of the door opening 3 u2014 over the lateral door delimitation which lies on the right with respect to the position in fig7 the laser radar scanning detector 12 signals a radial distance of the obstructing object 21 from the laser radar scanning detector 12 of r w cos u03b2 when this alignment line is crossed if the arrangement is made such that obstructing objects 18 which approach from the left with respect to the illustration in fig7 are to be sensed during scanning in the clockwise direction and obstructing objects 21 which approach from the right with respect to the illustration in fig7 are to be sensed during scanning in the counter clockwise direction the geometric relationships which are given in the following table are obtained therefore if the ratio of successive distance measurements of the laser radar distance detector 12 is higher than the reciprocal value of the corresponding cosine values of the scanning angles during scanning in the clockwise direction this means that the distance measurement of a scanning orientation which follows in the scanning direction becomes smaller and the obstructing object 18 takes on a travel component in the direction of the door opening deviating from the path according to the arrow 30 in this case it is necessary to trigger a monitoring signal which deactivates a closing door or reverses it into the opening.