| 摘要 |
<p>A transmissometer for measuring atmospheric visibility in fog is provided. A transmissometer comprises a lamp (10), a collimating lens system (12), a beam splitter (14), an exit window (44) and a retro-reflector (47). A light beam is reflected back to the beam splitter prism (52) where it is turned to 90° to a photodiode (48). The level of light measured by the photodiode is determined by the atmospheric transmission. A comparative beam is deflected by the beam splitting prism (52) to a reflective mirror (54) and back to the photodiode (48). A rotating shutter (16) allows the light to be cut off in a particular cycle in order that the atmospheric transmission can be compared with the reference beam reflected by mirror (54). The rotating shutter is constructed with various openings to allow measurements of noise of the instrument to be detected in order that an accurate reading can be provided by the instrument.</p> |
