X-Ray System Diode Size: A Sensitive Issue?

In today’s globalized food industry, x-ray inspection system sensitivity, and particularly the size of the machine’s diodes, has become of increasing concern to the growing number of multinational manufacturers working to comply with the many stringent safety regulations in force in their target markets. But is it always necessary to select an x-ray system with the smallest diodes?
 
What Exactly Do We Mean When We Talk About Diode Size?
The diodes are, arguably, the most important part of the x-ray machine. Just like the pixels in a digital camera, lines of diodes capture the x-ray beams passing through the search head and transform them into an electrical signal, which are then transmitted to the system’s computer for conversion into a grayscale image ready to be inspected for anomalies.
    
In general, a detector diode size of 0.8 mm will create a new line of image data every time the product moves 0.8 mm through the x-ray machine, and a diode size of 0.4 mm will produce a new line of data for every 0.4 mm of movement, and so on. These lines build up sequentially and are stacked into a matrix of pixels to form an overall image of the product being scanned.

Why Has Diode Size Become Such a Concern to the Food Industry?

In the current climate of globalization, more manufacturers than ever before are exporting their products overseas, eager to take advantage of lucrative emerging markets, such as the BRICS nations of Brazil, Russia, India, China, and South Africa. In order to retain access to these burgeoning economies, manufacturers are now finding that they have to ensure that their production processes comply with multiple food safety regulations at the same time, rather than just those in force in their home territory.
    
This state of affairs has, obviously, led to growing concern among food manufacturers about the sensitivity and effectiveness of the x-ray inspection system they use to detect physical contamination. Many worry that if their x-ray machines are not sensitive enough, they may not be able to capture every potential contaminant on their line, raising the likelihood of a brand damaging product recall, with possible repercussions for their trade in overseas markets.
    
Now, there is a widely-held theory in the food and beverage industries that the smaller the x-ray system’s diodes, the better it will be at detecting the smallest foreign bodies in food and other products, and so minimizing the risk of contamination passing unnoticed. However, this is something of an over-simplification of the entire issue. While smaller diodes, of 0.2 or 0.4 mm in size, say, do provide more data, giving greater spatial resolution – the ability to distinguish the finer details of the product passing through the search head – in x-ray images compared to larger 0.8 or 1.6 mm diodes, there are a number of other factors that also have to be taken into account, beyond the simple theoretical sensitivity of the diode itself.

What Other Factors Affect X-Ray Sensitivity?
It’s fair to say that, in addition to spatial resolution, the overall sensitivity of an x-ray system is dependent on the radiographic contrast it offers, which in turn is reliant on the machine’s ability to overcome the product effect.
    
Radiographic contrast simply refers to the ability of the x-ray machine’s computer algorithm to distinguish between the contaminant and the surrounding product, which plays a major role in allowing the x-ray system to detect foreign bodies. The principal factor affecting radiographic contrast in x-ray detection is signal-to-noise ratio. All electrical devices create a degree of background noise and if this noise level is too great, relative to the strength of the signal, x-ray image quality will be poorer. Different diode sizes produce different signal-to-noise ratios, but generally the bigger the diode, the stronger the x-ray signal relative to background noise will be. This means the radiographic contrast will be higher, resulting in a better quality image and a higher likelihood of contaminant detection.
    
In addition to the signal-to-noise ratio, product effect – the amount of x-ray energy absorbed by the product and any contaminants as the pack passes through the x-ray system – affects the contrast in the x-ray image and therefore the likelihood of detection. The level of x-ray absorption is dependent on the product or contaminant’s thickness and density, and generally, the greater the thickness or density, the more x-ray energy is absorbed before reaching the diode. Dense contaminants for example, such as stones, glass or metal, will be easy to spot in a less dense product, such as bread, but much harder to detect surrounded by a denser food, like cheese. To overcome this, more x-ray energy is needed to penetrate dense products, impacting on the choice of diode size.
    
