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 crop canopies and individual leaves in agronomic applications for the natural environmental sciences.  This work has been done primarily for agricultural applications to determine optimum irrigation scheduling and conservation of water.  Much work has been done by researchers in universities and federal facilities and in many foreign countries on such theories as the Crop Water Stress Index (CWSI) and Stress Degree Day (SSD) concepts.  However, one important concept has often been overlooked in these studies.  That is the effect of sky radiation and the employment of a means to compensate for the reflection of the sky from plants or other objects that the infrared thermometer is measuring. 

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 vegetation 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 received from a skyward direction may be taken into account to measure the actual surface temperature of the plant.  The purpose of this second sensor is to account for reflected sky radiation characteristics and compensate for them in the temperature reading from the plant itself in order to produce a more precise infrared measurement. 

In order to explain the inaccuracies that can be derived by measuring the vegetation without the use of a Sky Spy, assume that the leaf 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 leaf and the mirror radiation comprises only 2.0% of the total radiation (Surface Emissivity = 0.98) and the leaf 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 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 leaf down by that amount.  In a science where accuracies of ±0.5°C are desired, 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 SDD and CWSI measurements for irrigation purposes 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 CWSI application.  One such infrared thermometer is Everest’s AGRI-THERM II.  

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 thermometer 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.