Use of supplementary lighting top screens and effects on greenhouse climate and return on investment

Discomfort caused by light pollution from greenhouses that apply supplementary lighting is an issue in Dutch society nowadays. At this moment Dutch legislation requires an opaque screen that reduces light transmission of the greenhouse wall by 95%. In 2008 also the light transmission of the greenhouse roof must be reduced equally and supplementary light will be limited to 15,000lx(180¿mol/m2/s), unless light emission is totally prevented. The objective of this research was to calculate the economic consequences of installing reflecting, light emission reducing or blocking screens by considering crop yield and costs. A mathematical correction equation was developed to approach the light gain for the crop as a result of internal reflection. Greenhouse climate and tomato crop growth were simulated for a reference greenhouse with supplementary lighting and without an emission blocking screen and for a low-light-emission greenhouse with a blocking screen. The supplementary lighting level was set at 180¿mol/m2/s. Results show that the greenhouse climate below the screen remained manageable, but that the desired DIF of 2°C was affected. The light gain was on average about 3% and resulted in production increase. A small net yearly profit resulted based on direct and indirect effects of the screen. In conclusion, the simulation suggested that stopping light emission at the source with help of reflective opaque screens is economically feasible if screen operation is included in planning the lighting scheme.