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Box 2 Explanation

2. 4.

Box 2 represents what the detector "sees" as a function of time as X-ray photons from the off-axis point source indicated in Box 1 pass through the grids. The bottom grid is directly above the detector and the two rotate together. The detector "sees" the bottom grid as a stationary object with bars. The shadow of the top grid slides backward and forward with simple harmonic motion over the bottom grid once per half rotation. The slits in the bottom grid are partially blocked by the projection of the slats from the top grid. The degree of blocking changes with time to produce a variation in the transmitted number of photons. The shadow of the top grid moves in a circle on the bottom grid. The component of this motion parallel to the slits has no effect on the fraction of the photons that get through the grids. The component of the motion perpendicular to the slits causes the effective slit width of the grids to increase and decrease with time. This variation in effective slit width modulates the photon flux hitting the detector. That intensity variation as a function of time is represented in Box 4.

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This site last updated November 10, 2008.