How to Improve the Accurcy of Infrared Temperature Measurement

Infrared thermometers and temperature sensors, such as the AGRI-THERM II manufactured by Everest Interscience have been used for a few decades now to measure the surface temperature of targets out-of-doors. This work has been done primarily for agricultural applications to determine optimum irrigation scheduling and conserve water. Another use for infrared thermometers where temperatures are taken out-of-doors includes sports medicine applications. In many of these applications, it has been found that the temperatures were lower than what was meaningful. Any infrared temperature measurement that is taken out-of-doors can be found to have the same problem. One important concept has often been overlooked in these studies. That effect is sky radiation. A means to compensate for the reflection to the sky from plants or other objects that the infrared thermometer is measuring must be employed in order to insure accurate readings.

The unique solution of the phenomenon of sky radiation is the addition of the Everest Interscience SKY-SPY to the infrared thermometer or temperature sensor. The SKY-SPY “… provides a novel means for sensing the level of infrared radiation received by the object being measure from a skyward direction,” according to Everest’s U. S. Patent No. 4,420,265. With this invention, a second detector is utilized wherein the reflection by the vegetation or target that is received from a skyward direction may be taken into account to measure the actual surface temperature of the target. The purpose of this second sensor is to account for reflected sky radiation characteristics and compensate for them in the temperature reading from the target itself in order to produce a more precise infrared measurement.

In order to explain the inaccuracies that can be derived by taking out-of-doors infrared temperature measurements without the use of a SKY-SPY, assume that the object that is being measured is a first surface mirror and that the temperature reflected by that mirror is the temperature of the radiation from the sky. This temperature could be as much as -80°C under clear sky conditions. Then, assume that the mirror effect is fading into the object and the mirror radiation comprises only 2.0% of the total radiation (Surface Emissivity = 0.98) and the target forms 98%. The secondary mirror radiation, or the temperature of the sky radiation reflected on the secondary mirror, is now only two (2.0) percent of the temperature of the radiation or -1.6°C. The temperature of a leaf or is usually in a range of no more than minus ten degrees compared to the surrounding ambient air. So, the -1.6°C could be sixteen (16) percent of the total or as much as -1.6°C, bringing the effective temperature of the target down by that amount. In a science where infrared temperature-measure accuracy is critical, this error is intolerable.

Of course, at low-elevation conditions, perhaps in the presence of high water vapor and CO2 layers, the apparent sky temperature is much warmer than -80°C. Under these conditions, the SKY-SPY instantly and automatically corrects for all sky temperatures from ambient to -100°C or beyond.

This unique SKY-SPY, coupled with the infrared thermometers for making infrared temperature measurements with ±0.5°C accuracy that is covered in Everest’s U. S. Patent No. 4,301,682, provide a more accurate means of making the precise temperature measurements needed for agronomy applications. These instruments include Everest’s AGRI-THERM II handheld infrared thermometer, and their Model 4000 and their new Model 3800ZL Fixed-Mounted Infrared Temperature Sensors.

The AGRI-THERM II has been used extensively worldwide to measure crop canopy temperatures in an attempt to control irrigation with the goal of improving crop yields while reducing the amount of irrigation needed. More information on this subject can be read in M. B. Kirkham’s book, Principles of Soil and Plant Water Relations, 2004.

The AGRI-THERM II infrared thermometers are capable of taking surface temperature measurements in a highly dynamic environment such as having the thermometer taken from an air conditioned truck into the heat of the desert without giving them time for equilibration before taking the measurements. Under these environmental conditions, they respond with accuracy of ±0.5 degrees C or ±1.0 degree F.

Everest also markets infrared temperature sensors, the Model 4000.4ZL and the Model 3800ZL, infrared temperature sensors that can be mounted in the field and run continuously.

If you are planning to attend 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, please stop by Booth No. 1121 to learn more about the SKY-SPY.