PAR (Photosynthetically Active Radiation, 400 – 700 nm) is the driving force for photosynthesis and thereby also for crop growth and production. Light interception by the crop is an important trait. In general, growers aim to maintain such a leaf area that the crop has (nearly) full light interception. In order to estimate this, they take manual measurements of the Leaf area index, which is the area of leaves per area of floor surface. That implies that they manually measure the number of leaves and the area of these leaves, which is a time consuming activity and prone to (measuring) errors. It would be a huge improvement if it would be possible to collect data of crop light interception automated on a daily basis.
In the AGROS project “Towards an autonomous greenhouse”, we installed PAR line sensors, which measure the light intensity at three heights in the greenhouse: just above the canopy, in the middle and just below the lowest cucumber leaves. Each line sensor consists of 10 individual PAR sensors. The output is an average from all individual sensors’ measurements. In this way, spatial differences of the presence or not of a single leaf above a single point measurement are minimized.
Measuring PAR light intensities at different levels allows the estimation of the light penetration into the crop canopy. The difference between the PAR measurements at the different height are a good indicator of the light intercepted by the crop. Experimental results in a cucumber crop showed that the data of the PAR line sensors showed a comparable pattern as the manually recorded values of leaf area index. Therefore the measurements of PAR at different heights can be used as an automatically and continuously recording of the canopy light interception. Thereby, the PAR line sensor can be used as a sensor to replace the manual, labour intensive measurements of leaf area index.