A scanning tunnel comprising: a frame; a conveyor structure having a generally planar surface and being moveable with respect to the frame in a direction of travel; and a scanning system in a position with respect to the frame that is fixed during movement of the conveyor structure in the direction of travel, wherein the scanning system comprises a laser light source, a rotatable, multi-faceted wheel disposed with respect to the laser light source and the conveyor structure so that when the wheel rotates, and a facet receives light from the laser light source, the facet reflects the light to the conveyor structure in a scan across the conveyor structure in a direction transverse to the direction of travel, receives light reflected from the conveyor structure and objects carried by the conveyor structure, and reflects the reflected light to a detector, characterised in that the scanning system comprises an aperture that is disposed between the wheel and the conveyor structure so that light from the laser light source that is reflected from the conveyor structure and the objects to the facet passes through the aperture, the aperture having a first area that receives and transmits to the facet light from the laser light source that is reflected from the conveyor structure and the objects, and a second area (428, 430, 432, 434) that receives and blocks or diffuses light from the laser light source that is reflected from the conveyor structure and the objects, wherein the second area (428, 430, 432, 434) comprises a first portion (428, 430, 432) and a second portion, wherein the first portion (428, 430, 432) has a width in the direction perpendicular to the transverse direction that does not vary over the transverse direction, and wherein the second portion has a width in the direction perpendicular to the transverse direction that varies over the transverse direction. a frame; a conveyor structure having a generally planar surface and being moveable with respect to the frame in a direction of travel; and a scanning system in a position with respect to the frame that is fixed during movement of the conveyor structure in the direction of travel, wherein the scanning system comprises a laser light source, a rotatable, multi-faceted wheel disposed with respect to the laser light source and the conveyor structure so that when the wheel rotates, and a facet receives light from the laser light source, the facet reflects the light to the conveyor structure in a scan across the conveyor structure in a direction transverse to the direction of travel, receives light reflected from the conveyor structure and objects carried by the conveyor structure, and reflects the reflected light to a detector, characterised in that the scanning system comprises an aperture that is disposed between the wheel and the conveyor structure so that light from the laser light source that is reflected from the conveyor structure and the objects to the facet passes through the aperture, the aperture having a first area that receives and transmits to the facet light from the laser light source that is reflected from the conveyor structure and the objects, and a second area (428, 430, 432, 434) that receives and blocks or diffuses light from the laser light source that is reflected from the conveyor structure and the objects, wherein the second area (428, 430, 432, 434) comprises a first portion (428, 430, 432) and a second portion, wherein the first portion (428, 430, 432) has a width in the direction perpendicular to the transverse direction that does not vary over the transverse direction, and wherein the second portion has a width in the direction perpendicular to the transverse direction that varies over the transverse direction.