An improved method to produce artificial light for plant cultivation, an illumination device with a semiconductor light emission solution and device suited for plant cultivation in a greenhouse environment are described. The best mode is considered to be a lighting device with binary alloy quantum dots (110, 120, 130, 140, 150, 160) made by colloidal methods to produce a size distribution of quantum dots that produces an emission spectrum similar to the photosynthetically active radiation (PAR) spectrum. The methods and arrangements allow more precise spectral tuning of the emission spectrum for lights used in plant (310, 311) cultivation. Therefore unexpected improvements in the photomorphogenetic control of plant growth, and further improvements in plant production are realized.