Infrared thermometers measure the infrared radiation emitted by an object that is beyond the sensitivity range of the human eye. The infrared (IR) radiation is the electromagnet radiation have a wavelength interval from 0.75 microns to 1000 microns. The visible range for the human eye is roughly from .01 to 1 micron. If two objects are at different temperatures the net infrared radiation will flow from the hotter to colder of the two in an amount following the Stefan-Boltzmann"s law:
W= rT^4 where : W is the total radiation express in Watts/cm^2 T is the absolute temperature of the body and r is the Stef-Boltzmann Constant.
In the infrared thermometer the optics then collects this infrared radiation and focuses it using a lens on the infrared Detector which converts it to an proportional electrical signal proportional to the hot temperature object.
The electrical single is amplified , converted to a digital single where is linearized and then the digital number displayed on the output.
The back of the thermometer has a dial setting for emissivity which is normally set to 0.98 for agricultural measurements When radiant energy emitted by one objects strikes another object part of that energy is reflected, absorbed and transmitted. The infrared thermometer is calibrated to take the amount of absorbed radiation and convert it to the temperature that a black body would be emitting infrared radiation at that temperature. A black body is a surface that emits all the radiation generated based on its temperature. However, in the real world some of the energy from a surface is absorbed by the surface and the objects radiates less energy than a black body. The emissivity is a measure of the efficiency of surface and for most plants two percent of the energy is absorbed. The thermometer is calibrated for a black body and the output temperature has to be corrected for the emissivity of the plant material.
Use of infrared thermometers
To use the infrared thermometer hold the instrument at a 45 degree angle and face north, take 10 readings and then face south and take 10 readings. Average the readings from both directions. To Calculate the Crop water Stress index using the infrared thermometer click here
Last modified 01/15/96