Grade Mark Reader is both optically and electrically filtered from ambient lighting conditions to achieve the greatest signal to noise ratio possible. In most cases a shroud is placed around the head to minimize any interference from direct sunlight or some mill lighting. The GMR applies the equivalent of 240 watts of light to the board during scanning. In rare cases, sunlight coming through a doorway during a few weeks in a year can cause problems if it reflects off steel underneath the lug chain. When mounted properly with skirt, GMR is not affected by mill lighting, and in some cases can be operated without the skirt.