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A note on the measurement of focal spot sizes utilizing star test patterns at low magnification factors.

The validity of focal spot size measurement utilizing the star test pattern at magnification factors less than 1.4× has been confirmed by the use of a 0.4° vertex angle star test pattern and a traveling microscope. It has been found that the apparent nonlinearity of the star test pattern method in the measurement of focal spot sizes at low magnification factors is introduced by the observers as a systematic error due to the difficulty in determining the blur diameter within reasonable accuracy.