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.