Growing crops under artificial conditions need a very favourable environment, especially the spectral composition of radiation influencing the plant biometry greatly. The study objective was to find how to assess the closeness of real growing conditions to the optimal ones using a single coefficient, which would reflect several time dependencies of individual growth indicators. The plant growth friendliness factor (KG) was proposed for this purpose. Tomato transplants (Lycopersicon Esculentum Mill., ‘Polonaise F1’) were grown in a peat substrate under two lighting systems with different light quality.One system consisted of eight fluorescent lamps OSRAM L58W / 840 LUMILUX Cool White and eight lamps L58W / 77 FLUORA mounted on the standard frame, alternating the lamp types (Type I spectrum).In theother lighting system, the PCB Star LEDs with wavelengths of red 630 nm and far-red 735 nm were added (Type II spectrum). The irradiance level was maintained at 140 μmol m-2 s-1, the photoperiod was 16 h. The ratio of long-wave flux to the total flux KL was calculated for these lighting systems (0.37 rel.units for Type I spectrum and 0.50 rel.units for Type II spectrum) and KG factor was determined by the proposed formula. The value of KG was found to be twice as small for Type I spectrum than for Type II spectrum. The significant difference in biometric parameters of tomato transplants grown under Type I and Type II spectra was revealed. The plants grown in the environment characterized by higher KG, were higher; they had more significant wet mass and stem neck diameter.