Do You Need Diodes Sizes? Choosing The Right X-ray Inspection Machine

Author Steve Dowd

Diode size is the subject of much debate. It’s widely believed that the smaller an x-ray inspection machine’s diodes, the better the system will be at detecting the smallest physical contaminants. However, this is an over-simplification and ignores other factors affecting sensitivity of contamination detection.

Other factors must be taken into account, such as radiographic contrast caused by smaller vs. larger diodes, a product’s effect (i.e. density, texture, uniformity) and conveyor line speed.

As x-ray inspection technology has advanced over the years, a new consideration has emerged for those shopping the detector market: do you need diodes at all? While traditional scintillator detectors rely on the presence and configuration of photodiodes for detecting contaminants and foreign bodies, more modern photon counting detectors eliminate diodes from their construction altogether.

With more options available now than ever before, understanding which type of x-ray inspection technology can meet your desired capabilities is crucial to making the most useful and cost-effective choice for your business. In today’s blog we’ll be covering the 3 major types of x-ray inspection technologies and the factors which impact detection across the board.

Types of Scintillator X-Ray Detectors

Traditional scintillator style detectors rely on a process called indirect detection. In indirect detection, the x-ray beams, made up of photons, are converted once into light signals, then converted again into electrical signals. There are two common configurations for scintillator x-ray machines: linear array detectors, and TDI detectors.

Linear Array: Basic Detection With Diodes

A linear array x-ray machine contains a simple scintillator detection system. It’s called a linear array because the photo diodes are arranged in a single row alongside each other. This configuration is the most compact, lightweight, and cost friendly option. The lower cost of entry comes with some limitations like a limited field of view and slower scanning speeds, but mass measurement is where a linear array detector really shines. This technology provides the most accurate mass measurement of ±1.0-1.5%. While effective on its own in certain applications, this technology also serves as the basis for Time Delay & Integration (TDI) x-ray detection.

Advanced Detection With Diodes

A TDI detector functions on the same principle as a linear array detector, but with multiple rows of diodes stacked successively. With a higher quantity of diodes comes the possibility of greater image resolution—the ability to distinguish the finer details of the product passing through the search head. At the same time, this also presents more opportunities for missed contaminants and false positives if set up incorrectly. The diodes must be carefully aligned with one another to properly process the x-ray beam in order to render an accurate image of the target object. When set up correctly, a TDI detector allows operators to generate images with higher resolution and better quality using less x-ray power compared to a linear array detector. In general it does come with a higher operational cost as a result.

The Effects of Diode Size on X-Ray Detection

Smaller diodes (0.2 mm or 0.4 mm) do provide more data, giving greater spatial resolution      in x-ray images      compared to larger (0.8 mm or 1.6 mm) diodes. However, detectors with smaller diodes cover less surface area, meaning they need more time to collect sufficient data to provide quality images. A smaller diode pitch can also create a situation in which the signal to noise ratio is greater than that of the potential target. In this situation, achieving detection would require either a greater x-ray power– which comes with a higher cost of shielding and a shorter tube life– or a larger diode pitch.

For this reason, there’s often a trade-off as higher energy x-ray inspection machines have a greater total cost of ownership as the x-ray tubes have a tendency to burn out and are costly to replace.

PXT™: Advanced Detection Without Diodes

Unlike linear array and TDI detectors, PXT™ uses a method called photon counting, a form of direct detection. In this process, the photons emitted by the x-ray generator are converted directly into electrical signals. In doing so, the intermediate conversion to light signals is eliminated. This allows PXT™ to produce results more quickly and with greater accuracy than indirect detection technologies. It is also the most successful at distinguishing organic and inorganic materials like calcified bone to inorganic materials like low mineral glass.

Exclusive to Eagle PI, PXT™ photon-counting technology exhibits the most advanced detection capabilities on today’s market. CLICK HERE TO FIND OUT MORE >>>

Application Factors Which Influence Detection

Radiographic Contrast

Radiographic contrast refers to the ability of the x-ray inspection machine’s computer algorithm to distinguish between physical contaminants and the surrounding product. The principal factor affecting radiographic contrast in x-ray detection is signal-to-noise ratio. If the noise level is too great, relative to the strength of the signal, x-ray image quality will be poor. 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 higher chance of contamination detection. The key is to have the proper image resolution to find the given contaminant and not have a high signal to noise resolution, which could result in more false positives.

Product Effect

Product effect (the amount of x-ray energy absorbed by the product and any physical contaminants as the pack passes through the x-ray inspection machine) also affects the contrast in the x-ray image and likelihood of contamination detection. X-ray absorption depends on the product or contaminant’s thickness and density. Generally, the greater the thickness or density, the more x-ray energy is absorbed before reaching the diode. Dense physical contaminants will be easier to spot in a less dense product like a two-inch block of cheese, compared to a denser four-inch block of cheese. More x-ray energy is therefore required to penetrate dense products, which impacts on the choice of diode size. A machine with larger diodes is better suited to inspecting high-density products.

A product’s texture and uniformity must also be taken into account when considering product effect. Homogeneous packs provide a constant x-ray signal, meaning small changes in absorption of x-ray energy are 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 low-density physical contaminants. Advanced technologies, such as dual energy detectors, use image filtering techniques which reduce the variability in the image      to make it easier to detect physical contaminants in products with variable densities as the radiographic contrast will be better.

Want to learn more about advancements in dual energy detection? Click below to unlock access to our Resource Center.

Which X-Ray Detector Technology Is Best for You?

Think about what you want to prioritize in your application. When choosing an x-ray detector, there are 4 key areas to focus on:

Application

• Is it important to achieve a higher image resolution for detecting foreign objects, or to obtain more data for verifying chemical lean values?
• What drives your x-ray requirements: customer demand, due diligence, or a combination of both?

Specification

• Are you inspecting products of uniform or varied densities?
• What are the dimensions of your product?
• How do the density and dimensions of common contaminants compare to those of your product?

Production Environment

• How will your production environment affect wear on the system over time? Consider how factors like line speed can alter requirements for x-ray intensity.

Cost of Ownership

• How often can you perform maintenance on your x-ray inspection system? Think about the effects of downtime on your line.

Answering questions like these will help guide you to the right choice in x-ray detection for your business’ exact needs. There is no “one-size-fits-all” solution and it’s important to be mindful that while spatial resolution improves as diode size decreases, smaller diodes require higher x-ray energies and lower line speeds to maintain image quality. If you’re ready to start exploring your options or get recommendations from one of our x-ray experts, contact Eagle PI today to get started.