Determining the visual resolution Slitcamera Determining the focal spot

On the resolution measurement and electron-optical adjustment of image intensifier systems

It is common knowledge that flawless (image or electron) optics cannot exist due to theoretical reasons. The quality of optics reflecting one point in one level of the table, is basically dependent on how well the correction of the following image artefacts has turned out: spherical aberration, chromatic aberration, image field curvature, coma and astigmatism.
Especially the last three errors result in:
  • The resolution of one point of the image field depends on its distance to the middle of the image.
  • The measured resolution of points outside of the image's middle also depends on the line direction that the grids used for the measurement have. In fact, there are extreme values (maxima and minima) of the resolution obtainable, if the lines are directed to the center (spoke direction) or vertical in this direction (rim direction).
Furthermore, it is to be mentioned that with every artificially manufactured optics, deviations from the rotational symmetry (which is assumed in its calculation) can never be fully avoided. They induce:
  • Even points with the same distance to the middle of the image, i.e. points located circularly around the middle, do not often have the same resolution values.
Thus, it is necessary for the evaluation of the image intensifier's quality to measure the resolution of as much points of the image field in the spoke and rim direction as possible.
For this purpose, (at least) 13 test plates are used as far as modern image intensifiers are concerned. One is positioned in the middle of the image and 4 at a time on circles around the middle. Each of the test plates should have line groups in an appropriate resolution range with two line directions that are vertical to each other. Whereas, it is important to note that the actual test field (especially in the spoke direction) is fairly small so that the resolution of certain "points" can be measured.
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