1. A method of reducing unevenness in optical density of an exposure device suited for an electrophotographic printer, said exposure device comprising a plurality of light-emitting elements arranged in a row, said method comprising the steps of: energizing selected light-emitting elements according to a selection scheme, the selected light-emitting elements being a plural number of the plurality of light-emitting elements arranged in a row; using a pre-determined energy level for energizing each selected light-emitting element and measuring a corresponding exposure intensity distribution of the energized plural number of the plurality of light-emitting elements arranged in a row from the exposure device; predicting a toner area coverage distribution on a medium, based on the measured exposure intensity distribution of the energized plural number of the plurality of light emitting elements arranged in a row and on a pre-established transfer function; and obtaining an average of the predicted toner area coverage distribution on the medium of the energized plural number of the plurality of light-emitting elements arranged in a row; and determining setting values for the energy levels for energizing each selected light-emitting element such that the obtained average becomes a target attribute.