For a Dutch monitoring project, measurements were made in various greenhouse projects with thermal imaging cameras to determine the temperatures of plants or individual plant parts. With a thermal imaging camera, the differences between plant and air temperature can be made clear. This provides valuable information for a grower.
With the data from a thermal imaging camera, it is possible to assess whether the plant has a chance of condensation and whether the plant has stress during the day (when the plant temperature is much higher than the air temperature). The great advantage of a thermal imaging camera compared to an infrared plant temperature sensor is that the thermal imaging camera can measure the temperature of individual parts of the plant. For example, in a Gerbera experiment, it has been shown that the flower can be 0.5° C colder at night than the leaf, and that the leaf temperature may be about 0.5-1.5° C colder than the air.
Watching online
Because the application of the thermal imaging cameras is, in practice, a relatively new development, the researchers of Kas als Energiebron shared their findings. The Thermoview 48 camera, developed by Sensor BV in collaboration with LetsGrow, was chosen for the experiments. The advantage of this camera is that data is accessed directly via LetsGrow. Growers can look online at the plant temperature measurements. In LetsGrow, every 5 minutes the temperature measurement and every hour a picture is shown (see example below).
Since the Thermoview 48 camera has a measurement accuracy of +/- 2° C, a reference measurement was added in the experiments. In the display screen of the camera, an object was placed with an external temperature measurement, to determine the reference temperature. The temperature of this object is then measured by the Thermoview camera and by the external temperature sensor. The difference between both measurements is used to correct the camera.
Measurements do not match
Correcting the measurements of the thermal imaging camera worked very well. During the night, the corrected temperatures of the thermal imaging camera closely matched the temperatures determined by the infrared plant temperature measurement. During the day, these measurements did not match well. The cause of this is still being investigated. However, the corrected measurements are not yet available in LetsGrow. Therefore, a grower should always take the accuracy of the camera into account when analyzing his measurements.
The measurement accuracy between pixels in the thermal image is 0.1° C, so that temperature differences of objects in the image can be determined accurately. That is why the temperature difference between the flowers and leaves could be determined very accurately. The temperatures of the individual parts of the plant can be followed by linking in the software points and squares to specific plant parts. In practice, however, this does not always work as well because some plant parts (such as flowers and heads of plants) move so that the points and boxes are no longer linked to their specific plant parts. Recently, it has been made possible to analyze the images afterwards so that it is possible to determine the temperature of each specific object later. If, in the future, one would want to follow the plant parts automatically, then this could be realized with image processing.
Taking movement into account
In summary, one can say that the thermal imaging camera is a good tool for determining the temperatures of different plant parts. This provides more information about the plant status compared to current climate registration using only temperature and RV measurement boxes. However, the accuracy of the camera must be taken into account and, depending on this, it may need to be corrected. It must also be taken into account that plant parts do move. If this is the case, one has to determine the temperatures of specific components manually.