The invention relates to an illumination device with a semiconductor light emission solution that has a wavelength up conversion feature and is suited for plant cultivation in a greenhouse environment. The best mode of the invention is considered to be a light device with at least one blue LED (101) with wavelength up conversion binary alloy quantum dots (110,120, 130, 140, 150, 160) made by colloidal methods that are arranged to produce an emission spectrum similar to photosynthetically active radiation (PAR) spectrum with the blue LEDs. The methods and arrangements of the invention allow more precise spectral tuning of the emission spectrum for lights used in plant cultivation. The invention therefore realises unexpected improvements in the photomorphogenetic control of plant growth, and further improvements in plant (310, 311) production. The invention is especially advantageous for greenhouses that already have legacy LED systems. These LED systems can often just be upgraded by addingthe quantum dot array of the invention to arrive at the lighting solution of the invention.
