A speckle readhead includes a
light source that outputs light towards an optically
rough surface. Light scattered from this surface contains speckles. The
scattered light is imaged onto an
image detector, captured and stored. Subsequently, a second image is captured and stored. The two images are repeatedly compared at different offsets in a displacement direction. The comparison having the highest value indicates the amount of displacement between the readhead and the surface that occurred between taking the two images. An optical
system of the readhead includes a lens and an aperture. The aperture can be round, with a
diameter chosen so that the
average size of the speckles is approximately equal to, or larger than, the dimensions of the elements of the
image detector. The dimension of the aperture in a direction perpendicular to the direction of displacement can be reduced. Thus, the imaged speckles in that direction will be greater than the dimension of the
image detector elements in that direction. Such a readhead is relatively insensitive to lateral offsets. The lens can be a
cylindrical lens that magnifies the
relative motion along the direction of displacement but does not magnify relative motions in the direction perpendicular to the direction of displacement. The optical
system can also be telecentric. Thus, the readhead is relatively insensitive to both separation and relative motions between the readhead and the surface. The
light source can be modulated to prevent smearing the speckles across the image
detector. The
light source can be strobed to freeze the image.