Method of generating a correction function for a light-emitting diode (LED) testing process, including: detecting light emitted by a reference LED and reflected from inactive LEDs on a panel within a field of view of a detector; varying a number of the inactive LEDs to derive uncorrected values of an optical parameter as a function of the number of inactive LEDs; detecting light emitted by the reference LED, or by an active LED having identical optical properties, in the absence of any other LEDs, to determine at least one reference value for each optical parameter; and calculating differences between the uncorrected values and each reference value to generate the correction function, the correction function being based on the number of inactive LEDs which are arranged within the field of view of the detector in the light detecting step.