Patent ID: 8325344

Claim:
An optical density determination apparatus comprising: a first light source to emit a first light beam in a first direction towards a substrate; a second light source to emit a second light beam in a second direction towards the substrate, the second direction being different than the first direction; a first sensor to sense diffused light of the first light beam reflected from an illuminated spot on the substrate at a first distance from a center of the illuminated spot on the substrate, the first light source to emit the first light beam at a first angle relative to a normal vector from the center of the illuminated spot on the substrate, the first sensor tilted relative to at least one axis at a second angle relative to the normal vector from the center of the illuminated spot on the substrate to reduce a specular reflection of the first light beam sensed by the first sensor from the substrate relative to the first sensor at the normal vector from the center of the illuminated spot on the substrate and at the first distance from the center of the illuminated spot on the substrate; a second sensor to sense diffused light of the second light beam reflected from the illuminated spot on the substrate at a second distance from the center of the illuminated spot on the surface at a third angle relative to the normal vector from the center of the illuminated spot on the substrate to reduce a specular reflection of the second light beam sensed by the second sensor at the third angle relative to the second sensor located at the normal vector from the center of the illuminated spot on the substrate and at the second distance from the center of the illuminated spot on the substrate; and the first and second sensors to provide signals indicative of the light sensed by the first and second sensors, the signals useable to determine an optical density associated with the substrate.