The cones used to position poultry carcasses on poultry processing lines must be cleaned frequently. A hologram element is installed in the optical system so that test beams diffracted by the hologram element travel along a same optical path as test beams incident on the optical system. The first guide is moveable towards or away from the second guide between a first position, a second position and at least one intermediate position The disclosure is particularly directed towards an adjustable width guideway assembly for use in an air conveyor conveying light plastic bottles suspended in the guideway by a flange located on the bottle neck. The reflected light is returned so as to be condensed and focused on a light receiving surface of sensor in a light measuring device. The polyurethane foam has a thickness in a range of 0.