*2.2. Light Source*

A LED array system was used for irradiation of cells in vitro. This system was composed of a 20-LED-lamp-array model XL001WP01NRC660 (Shenzhen Sealand Optoelectronics, Ltd., Shenzhen, Guangdong, China) attached to a metal cooling unit and controlled by a constant current LED driver (Recom Power, Inc., Dietzenbach, Germany) model RCD-24-0.35/W. The energy fluence (J/cm<sup>2</sup> ) was adjusted according to the following formula: [power (W) <sup>×</sup> irradiation time (s)]/area of the light sensor (cm<sup>2</sup> ). LED spectral emission was recorded with a portable spectrometer (Ocean Optics, Inc., USA) with a spectral resolution of 0.2 nm in a range of 600–700 nm. A light power meter (Fieldmax II, Coherent Inc., Santa Clara, CA, USA) with a 1.9 cm diameter circular light-sensing area was used to measure maximum power and power as a function of the distance between the illumination

system and the target. This system has a maximum power of 800 mW and a maximum fluence rate of 55 mW/cm<sup>2</sup> inside the 2.5-cm-radius illuminated area. The actual power used was equivalent to 122 mW. The spectral output is limited to the 660 nm red band.
