To obtain a homogenous light field at appropriate wavelengths and within a range of irradiance values, light sources based on a light emitting diode (LED) array were used for this study.23 Each lamp consists of an array containing 247 ( diodes, 624 nm, illumination field ) or 260 ( diodes, 410 nm, illumination field ) LEDs (Roithner LaserTechnik, Vienna, Austria) connected in parallel. The irradiance of these LEDs is adjusted by the current applied as described by Pieslinger et al.23 The assembly of the LEDs and the distance from the LEDs to the illumination field are optimized to achieve a homogenous light field (maximal variation 10%). The LED light source for blue light illumination (LED type VL410-5-15, dominant wavelength 410 nm, spectral half-wave width 18 nm) allows for irradiance values up to , and the red light illumination source (LED type R5CA5111P, dominant wavelength 624 nm, spectral half-wave width 20 nm) delivers up to . The emittance spectra can be found in 24 and the light viewing angle is 15 deg. Irradiance was measured in a grid pattern (covering the whole light field) by an LI-189 light meter equipped with a PY pyranometer detector (LI-COR, Lincoln, Nebraska, diameter 7.5 mm) and a LabMaster Ultima laser measurement system with a vis-sensor (Coherent Inc., Wilsonville, California, diameter 8 mm). Both light meters had a typical accuracy of for the total system. The light transmission of the microplates used is reported to be close to 100% at wavelengths according to information from the manufacturer. The transmission spectrum should, therefore, be close to that of the lamp itself.