Infrared Radiometry for Measuring Plant Leaf Temperature during Thermal Weed Control Treatment

One side of a plant leaf was exposed to liquefied petroleum gas flames, while the temperature of the other side was measured continuously with an imaging infrared radiometer. Temperature histories of leaves and the performance of the measuring system were studied by flaming over 200 leaves and recording the measurements for off-line analysis. Flaming raised the temperature of the leaves very rapidly. The peak heating rate was typically around 180°C s−1. The imaging speed of the radiometer, 50 fields s−1, was sufficient to evaluate the temperature histories. Flaming induced marked temperature differences in the flamed leaves, with veins heated to lower temperatures than the thin areas between them. Thermograms with good spatial resolution were essential to characterise the temperature distribution with gradients as steep as 50°C over a 1 mm interval. With a factory calibrated radiometer an accuracy of ±2·1°C in absolute temperature values for a temperature range of 20–90°C was achieved by determining the target emissivity with the radiometer. By calibrating the radiometer for the actual conditions under measurements the error could be reduced to ±1·0°C. In conclusion, an imaging infrared radiometer is a superior instrument for measuring fast, spatially distributed temperature changes in plant leaves during thermal weed control treatment