Method of making characteristic line

A method of making a characteristic line representing a relation between a signal value of a photodetector element which photoelectrically converts light it receives and outputs a signal a value of which depends on the amount of light it receives and the amount of light the photodetector element receives wherein the improvement comprises the steps of

• causing a plurality of small amount reference light beams which have different predetermined amounts of light in the range of small amount side in the light receiving range of the photodetector to impinge upon the photodetector element,
• obtaining a zero-light signal value which is estimated to be output from the photodetector element when the amount of light impinging upon the photodetector element is zero by extrapolation of the relation between the small amount side reference light beams and the signal values which are respectively output from the photodetector element in response to the small amount side reference light beams,
• obtaining a large amount reference light signal value which is output from the photodetector element when a large amount light beam having a predetermined amount of light in the range of large amount side in the light receiving range of the photodetector element is caused to impinge upon the photodetector element, and
• making a characteristic line representing a relation between the amount of light which the photodetector element receives and a value of the signal which is output from the photodetector element in response to its receipt of the amount of light on the basis of the zero-light signal value and the large amount reference light signal value.