Infrared Thermometers with Improved Accuracy for Use in The Natural Environment

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Scientists have found that infrared temperature measurements taken outdoors can be in error by as much as 2 Degrees Celcius. This is due to cold sky reflections off of the target's surface, combined with the target's radiation signal, being received by the infrared thermometer. Everest Interscience has solved this problem by the addition of their Sky-Spy System to their infrared thermometers for the natural environment.

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…provides a novel means for sensing the level of infrared radiation received by the vegetation from a skyward direction

Researchers and scientists have found that outdoor infrared temperature measurements are in error by as much as 2.0C due to sky radiation effects. Infrared thermometers that are calibrated to 0.25C or 0.5C under laboratory conditions won’t hold anywhere near that accuracy when used outdoors. Sky radiation noise will contribute from 0.5C to 2.0C random uncertainty to the end reading. The reason for these errors and a solution to the problem has been found. A U. S. Patent co-authored by the late Dr. Graham K. Walker, a Canadian Agrometeorologist who contributed to excellence in research and operational agrometeorology, and Charles E. Everest, Design Engineer and Co-Founder of Everest Interscience (U. S. Patent No. 4,420,265), explains what is called the SKY-SPY System for infrared thermometers. This system provides the solution to this problem of inconsistent readings outdoors.

The inconsistent readings are the result of the fact that 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 0.5C to 2.0C 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, 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 vegetation from a skyward direction,” according to their U. S. Patent No. 4,420,265. This same radiation might be received by the snow bank or the human skin, as well. With this invention, a second wide- angle optical system and detector are 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 entitled “Infrared Thermometry of Vegetation.”

In their work, Drs. Fuchs and Tanner reported that, “Infrared thermometers with bandpass filters from 8? to 13? 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.” They concluded that, “The apparent radiative temperature may differ as much as 2C from the real temperature.”

Emissivity corrections are easily made 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 (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 Series 4000 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 surfaces, such as leaves, 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 Degrees 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 most 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.

For more information on the SKY-SPY and the instruments that incorporate it, please log on to for Contact Information. Also, instruments with the SKY-SPY System incorporated in them will be on display at the Joint Meeting of the Geological Society of America, the Soil Science Society of America, the American Society of Agronomy and the Crop Science Society of America at the George R. Brown Convention Center in Houston, Texas, on October 5 through October 8, 2008, please stop by Booth No. 1121 to learn more about them. Or, you can call Everest Interscience at (800) 422-4342 in the United States and at (520) 792-4545 outside the Toll-Free Area.


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Marilyn M. Everest
520-792-4545; 800-42
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