| 摘要 |
A light barrier in which a light transmitter transmits (19) light, in particular light outside of the visible spectrum, via a front objective (4) through a region (21), to be monitored onto a light receiver-either (13 min ) directly or via a reflector (13) is provided with an adjustment aid which makes use of background light in the visible spectrum to facilitate the alignment of the apparatus. The adjustment aid, which can be used with one or both of the light transmitter or receiver, basically comprises a plane mirror (14) with a visible reference feature disposed behind the front objective at the location of the real image of the reflector or transmitter and beam divider (15), which can conveniently be in the form of a partially transmitting or dichroic coated mirror arranged in front of the front objective (11) and inclined relative thereto. In operation the human eye (26) can directly observe the visible light reflected sideways from the beam divider and will be able to view the alignment of the image of the reflector or light receiver relative to the visible reference feature and to correct this alignment by displacement of one or other of the pieces of apparatus. The plane mirror (14) in front of the light transmitter can either have a central aperture (17) for transmitting the operational light beam of the light barrier or can itself have a dichroic coating so that it reflects visible light but transmits the operational light beam for the light barrier. |
