Abstract: Apparatus for measuring the length of an elongate product, such as a rolled steel bar or shape. The product is inserted between a light source and a light receiver, and thereby throws a shadow on the light receiver. The receiver comprises an outer, stationary cylinder which has a longitudinal slit, and an inner rotatable cylinder which has a helicoidal or triangular or similar slit. The outer and inner slits jointly define a window of changeable form or position. Through this window, incident light from the source, free of the product's shadow, falls on a light conducting rod or system, thereby controlling a photo-sensitive system which provides a measurement of the exact end of the shadow's position, and thereby of the length of the product.