A printing unit comprises a movable carriage being movable along a path and having a plurality of energy emitters arranged in an array, a controller to control a supplied energy level of the emitters, and a sensor unit to measure an emitted energy level of each emitter, wherein the controller is to compare the measured energy level with a target energy level associated with the supplied energy level to obtain and to generate a matrix indicating a compensation value for each emitter based on the comparison result so as to calibrate the energy level of comparison result.