User's Guide _______________________________________________________________________
20____________________________________________________________________ M210543EN-B
Optical Measurements
Optical Arrangement
0104-026
Figure 3
PWD22 Optical System
PWD22 measures light scattered at an angle of 45°. This angle
produces a stable response in various types of natural fog.
Precipitation droplets scatter light in a different manner as fog and
their contribution to visibility must be analyzed separately. PWD22
can detect and measure precipitation droplets from the optical signal
and use this information in processing the scatter measurement results.
PWD22 has a small sample volume of about 0.1 liters (see Figure 3
above). This allows for independent particles to be measured even at
relatively heavy precipitation intensities. The signal levels from even
the smallest precipitation droplets can also be detected.
Transmitter Unit PWT11
The transmitter unit consists of an infrared LED, control and
triggering circuits, LED intensity monitor, and backscatter receiver.
The transmitter unit electronics pulses the IR-LED at a frequency of
2 kHz. A PIN photodiode monitors the transmitted light intensity. The
transmitter intensity level measurement is used to keep automatically
the LED's intensity at a preset value. The "LEDI" feedback voltage is
monitored by the CPU to get information of the aging of the LED and
possible defects.
The feedback loop compensates for temperature and aging effects of
the light emitting diode. On the other hand, the active compensation
slightly accelerates the LED aging. Thus, the initial LED current is set
to a value, which ensures several years of operation without
maintenance.
