| 摘要 |
<p>The system consists of a light source (A) and light sensor (B) which have a pillar-shaped opaque obstacle (C) between them. All three items are mounted in fixed position on a rail. The light source (A) is a pulsed gallium aluminium arsenide diode, which emits a beam, 7 deg. wide, at a wavelength of 800 nanometres. The obstacle (C) is an elliptical section pillar, with vertical U-shaped grooves down the sides. It is so placed, that no light from the source can reach the sensor in direct 'line-of-sight'. The sensor can only detect the pulsed light if it is diffracted around the pillar by water droplets or other particles. The pillar has an internal electrical heater which prevents the build-up of ice or snow. Versions can be produced when will sample the mist at pre-selected densitites, or will only sample so-called 'freezing fog'.</p> |
