Abstract: A method is provided for setting up a photon counting detector, to enable it to carry out a centroiding procedure in which a photon event occurring in a given range is allocated to one of a plurality of channels into which the range is divided. The method comprises the steps of: a) sub-dividing the range into a plurality of channels of preferably equal width and loading the channel boundaries into a look-table; b) performing an integration on a flat field and allocating photon events to the channels defined in step a); c) counting the number of events allocated to each channel; d) altering the channel boundaries in directions tending to equalize the number of events allocated to each; and, optionally, e) repeating steps b), c) and d) until the variation between the number of events held by the channels is at an acceptable level.