Image processing apparatus

An image processing apparatus comprising:

• a machine tool disposed for actuation along a line which is perpendicular to a plane along which a detection mark lies, wherein the detection mark also intersects said line which is perpendicular to said plane;
• a camera unit including a lens system through which light from the detection mark is received;
• and
• a photodetecting device disposed in the optical axis of said lens system;
• wherein the optical axis of said lens system is inclined at a predetermined angle to said perpendicular line to a plane along which the detection mark lies so as not to interfere with the actuation of said machine tool, and the detection mark intersects the optical axis of said lens system, wherein said photodetecting device is inclined at a predetermined angle to the optical axis of said lens system of said camera unit;
• wherein said predetermined angle of inclination of said photodetecting device to the optical axis of said lens system is determined so that (a+a')=(b+b'), where a is the length of a section of a light path between a portion of the detection mark on the side of said camera unit and said lens system of said camera unit, a' is the length of a section of the same light path between said lens system and said photodetecting device, b is the length of a section of a light path between a portion of the detection mark diametrically opposite the portion of the side of said camera unit with respect to the center of the detection mark and said lens system, and b' is the length of a section of the same light path between said lens system and said photodetecting device.