PRECIPITATION DETECTION AND CHARACTERIZATION

SCIENCE OBJECTIVE:

INSTRUMENT DESCRIPTION: I have two ideas for optical precipitation detectors based
on commercial units.  One is similar to the sensors some luxury cars use to detect rain on the
windshield that can then activate the windshield wipers.  It works by focusing infrared light from
an LED on the inside of the windshield at an angle of 45 degrees and detecting the reflection with
a photodiode also aimed at 45 degrees to make a 90 degree arrangement.  When water accumulates
on the outside of the windshield the refraction changes and less light is reflected back to the
photodiode.  This project involves determining the mounting geometry of the LED and sensor,
determining the best IR wavelength to use given the limited commercially available IR products,
the thickness of glass, the area that can be sensed, how to deal with dirt build up, how to deal
with ambient light especially the sun, and how to monitor this electronically.

For a brief description see this web page:

http://www.rainsensors.com/Optical.htm

The other detector is much more sophisticated and capable.  Whereas the above detector notifies
the presence of precipitation, this sensor can also characterize its form and amount.  It provides
information about the type of precipitation and the amount falling per unit of time.  It does this by
generating a plane of laser light which is focused on an optical linear array.  The linear array is an
optical detector similar to the sensing element of a digital camera but only a single line, not an x-y
imager.  When precipitation falls into the plane of light some of the light is attenuated before it
reaches the linear array and by repeatedly scanning the array a profile of the precipitation can be
generated.

This project involves generating a plane of laser light from a laser pointer.  In other words, turning
a point of light into a relatively uniform plane of light.  One way to do this is with a prism.  How best
to do that would have to be determined.  The size of available sensing elements determines the width
of the plane of light.  The other variable to be determined is the length of the plane.  Finally, the
information read from the array will have to be processed and correlated to different forms of precipitation.

An example of a commercial sensor can be seen at:

http://www.biral.com/datasheets/laserprecipitation.pdf

SPECIFICATIONS:

PROJECT STATUS:

INSTRUMENT COSTS:

APPLICABLE WEB SITES:

http://www.rainsensors.com/Optical.htm

http://www.biral.com/datasheets/laserprecipitation.pdf

DATE: July 6, 2008