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The collimator and the X-ray beam are tilted slightly upwards due to the adjustment of the bolts at the base. Because the sample is held vertically, this means that the beam does not pass through it at exactly 90 degrees – the angle is about 87 degrees. In theory the path length is therefore slightly longer than the thickness of the sample but the difference is negligible. There are reports that when the beam is exactly aligned with the axis of the wood cells the attenuation is reduced, by up to 10-20%. We have modelled this effect and found that the alignment would really need to be very exact, within about 1 degree. The cell orientation is statistically variable so it is likely that only small parts of the sample would be accurately enough aligned for this effect to be visible. But we wondered if perhaps the upward tilting of the beam was intended to prevent it.