The texture and uniformity of the product also have to be taken into account when considering product effect. Homogeneous packs provide a constant x-ray signal, and small changes in absorption of x-ray energy are therefore easily detected. However, in ready meals and foods, such as salad bags or packs of frozen vegetables, there are areas of varying absorption caused by different food types and pockets of air between items. These can reduce the contrast between product and contaminant in the final x-ray image, making it harder to detect foreign bodies, particularly those with a lower density, such as glass, stones or bone.
    
With all this in mind, it is clear that, in addition to considering the spatial resolution offered by the x-ray system, it is important to take into account the specific properties of the product to be inspected. Plus we need to consider its effect on the radiographic contrast of the final image to ensure maximum contaminant sensitivity.

How Can Manufacturers Decide What Diode Size Is Right for Their Production Line?
There is no “one-size-fits-all” solution here. If the manufacturer is simply inspecting thin, homogenous products, like sachets of powder or sauce pouches, then an x-ray system with smaller diodes will indeed be the best choice for their line. However, if the product in question is not uniform, such as a bag of frozen vegetables, then a machine with larger diodes will be more appropriate. Likewise, if the principal contaminant threat to the product is from foreign bodies significantly denser than the surrounding product, such as metal shards in a block of cheese, then a system with a small diode size will be suitable. If the threat is from less dense contaminants, such as stones or bone fragments, then a larger diode size would be recommended.
    
Ultimately, the choice of diode size will depend both on the specific properties of the product to be inspected, and nature of the contaminant threat. Carrying out a Hazard Analysis and Critical Control Points (HACCP) audit will enable manufacturers to identify not just the Critical Control Points (CCP) where the line is most vulnerable to contamination, but the most likely contaminant risk as well. This can help them to determine the magnitude of the product effect and help them to decide which diode size is most appropriate for their needs.

What else should manufacturers look out for when selecting x-ray technology?
There are a number of other factors that manufacturers need to take into account when selecting x-ray technology, as well as its sensitivity.

Systems capable of offering sensitive contaminant detection at high speed, for example, can help manufacturers to maintain high throughput rates throughout their production line, maximizing productivity. There are even machines now that can be set to operate at varying speeds, enabling manufacturers to respond efficiently to changes in market demand. X-ray systems that offer pre-set menus can also help to speed up the changeover process, reducing downtime to further enhance efficiency.
    
It is also important to look at the nature of the product packaging when selecting an x-ray system, as this can affect the choice of rejecter system. Small, lightweight packs may require an air-jet rejecter, whereas tall packs, such as bottles or jars may need a sweeper system to minimize the risk of breakages.
    

Whatever diode size the manufacturer selects, it is vital to consider the broader needs of the production line as well to ensure the most suitable x-ray system is chosen.

Is there any help out there for manufacturers to help them make a decision?
There is plenty of support available for manufacturers to help them select the right diode size and the most appropriate x-ray solution for their production line. They can take advantage of white papers on the subject, as well as advice on the internet.
    
However, to get the most up-to-date information, as well as the best guidance, it is important to talk to product inspection experts. This way they can gain a deeper understanding of the issue of diode size, and of their production line’s specific needs, helping them to choose the right product inspection solution for their requirements.
    
X-ray sensitivity is dependent as much on the properties of the product being inspected and the contaminants to be detected as it is on the diodes in the x-ray system in question. But with advice from experts, manufacturers can strike a balance between spatial resolution, radiographic contrast, and product effect, to ensure optimum foreign body detection on their line. Doing this, they can maintain product safety, safeguarding both consumer wellbeing and brand reputation in both their home territory and in lucrative international markets.

    Norbert Hartwig is director R&D, Eagle Product Inspection. He holds a Diploma-Ingenieur Elektrotechnik (Master of Science) from the Technical University of Darmstadt/Germany. He has more than 20 years experience focusing on high technical x-ray technologies. For more information on Eagle Product Inspection, Tampa, FL, call 877-379-1670 or visit www.eaglepi.com.

For related articles, news, and equipment reviews, visit our Screening & Separation Equipment Zone