How do x-ray systems work?
Modern x-ray systems are very advanced and effective pieces of equipment and are routinely used in a range of industries for inspection purposes.
Most people are familiar with medical diagnostics and security x-ray systems but are unaware of their significant presence in the food, pharmaceutical and electronics industries where they are quietly working away 24/7 to ensure product quality.
But how do x-ray systems actually produce an image and allow us to see foreign bodies and other product defects?
X-rays are electromagnetic radiation, part of a single continuum known as the electromagnetic spectrum, this is arranged according to frequency and wavelength, and runs from radio waves at one end (which have a long wavelength) to gamma rays at the other (which have a short wavelength).
The short wavelength of x-rays enables them to pass through materials that are opaque to visible light, however they do not 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. Hidden contaminants and other dense internal structures, including foreign bodies like glass, bone and metal, show up because they absorb more x-rays than the surrounding product.
In simple terms, an x-ray system uses an x-ray generator to project a beam of low- energy x-rays onto a linear x-ray sensor or an x-ray imaging intensifier.
The system is typically configured with the x-ray generator above the conveyor, which is used to transport the product through the system and the x-ray sensor positioned under the conveyor which collects and converts the x-ray “shadow graph” signals into a visible image.
The x-ray shadow graph is generated as a result of the different attenuating properties to the product under inspection. For instance a fragment of glass in a pack of cheese has a higher attenuation (adsorption) than the cheese in which it is embedded therefore as that section of the pack is inspected the area of glass generates a “shadow” or lower beam intensity that the detector can record and display.
It is by this very simple principle that all x-ray systems operate.
Although generally with food products the contaminant has higher absorption and therefore generally displays as a darker than the background image it is still possible to detect material that has a lower absorption that the background. This allows the detection of voiding in filled packs and the detection of missing components in multiple unit packs.
The same shadow attenuation principles apply for the inspection of pharmaceuticals, missing, broken tablets, missing instruction leaflets etc. and across a whole range of industries and quality applications that are beyond the scope of the brief introduction.
Image generating detectors
There are numerous types of x-ray image generating devices but broadly they fall into two categories a line (or linear) array or an area sensor.
- A line sensor, also often referred to as a linear array, photo-diode array or line scan detector. As its name implies it is a line of x-ray sensitive detectors which is positioned under the conveyor and across it at 90 deg to the direction of travel of the product. It relies on the motion of the conveyor to generate the image in the travel direction and is the most commonly used method in automated systems.The advantage of these detectors is that they produce a high contrast low, noise image and operate over a wide energy range. The disadvantage is that as they are made up of a single line of individual sensing elements the spacial resolution becomes a function of the sensor element size.
In security applications such as baggage screening the typical size is 1.6 x1.6mm, in food and industrial inspection, typically 0.8 x0.8mm , 0.4 x 0.4mm and in some cases 0.2 x 0.2mm. There are higher resolution sensors than this but it is unusual to need to deploy them in most applications.
- Area sensors, these can either be image intensifiers of flat panel arrays. In fully automated systems these sensors are unlikely to outperform the line sensors. They were developed for and are generally used in the medical sector for heart procedures and as a replacement for x-ray film. The main advantage they have is that you a full frame image generated which is not dependant of movement of a conveyor; in fact movement is generally detrimental to the image quality. The other advantage of the image intensifier is that it is a very high gain device with high spacial resolution but at the cost of significant signal noise.
In conclusion both techniques have their advantages and at AIS you will always have access to the most appropriate system for your requirement.