Improved LED artificial sunlight source system available for sunlight-effect research in plant sciences

Abstract

We have improved a light-emitting diode artificial sunlight source system that we developed in 2013. The 2013 system can produce light with various spectral distributions for wavelengths of 380-940 nm that approximates those of ground-level sunlight and produce light with arbitrarily modified spectral distributions. Moreover, this system can produce time-varying light with different spectral distributions. However, this system’s utility is limited because the light it produces has low maximum irradiance and low time-stability for sunlight-effect research experiments. Hardware and software improvements to that system allow it to now produce a maximum irradiance of approximately 1.54 kW m-2 for 380-940 nm at a 7.1 cm2 light outlet with greater time stability and make it easier to produce time-varying light having a large number of different spectral distributions. First-step operational tests showed that this improved system can accurately produce single light with spectral irradiance distributions (SIDs) approximating: (1) SIDs of ground-level sunlight measured in Tokyo at two-hour intervals on a clear day; (2) various magnifications (1.33, 1.2, 1, 1/10, 1/100, and 1/1000 times) of a reference terrestrial solar SID, which is defined by the International Electrotechnical Commission; and (3) various geometric-shape SIDs as arbitrarily modified ones, except for a rectangular shape. For the second-step operational test, time-varying light with the three SIDs described above was produced in various sequences at three-second intervals. The third-step operational test verified the 30-min time stability of SID at the light outlet. Operational tests indicate that the improved system can facilitate various sunlight-effect research. Our improved system enables sunlight-effect research experiments that were previously impossible, such as investigations of naturally fluctuating sunlight effects on plant response, growth, and development. Moreover, experiments comparing the effects of ground-level sunlight spectral distribution and conventional artificial lamps on plant growth and development are now possible.