| 摘要 |
The illuminated field, which is formed, for example, by a hairline (4), is projected by means of a lens (5) onto a reflecting system consisting of a prism (6) and a plane-parallel mirror (7). The hypotenuse face (8) of the prism is half-silvered. The mirror (7) is provided on its rear with a reflective coating (11) and the non-silvered front (12) is cemented onto the said hypotenuse face. The first short face (13) is the light-entry face and the second short face (14) is the light-exit face of the prism (6). Arranged immediately adjacent to the second short face is a differential photoreceiver (15) having two photodiodes (16, 17). Owing to the reflecting coating (9) of the prism, and to the reflective coating (11) of the mirror (7), the illuminated field is split into two halves, each half being focused onto one or the other photodiode. The positioning of the illuminated field can be determined from the difference between the currents generated in the photodiodes. <IMAGE> |
