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Instrument: SUN PHOTOMETERS
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Description
A Sun photometer (also spelled Sunphotometer) is an instrument
that measures the intensity of sunlight arriving directly from
the Sun. Since it is designed to be pointed directly at the Sun,
a Sun photometer measures only direct sunlight, not the diffuse
light scattered from the sky, haze and clouds. Since haze blocks
some direct sunlight, a Sun photometer is an ideal instrument
for measuring haze.

Scientists have used Sun photometers of various kinds for more
than a century. The modern generation of hand held Sun
photometers was pioneered in the late 1950's by Frederick Volz
[1], who has made important discoveries about the effects of
natural and volcanic haze on the environment.

Although some Sun photometers respond to a wide range of colors
or wavelengths of sunlight, most include special filters that
admit only a very narrow band of wavelengths. These filters are
expensive. They also have a limited life. While some filters may
work well for a decade or more, others may last only a few
years.

Additional information available at
http://www.concord.org/haze/spwhat.html

[Summary provided by The Concord Consortium]