In a modular scanning unit, a laser diode source is associated with a rotary body on which hologram facets are arranged at an angle for diffracting the laser beam on to a bar code objective. The beam from the source is deflected by a mirror through a ring of slits in the top of the body. Light scattered from the ojbective is again diffracted by the hologram facets and is sensed by an optical detector on to which it is focussed by a concave lens at the end of the body.
