Embodiments of the invention provide a method for setting a light source parameter in order to recognize an identification mark on an object surface. In one embodiment, a grayscale image is generated by setting the light volume of the light source to a given value. Plural thresholds each for binarizing the grayscale image in density are set. The inter-class variance values to be produced respectively according to the thresholds are calculated and the largest inter-class variance value is selected as the peak inter-class variance value from the inter-class variance values calculated. The light volume is changed to a plurality of different values. Peak inter-class variance values are obtained respectively for the values and the largest peak inter-class variance value is selected as the maximum inter-class variance value from the peak inter-class variance values obtained. The object surface is irradiated by the light source whose light volume is set to a value which would give the maximum inter-class variance value.