Rotary beam light curtain

Apparatus for scanning an area bound by sides that are angularly inclined relative to each other, the area defining first and second, generally diagonally opposed corners and first and second sides which intersect at the second corner, the apparatus comprising:

• a light transmitter-receiver disposed in the first corner including means for scanning a light beam over substantially the full length of the first and second sides;
• first and second retroreflector means for reflecting back to the transmitter-receiver light incident thereon at an angle of incidence up to a predetermined critical angle, the first and second retroreflector means being disposed along portions of the first and second sides so as to limit the angle of incidence of light from that source striking the retroreflector means to the critical angle of incidence;
• and plane mirror means disposed along a remaining portion of the first side between the second corner and an end of the first retroreflector means;
• whereby light from the source strikes the mirror means at an angle of incidence which is greater than the critical angle;
• the second retroreflector and the mirror means being oriented with respect to each other so that light from the source striking the mirror means is reflected by the mirror means towards the second retroreflector means and strikes the latter at an angle of incidence that is less than the critical angle;
• whereby the second retroreflector means reflects such incoming light via the mirror means back to the transmitter-receiver.

Apparatus for scanning a generally rectangular area bound by perpendicular sides and having first and second diagonally opposed corners, and a long side and a short side which converge at the second corner, the apparatus comprising:

• a light transmitter-receiver disposed in the first corner including means for scanning a light beam over substantially the full length of the long and short sides to thereby scan the light beam over the full area;
• a first retroreflector placed along the long side, extending from an end of such side opposite the second corner to an intermediate point thereof, and having a length so that the light beam from the source strikes it at an angle of incidence that is less than a predetermined critical angle of incidence for the retroreflector;
• a second retroreflector placed along the short side, terminating at the second corner and having a length so that the light beam from the source strikes it at an angle of incidence that is less than a predetermined critical angle of incidence for the second retroreflector;
• and a plane mirror placed along the side and extending from the point to the second corner;
• whereby a light beam directly striking the retroreflectors is directly reflected back to the transmitter-receiver and a light beam striking the mirror at an angle greater than the critical angle is directed by the mirror to the second retroreflector and strikes the latter at an angle of incidence which is less than the critical angle for reflection of such light beam by the second retroreflector via the mirror back to the transmitter-receiver.