Food manufacturers across the globe have been relying on x-ray technology since the early 1990s to protect consumers, reduce the risk of product recalls and safeguard their brands. But what exactly is x-ray inspection and how does it work?
This article takes food x-ray inspection back to basics by explaining what x-rays are, the main components and operating principles of an x-ray system, and the types of foreign bodies that can be detected.
What are X-rays?
X-rays are a naturally-occurring source of radiation and 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 short wavelength of x-rays enables them to pass through materials that are opaque to visible light; however they don’t 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.
Principles of X-ray Inspection
In simple terms, an x-ray system uses an x-ray generator to project a beam of low energy x-rays onto a detector. X-ray inspection involves passing a product or pack through the x-ray beam before it reaches the detector. The amount of x-ray energy absorbed during an x-ray beam’s passage through a product is determined by the product’s thickness, density and atomic mass number. When a pack or product passes through the x-ray beam, the beams 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 the product and foreign body is the basis of x-ray inspection.
What Makes up an X-ray System?
An x-ray system consists of three key components:
- An x-ray generator
- A detector
- A computer
An x-ray system is essentially a scanning device. When a product passes through the system at a constant speed, the x-ray detector captures a grayscale image, which is generated by measuring the amount of x-ray energy reaching the detector. Each image is made up of pixels and the x-ray energy absorbed by each pixel creates a value on a grayscale. As the product or pack passes over the detector, each line of gray level data is added to previous lines, just like slices of bread can be added to form a loaf, resulting in a complete product image. Eagle’s SimulTask™ PRO imaging software generates powerful image analysis routines, has autolearn functionality for faster product set-up and changeovers, on-screen diagnostics and safety system status visualization. The software within the x-ray system analyzes the image and compares it to a pre-determined acceptance standard. On the basis of this comparison, the system either accepts or rejects the image. If the image is rejected, software sends a signal to an automatic reject system which then removes the product or pack from the production line.
The x-ray beam is generated by an x-ray tube encased in the x-ray generator and passes through the product or pack being inspected, before finally reaching the detector. Standard x-ray systems have one detector containing individual elements called diodes, which convert the level of detected x-ray energy into an electrical signal that’s sent back to the machine’s on-board computer.
X-ray is All About Absorption Difference
A process known as ‘relative absorption’ is at the heart of x-ray inspection. It refers to the different amounts of x-rays that different materials absorb and allow to pass through them. Food products typically contain low atomic mass elements and have a low density, while foreign bodies usually contain high atomic mass number elements and have a high density. For this reason, it’s convenient to use density as the benchmark for foreign body detection.
Which Foreign Bodies Can Be Detected?
As a general rule, if a contaminant floats on water, it is not detectable by x-ray. Usually, contamination detection is only possible when a foreign body is denser (i.e. has higher specific gravity) than the food product in which it’s embedded. Many foods are water-based which means they have a relative density similar to water. Expressed in specific gravity terms, this is an SG of 1.0. Some foreign bodies aren’t detectable because their density is less than (or too close to) that of the food product. As densities increase, physical contaminants which absorb more x-ray energy are more easily detected. This also means smaller particles of these kinds of foreign body contaminants can be detected with greater ease by x-ray inspection systems
Wood, for example, is very hard, but not very dense, and therefore isn’t detectable. Most plastics are also very hard but show densities similar to water, making them difficult to detect in products with densities similar to water. Ferrous metals, most non-ferrous metals and stainless steel all have specific gravities between 7.0 and 8.0 which means they’re detectable at the same sensitivities (sizes). In contrast, aluminum – a low-density metal with an SG of 2.71 – is x-ray detectable at sizes similar to glass and stone, which both have similar densities.
X-rays are extremely good at detecting dense foreign bodies, especially ferrous and non-ferrous metals, stainless steel, glass and mineral stone. However, x-ray systems are not only capable of just contaminant detection they are also capable of performing numerous quality assurance checks for bulk, loose, packaged or unpackaged products. Checks such as fill level, package integrity, component count and mass measurement.
See table below for a general overview of contaminant detectability, does not include variability for throughput or product density.
|Typical Food Contaminant||Typical Density [kg/m³]||Detectability|
|Stainless Steel||7.93||Easily Detectable|
|Glass||2.40 – 2.80||Detectable|
|Stone||2.30 – 3.00||Detectable|
|Polypropylene||0.90||Typically Not Detectable|
|Wood||0.65||Typically Not Detectable|
|Insects||0.59||Typically Not Detectable|
|Cherry Pit||0.56||Typically Not Detectable|
|Hair||0.32||Typically Not Detectable|
Christy Draus, Eagle Product Inspection Marketing Manager