Spot Size Measurement and Analysis
A key parameter in the clarity of an x-ray image is spot size. Physically, this is the size of the region where the x-rays are generated. The larger this region, the fuzzier the resulting image will be. Likewise if this was an infinitely small area; image clarity would be the best. In the real world there are limits on how small this region is, with areas (diameters) of 1-1.5 mm being typical for the machines I describe here.
Since this is a critical parameter for overall performance, we have methods to measure the spot size. Two methods I use commonly are the roll bar and pin hole methods.
Roll Bar
The first method is the roll bar. It is simple, with the roll bar itself shown below. Just a block of tungsten with one face crowned.
It is used as shown below to obtain either a film image that is then digitized, or a direct digital image
The Roll Bar
How the Roll Bar is used in test
From the size of the transition region between dark and light, on the film, it is possible to calculate the spot size.
To calculate the spot size from the edge data one needs to differentiate some very noisy data. A traditional method results in meaningless data
The computational code I developed performs the differentiation as well as a re-integration of the derivative, so a double check of assumptions is performed.
Grid Camera
The second method is the Grid Camera method. It involves a more complicated piece of equipment but provides a direct measure of spot size. The physics of the camera is simple but the construction is very complicated, and is the proprietary information of my current employer, so I can not disclose that.
However the physics is simple and I can describe that.
The Grid Camera is essentially a long narrow block of tungsten (or other dense metal) with a 2-dimensional pattern of long, thin, parallel holes through the length.
The Grid Camera
The hole pattern
It is used to capture an image like that shown on the left below. I have contributed in various ways to this effort. First I built our existing camera. Second I designed and built the fixturing for the camera that allows it to be used on all our machines. Third, by automating the analysis of these films. I have developed software that, with the setup of a couple basic parameters, automatically performs the analysis. The example below right shows the 50% spot size overlay.
