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You may have noticed that
some of your infrared thermometer measurements have been inconsistent and
non-repeatable under apparently identical conditions. There is a reason for
these errors. Here is that reason. When an infrared thermometer is
calibrated indoors, it will read low by 1°-2°
when used outdoors. This is because the cold sky reflects off of the
target’s surface and into the thermometer. This reflection combines with the
target’s radiation signal, causing an error or, what is called in infrared
technology, “noise” in the temperature reading. The big problem is not so
much that the error is there, but that it is so dynamic and unpredictable – it
can vary from 1/2°
to 2°
in a matter of minutes – making reproducible readings impossible. It is true
statistical “white” noise and causes identical temperatures to be read
differently from hour to hour.
“White Noise” is an unwanted
signal that is unique in time and never repeats. It is noise that can make
the temperature readings change unpredictably. This makes realizing
repeatable infrared surface temperature measurements impossible. This
unpredictability is due to the fact that the “white noise” is uncorrelated in
time.
Most infrared thermometry
measurements in the natural environment involve temperatures which are within
a few degrees of ambient. These measurements would include such targets as
snow banks, ocean water, leaves or vegetal surfaces or, possibly, human
surface temperatures. Because of the normal narrow temperature range for such
targets, downscale errors as high as 20% or more of the readings are
commonplace. Errors of this magnitude are totally unacceptable in scientific
research or medical analysis. Now a patented, time proven, solution to this
problem is available.
Everest Interscience
“…provides a novel means for sensing the level of infrared radiation received
by the target from a skyward direction,” according to U. S. Patent No.
4,420,265. This same radiation might be received by a snow bank, water
surface or the human skin, as well. With this invention, a second independent
wide- angle infrared thermometer is utilized wherein the reflection by the
vegetal surfaces or other targets, received from a skyward direction may be
taken into account to measure the ”true” surface temperature of the target,
not the “apparent” temperature, thereby reducing inaccuracies caused by sky
radiation. When used in this sense, “true” temperature measurements are the
actual temperature measurements and “apparent” temperatures are the
uncorrected measurements being taken when sky radiation is not taken into
account. These terms were coined by Dr. M. Fuchs and Professor C. E. Tanner
in their discovery documentation of the sky radiation phenomena in the
AGRONOMY JOURNAL, VOL. 58, November-December 1966, entitled “Infrared
Thermometry of Vegetation.”
In their work, Drs. Fuchs and
Tanner reported that, “Infrared thermometers with bandpass filters from 8µ to
13µ can be used to measure the real temperature of vegetal surfaces
with errors in the range of 0.1C to 0.3C. To do this the emissivity must be
either known or determined and a correction accounting for the reflected
radiation from the surroundings must be made.” Emissivity corrections are
found on most infrared thermometers on the market today, but only Everest’s
infrared thermometers for the natural environment are capable of “…accounting
for the reflected radiation from the surroundings,” as Fuchs and Tanner deemed
necessary.
The necessary corrections
needed to give the “true” temperatures and not the “apparent” temperature
readings is provided by use of Everest’s “SKY-SPY” System which
continually corrects for these errors, in real time, as explained in U. S.
Patent No. 4,420,265. The SKY-SPY System is incorporated into
Everest’s hand-held AGRI-THERM
II™infrared thermometers and their
fixed-mounted infrared temperature sensors for use in the natural environment or
out-of-doors. This system continually and effectively corrects for overhead
radiative conditions from clear skies atop a high mountain, such as would be
the case when measuring snow pack temperature, to ambient conditions in a room
where required correction is near zero.
Plant tissue and sea water
surfaces are Lambertian or diffusive, and, hence, radiate and reflect in all
directions. Diffusion is the scattering of incident light by reflection
from a rough surface.
Since the surface radiates
and reflects in all directions, all segments of the sky or surrounds
contribute to this radiative error. Because of this, it is not practical to
take a sample of the sky temperature with an ordinary infrared thermometer
with a narrow field-of-view (15°
or less) and try to apply the reading as a correction. A special, wide angle,
optic is needed to cover the entire sky. In addition, the sky radiation
conditions change by the minute and must, therefore, be tracked and corrected
in real time with less than one second time constant. This is an absolute
necessity for accurate, consistent and repeatable temperature measurements.
And, it is an absolute necessity in order to read the “true” surface
temperature versus the “apparent” surface temperature of targets out-of-doors
with an infrared thermometer. This correction is so important that Everest
had to invent a calibration source to simulate hemispherical, -80°C
radiative sky conditions, to enable the SKY-SPY System.
This SKY-SPY error
correction system is now embedded in all of Everest Interscience infrared
thermometers designed to be used in the natural environment except for their
Model 3800, at no increase in price. If, for any reason, the user wishes to
disable the SKY-SPY, all that is required is to put a piece of tape
over the SKY-SPY window, and the instrument behaves like any ordinary
infrared thermometer.
Log on to
www.EverestInterscience.com to learn more about
the instrumentation offered by Everest Interscience.
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