One of the most critical aspects when beginning to understand industrial x-ray inspection is the importance of resolution and how it should be applied. One of the first questions I receive when discussing x-ray is- “what is the best resolution your system can achieve?” My answer 100% of the time is- it depends! The top two factors when discussing resolution are geometric magnification and the basic spatial resolution of the detector (SRb).
The pixel pitch of the digital detector array (DDA) is typically declared by the manufacturer and is specific to each model number. It would be ideal if the manufacturer’s pixel pitch translated exactly to the SRb of the detector, but that is just not the case. Luckily, there are explicit instructions discussed in ASTM E2002 and with the help of a duplex wire gauge you can determine the basic spatial resolution of the detector you are using.
Once the SRb of the DDA is determined, you can figure out the geometric magnification based on your inspection technique. Geometric magnification is the relationship between the Source to Detector Distance (SDD) and the Source to Object Distance (SOD). By dividing the SDD by the SOD, you can calculate your geometric magnification (EX: 1200mm/600mm=2X Geo. Mag). Based on this equation, you can increase your magnification by decreasing your SOD or by increasing your SDD (EX: 1200mm/300mm=4X Geo. Mag. OR 2400mm/600mm=4X Geo. Mag.).
Typically, with industrial digital x-ray, we prefer to bring the DDA as close to the source as possible and decrease the SOD to an ideal amount. This is done based on the principals of cone-beam x-ray, which states the closer the DDA is to the source, the more flux penetrates the part and reaches the imaging plate. By allowing more flux to reach the plate, you allow the opportunity to reduce the wattage, therefore theoretically allowing you to increase the sharpness of your image.
Now that the SRb and the magnification have been determined based on your specific x-ray technique, it is time to put them together. Like the relationship betwixt SDD and SOD, finding the resolution of your technique is determined by the relationship between the SRb and the magnification. By taking the SRb and dividing it by the magnification, you can determine your resolution (SRb/Geo. Mag.). For example, if you have determined the SRb of your DDA is 220 micron and your technique places the component at 2X magnification, your resolution would be 110 micron.
While resolution is an important factor to discuss while developing a proper x-ray inspection technique, but it is not the only factor. There are many more variables to consider that are dependent on the part you are attempting to inspect. There is no one technique that can be used with all products being inspected. Resolution, along with the other variables, are dependent on material size, geometry, alloy type, and purpose of inspection.
For more information, please reach out to our PiXL team at [email protected].