When talking about x-ray inspection, we have noticed over the years that magnification and field of view (FOV) are characteristics not well understood. To set the record straight, in this post we will describe in detail what each one of these parameters mean. The following figure shows a simplified diagram of an x-ray tube. Note that the modern tubes used in our systems are far more complex, but this diagram is very useful to illustrate magnification and FOV. The x-ray tube is the device inside the x-ray source that generates the x-rays that are used to project an image onto the x-ray sensor. The electron beam generated by the cathode is rapidly accelerated against the anode. Upon colliding with the anode, a beam of x-rays is generated.
The random nature of the collision of the electron beam on the anode target creates an x-ray beam that is cone-shaped. As the x-ray beam moves farther from the anode target, the diameter of the beam increases proportionally. The angle of the x-ray cone beam, α, is determined by the angle of the anode target. The following figure shows the FOV at different distances from the source. This measurement is called source to object distance (SOD). It is important to note that the SOD is not measured from x-ray window to the object. Instead, the measurement starts from the target inside the tube to the object. X-ray source manufacturers give us that distance, so we can add it to the distance from the top of the source to the object.
What the previous image shows is that the diameter of the x-ray beam increases as it moves away from the source. For example, at 2” from the source, the diameter of the x-ray beam is 1.4”. To better illustrate this discussion, let us use the TruView Prime x-ray inspection system as an example. The minimum distance between the x-ray source and the x-ray camera in the TruView Prime is 6”. This measurement is called the SID – source to imager (x-ray camera) distance. The SID was designed to accommodate TruView Prime’s large 3”x4” high definition flat panel x-ray camera. That means that we would see vignetting of the image if we placed the camera closer to the source. The following image shows an example of vignette – an x-ray image when the camera is too close to the source. The dark corners in the image represent the regions where the x-ray beam is not shining.
Now that we have a better idea of how to calculate the FOV based on the distance to the x-ray source and the angle of the x-ray beam, the next step is to connect it with the concept of magnification. We will use the TruView Prime as an example again. In the TruView Prime, the camera, or x-ray sensor, can move up and down to change the distance between the camera and the sample, as seen in the following figure.
The magnification, M, of the system is given by:
In the example shown in the previous figure, the magnification of the system is 2:
That means that within the boundaries of the 3”x4” x-ray sensor in the TruView Prime, the sample image will be magnified by 2X. Thus a 100um2 feature in the sample will be projected in a 200um2 area of the x-ray sensor.
We hope this post was able to clarify some of the topics related to x-ray inspection. As usual, we’d love to hear your feedback!