Evaluation of LED Lighting Systems in In Vitro Cultures

Many LED lighting systems for plant exposure are available on the market. The information to compare those systems varies between the numerous producers. There are still many descriptions of the systems that depend on photometric units like the luminous flux or light yield. But it turned out that radiometric units like the Photosynthetic Active Photon Flux Density (PPFD) are generally accepted to evaluate the electromagnetic radiation of an illuminant for plant exposure on a surface. Together with measurements of the required electrical power relative to one square meter the energy efficiency was calculated and described in mu mol s(-1) W-1. But the PPFD differs on the illuminated surface. Therefore the energy efficiency is affected by the position of the sensor. A solution of this problem is to work out the average PPFD (PPFDempty set) before calculating the energy efficiency. To compare the LED lighting system with conventional lamps including fluorescent tubes the PPFD on a shelf board (1500x500 mm) was measured on 161 measured points. With these data the PPFD distribution on the surface could be displayed and the PPFDempty set was calculated. The PPFD distribution strongly depends on the shelf board distance or the distance between the lamp and the PPFD sensor. In a LED lighting system it furthermore depends on the LED density and the location of the LEDs. Using the deviation squares from the PPFDempty set the PPFD distribution of the different lighting systems at various settings (LED density, shelf board distance) could be compared. This work shows a way to evaluate lighting systems. It points out the differences between the methods of evaluating lamps. Currently there is no consistent method to measure the output of lamps for the exposure of a shelf's surface and to make the lamps comparable with each other.