Detection of the edge of an object such as a sheet

摘要

An edge detecting device for detecting a position of an edge of an object includes a measuring circuit for emitting an inspection light to a linear inspecting area including the object, and measuring a first light quantity of reflected light of the inspection light reflected from the inspecting area in a first state and a second light quantity of reflected light of the inspection light reflected from the inspecting area in a second state, a calculation circuit for calculating a difference of the first light quantity and the second light quantity measured by the measuring circuit for each pixel with a first predetermined length, calculating first total differences of reflected lights from pixels with a second predetermined length located adjacent to an objective dot in one direction by totalizing differences between the first and the second light quantity reflected from each of the pixels, and calculating second total differences of reflected lights from the other pixels with the second predetermined length located adjacent to the objective dot in the other direction by totalizing differences between the first and the second light quantity reflected from each of the other pixels and a judgment circuit for judging the objective dot with the largest variation between the first total differences and the second total differences to be a position of the edge.