Leaf, Bud & Canopy Temperature

Infrared Radiometry - Canopy Temperature

An infrared thermometer measures radiant energy. This radiation is simply “light” that is slightly outside the human eye’s sensitive range. All objects radiate infrared energy. The intensity of infrared radiation is proportional to the temperature of the object.

Infrared thermometers produce no “intrusion error.” A hot object “target” is radiating its infrared radiation in all directions. The object’s radiation characteristics, and hence its temperature, are not disturbed by the presence of the infrared thermometer.


The infrared thermometer optics collect a sample of infrared radiation from the hot object (soil & plant) being measured and focus it on the tiny infrared detector.


The detector, in turn, converts it to a proportional electrical signal, which is the exact electrical analog of the incoming infrared radiation, and hence the hot
object’s temperature. This minute electrical signal is then amplified, converted to a digital signal, and digitally linearized and the resultant temperature either displayed or data logged.


Low temperature infrared thermometry (IRT) is technically quite difficult especially when measuring temperatures of crop canopies which have a very weak infrared signal and temperatures are needed to be resolved to 0.1 Deg C to make meaningful irrigation and management decisions. Continuous measurement of the transducer temperature and sky reflectance of infrared light must be undertaken.


Accurate measurements of plant canopy temperature, which, along with other environmental variables, allows estimation of canopy transpiration and crop stress using a calculation such as Crop Water Stress Index (CWSI).

Leaf Temperature

The THERM-MICRO Leaf Temperature Sensor is a very small thermistor that can be adhered to a leaf surface for the measurement of absolute temperature of the leaf at the surface. The THERM-MICRO’s small size means that it has almost no thermal mass, resulting in minimal boundary layer influence and measurements which are highly responsive to changes in leaf temperature.

Frost (Leaf & Bud Temperature)

Frost damage to plants can have large impacts on crop yield and quality. The SF-421-SS is a combination of two temperature sensors (precision thermistors) designed to mimic a plant leaf and the other a flower bud. Protection of crops during frost events is dependent on the accuracy of plant temperature predictions.

Often, air temperature is not a reliable predictor of timing, duration and severity of frost events because plant canopy temperatures can be significantly different than air temperature under certain environmental conditions. On clear, calm nights, plant leaf and flower bud temperatures can drop below freezing even if air temperature remains above 0°C. This is called a radiation frost and is due to the lack of air mixing (wind) near the surface, and a negative net longwave radiation balance at the surface.

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