Light sources and supply
The blue LEDs used had an emission range from 450 to 500 nm (13). On a LED panel (5×5 cm), 85 LED lamps were arranged in 10 rows and 9 columns and powered with DC power supplies (PA36-2A; Kenwood, Tokyo).
We generated flashing light by intermittently turning off the electrical supply with a high-speed switching device, which consisted of a signal generator (SG-4105; Iwatsu Test Instruments, Tokyo) and a photo-relay (TLP598A; Toshiba, Tokyo). Flashing light is characterized by its incident intensity (μmol m−2 s−1), frequency and duty cycle. The frequency (Hz) is defined as the number of flashing per second and expressed by (tLight+tDark)−1 where tLight and tDark (s) are the duration of light and dark periods in each cycle, respectively. The duty cycle (%) is defined as the fraction of the duration of the light period in one cycle, tLight/(tLight+tDark)×100. In addition, we defined the incident light intensity for flashing light (I0) as the product of the light intensity impinging on the surface of the illuminated vessel during the light period (IL) and the duty cycle/100.
I0=IL×tLight/(tLight+tDark)
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In this work the intensity of light impinging on the vessel during the light period was fixed at 12 μmol m−2 s−1, and the incident intensity of the flashing light was adjusted by changing the duty cycle. The frequency and duty cycle for the flashing light were varied in the ranges of 25–200 Hz and 17–67%, respectively. The continuous light was supplied at the incident intensity ranging from 2 to 12 μmol m−2 s−1. The incident light intensity at the surface of the vessels was measured with a waterproof photosensor using a thermopile (MIR-100C; DIA Instruments, Tokyo) (12). The light intensity was expressed as the number of photons passing through a unit area per unit time, namely the photon flux (μmol m−2 s−1) (4).
The response time of the LEDs to electricity input is several nano seconds, and, as a result, it was possible to control the on-off of the LEDs by changing the input voltage with the switching device, as shown in Fig. 1 at a frequency of 1 kHz (duty cycle = 50%). Although a slight delay was observed in the voltage-rising phase, the change in input voltage is approximated by a rectangular pulse.