In the stress analysis by X-rays, the detectable volume is the function of the area irradiated by X-rays and the depth of penetration of X-rays. It involves a number of crystals having the specific diffraction plane which satisfy the Bragg's diffraction condition in regard to incidental X-rays. Therefore, if the orientation of each crystal in the metal is statistically uniform, we can postulate that the macroscopic gauge length for the stress analysis equals the area irradiated by X-rays. In

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... photographic method, the irradiation area is limited mainly by the optical slit system. Since the irradiation area is the gauge length for stress analysis, it is necessary that X-rays with higher intensity and better parallelity are irradiated uniformly on the area limited by the measurement purpose. But, there are no practical slits with good characteristics, which was examined closely from such a view point, on the market. According to the geometrical condition, we classified the slit system into the following four classes; that of the source at inlet and outlet slit. For these four classes we calculated the dimensions of X-ray irradiation areas and their intensity distributions from the view point of geometrical optics. We made for trial the slit systems with different geometrical conditions to provide for our experiment. By using these slit systems, we measured the dimensions of X-ray irradiation areas and their intensity distributions. The experimental results were in fairly good agreement with the theoretical calculations. From this study it was concluded that the setting of inlet and outlet slit close to the X-ray source and to the specimen respectively, was effective to obtain a uniform distribution of incident X-rays and to reduce the exposure time.