The invention relates to an optoelectronic scanning arrangement, in particular, to an arrangement of light sources, light receivers and reflecting marks which result in a more reliable optoelectronic scanning operation.
The use of optoelectronic scanning arrangements for monitoring movement processes is generally known. The present invention is limited to those optoelectronic scanning arrangements where the light emitted by a light source is reflected by a reflection surface and received by a light receiver. Usually the light source and light receiver are accomodated in one housing and form a so-called "reflection light barrier" (photoelectric cell).
To ensure safe and reliable operation of the photoelectric cell, the cell must deliver two clearly distinguishable signals. A first signal to indicate that light reflected by the reflecting surface is being recorded, that is, that a reflecting surface exists, and second signal to indicate that no reflected light is being recorded, that is, that a reflecting surface does not exist.
It is conceivable to use for a reflecting surface a metallized surface, and for a non-reflecting surface a surface of light-absorbing material. It is further known to use as the non-reflecting surface a deep slit, which may furthermore be lined with light-absorbing material. However, diffused light and dirt on the reflecting surfaces reduce the difference between the two signals. Fouling of the reflecting surface results in the reflection of less light toward the light receiver and hence in a smaller signal. Futhermore, stray light may result in more light being received in the light receiver despite the presence of a non-reflecting surface which may create difficulties in distinguishing correct marks. In addition, a soiled non-reflecting surface also produces more light in the light receiver and hence a disturbance of the signal.