Electromagnetic calorimeters based on dense, scintillating crystals have the potential to achieve extremely good energy resolution. However, care must be taken to minimise systematic effects such as variation of light collection efficiency with depth in the crystal. A HPMT has been used to accurately measure the non-uniformity of light collection in lead tungstate crystals for the CMS detector. It is found that the non-uniformity of crystals measured in the detector configuration is not much larger than the maximum tolerable limit. Shading the chamfered edges of the crystals with a pencil can reduce the non-uniformity by approximately 0.1%/X0. Roughening the chamfers achieves a similar reduction.
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