The size, number concentration and inter-arrival times of cloud droplets in a size range of 3 – 35 microns can be determined with the M-Fast-FSSP (Schmidt et al., 2004). The M-Fast FSSP at IfT is a follow-up instrument of the FSSP developed by Particle Measuring Systems, Inc. and the Fast-FSSP built by Meteó France. In the following, the principle functionality of the M-Fast-FSSP will be explained.
A 5 mW Helium-Neon Laser (1) emits light with a wavelength of 638 nm. This light is focused by means of a lens (2), such that at the focus point (4) the waist of the laser beam has a width of about 200 microns. Using a mirror (3), the focus point is positioned outside the instrument, so that droplets passing the laser beam scatter light. The direct laser light is masked by the damp spot (5), whereas the forward scattered laser light between 3 and 13° is redirected by a prism (6) and focused by a lens (7). After the scattered light is led through a beam splitter (8), it is finally detected by the so called signal (9) and annulus (10) photo diode. The photo current produced by the photo diodes is converted into a voltage, which is digitalized into 256 channels and, together with additional parameters, is recorded by the data acquisition system. Using Mie-theory, the intensity of the scattered light can be related to the size of the droplets.
When droplets don’t pass the center of the laser beam but its edge, where the intensity of the laser light is lower, they scatter accordingly less light and their size is underestimated. To overcome this problem, two photo diodes, the so called signal and annulus diode are used to determine the intensity of the scattered light and the position of the scattering droplets within the laser beam. The annulus diode has a smaller diameter than the signal diode (d = 0.4 mm compared to d = 0.8 mm).
The decision whether a droplet is accepted for accurate sizing is based on the following procedure. If droplets pass the laser beam at the focal point, a sharp images is produced at the photo diodes, and the voltages recorded by the annulus and signal diode are the same. When a droplet passes the laser beam away from the focal point at another position on the x-axis (A), its image is blurred and the annulus diode receives less scattered light than the signal diode. Similarly, if droplets pass the edge of the laser beam (B), their image is displaced, which also results in a smaller voltage of the annulus diode compared to that of the signal diode. Because both, the signal and the annulus voltage is stored, both can be compared after the data is recorded, and droplets from the center of the laser beam, where the intensity is highest, can be selected to calculate an accurate droplet size distribution.
