A reed beam has mounted thereon a series of reed blades which have, in their beating edges, aligned U-shaped notches forming a transport channel for the weft. A stationary blowing nozzle is in position to project a weft thread through such aligned notches when the reed is retracted. An apparatus for projecting a substantially parallel beam of light is carried by the reed beam and is arranged to project, through the space between at least two adjacent blades at the end of the reed beam remote from said blowing nozzle, a beam of light covering substantially the entire area of a notch. A photoelectric receiver comprising a series of light-sensitive elements is arranged to receive at least a portion of such beam of light that extends substantially across the width of a notch.