The invention relates to an improved method to produce artificial light for plant cultivation. In more particular, the invention relates to an illumination device with a semiconductor light emission solution and device suited for plant cultivation in a greenhouse and/or dark growth chamber environment. The best mode of the invention is considered to be a lighting device with LEDs that produces an emission spectrum similar tothe photosynthetically active radiation (PAR) spectrumin a dark growth chamber. The methods and arrangements of the invention allow more precise spectral tuning of the emission spectrum for lights used in plant (310, 311) cultivation. The invention therefore realises unexpected improvements in the photomorphogenetic control of plant growth, and further improvements in plant production, especially in dark growth chambers, such as basements.
