Determining the emissivity of the leaves of nine horticultural crops by means of infrared thermography

The present study was carried out with the aim of analysing the variability of the emissivity values of nine of the most characteristic horticultural crops of the greenhouse productive system in the Mediterranean region. A thermographic camera was used for both qualitative and quantitative emissivity measurement by evaluating radiation emission from the leaves. The real temperature of the leaves was also measured with a contact probe in order to calculate emissivity. The differences in emissivity between crops for the upper side of leaves are below standard deviation values, the average values are all close to 0.98. For upper side of leaves we obtained the following average values of emissivity: 0.980 ± 0.010 for Lycopersicum esculentum Mill., 0.978 ± 0.008 for Capsicum annuum L., 0.983 ± 0.008 for Cucumis sativus L., 0.985 ± 0.007 for Cucurbita pepo L., 0.973 ± 0.007 for Solanum melongena L., 0.978 ± 0.006 for Cucumis melo L., 0.981 ± 0.009 for Citrullus lanatus Thunb., 0.983 ± 0.006 for Phaseolus vulgaris L. and 0.983 ± 0.005 for Phaseolus coccineus L. Considerable differences have been observed between the emissivity values on the opposite sides of the leaves in some horticultural crops, such as green bean and particularly red bean, with a difference of 0.029 in the average emissivity value. Emissivity values of 0.98 are recommended as a reference for measuring the temperature of horticultural crops other than those studied here whenever there is no other possibility to determine the emissivity.

► Emissivity values of 0.98 are recommended as a reference for measuring the temperature.
► Range from 0.973 for aubergine leaves to 0.985 for cucumber leaves.
► Considerable differences have been observed between the opposite sides of the leaves.
► Differences in emissivity of 0.03 can give rise to errors of up to 0.21 °C.