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.
Smaller diodes (0.2 mm or 0.4 mm) 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 mm or 1.6 mm) diodes. However, diode size is not the only factor, 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.
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.
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 a larger diode size to make it easier to detect physical contaminants in products with variable densities as the radiographic contrast will be better.
Which Diode Size is Best for You?
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. This is because detectors with smaller diodes cover less surface area, meaning they need more time to collect sufficient data to provide quality images.
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.