs and images—provides evidence of control and is essential for audits, customer reviews and potential legal defense.

Preventive measures reduce dependence on detection and should be addressed before or alongside x-ray implementation. These measures include supplier controls and raw material screening, robust equipment design and maintenance, handling and storage practices, housekeeping and hygiene routines, tool and small item controls, and staff training.

Preventing contamination at source limits the number and severity of incidents reaching x-ray CCPs, easing the burden on detection systems and reducing waste and rework. Prevention activities are typically documented in prerequisite programs and audited regularly.

Location of x-ray systems is determined by hazard analysis and process flow. Typical CCPs include raw-material reception, post-processing steps where contamination risk is high, prior to packaging, and at the final product stage.

Selecting CCPs involves evaluating where hazards can arise, where they can be effectively detected and removed, and how practical and economical it is to implement inspection at those points. In many cases, multiple control points are chosen to provide layered protection, such as bulk x-ray early in the process and packaged-product x-ray at the end.

Operating sensitivity must be defined in practical terms for each product and CCP, specifying the smallest detectable contaminant sizes by material type that are required and can be achieved consistently.

Sensitivity targets may be set by internal risk assessments, customer specifications or certification schemes. They should be validated under normal production conditions, considering product variability, line speeds, environment and acceptable false reject rates. Sensitivity settings must also support stable operation, ensuring that systems do not become overly sensitive to harmless product variations.

Proper installation and commissioning are essential for safe and effective operation. Mechanical placement must respect shielding, guarding and access requirements; electrical connections must be stable and compliant; and integration with conveyors, reject devices and line controls must be tested.

Commissioning includes verifying leakage levels, checking all safety interlocks, configuring products and inspection tools, testing reject timing and confirming that performance meets agreed sensitivity targets. Training should cover system operation, safety procedures, test routines, basic troubleshooting and cleaning; it should be repeated or refreshed as needed for new staff or updated systems.

Performance verification ensures that systems continue to operate at required levels over time. Programs define test methods, frequencies, acceptance criteria and responsibilities. Typical tests involve passing certified test pieces of known type and size through the system at specific positions, observing detection and proper rejection.

Auditing involves reviewing records of tests, alarms, maintenance, incidents and corrective actions to confirm that procedures are followed and effective. Audits may be internal or conducted by external parties such as certification bodies or customers.

Procedures for handling suspect or rejected product must prevent contaminated items from re-entering the saleable stream and ensure appropriate disposition.

Rejected packs should be collected in secure, clearly identified receptacles or areas, with access controlled and contents reconciled. Decisions on re-inspection, rework, destruction or downgrading should follow defined criteria and be documented. Repeated or unusual rejections should trigger investigation into root causes, such as equipment damage, supplier issues or process changes.

TCO analysis covers the full lifecycle of an x-ray system: purchase price, installation and integration, training, maintenance, calibration, parts, energy, test time, downtime, false rejects and potential costs avoided through effective detection.

Systems that are robust, stable and easy to test may have higher initial cost, but a lower long-term operating expense. Conversely, lower-cost systems that require frequent adjustment generate many false rejects or suffer from reliability issues can be more expensive over their lifetime. TCO should be evaluated alongside risk reduction and quality benefits when comparing options.

Due diligence requires demonstrating that all reasonable steps have been taken to prevent unsafe product reaching consumers. For x-ray inspection, this involves having appropriate systems in place, operating them correctly, and maintaining thorough documentation.

Key components include:

• HACCP-based hazard analysis and CCPs
• Appropriate selection and validation of x-ray systems
• Documented procedures for operation, testing and maintenance
• Training records
• Logs of inspections, alarms and rejects
• Evidence of corrective actions and continuous improvement

Compliance with recognized standards such as BRC, IFS, FSSC 22000 or SQF, and adherence to retailer codes of practice also support due diligence claims. Senior management must endorse and enforce the program and ensure that inspection performance is regularly reviewed.

Data from x-ray systems such as reject counts, test results, image archives, alarm histories and performance indicators provide rich information for improving both the inspection program and upstream processes.

Analyzing trends can reveal recurring issues by product, line, shift, supplier or equipment. Insights may lead to changes in raw material controls, equipment maintenance, process parameters or training. Image archives of rejected packs offer useful evidence for investigating complaints and understanding contamination events.

Integrating x-ray data with other quality and production systems supports holistic continuous improvement and enhances overall equipment effectiveness.