Far Red radiation - or Far Red light - designates the far end of the red light spectrum that is visible on the edge between red and infrared light. Usually referring to a wavelength between 710 and 850nm, Far Red radiation can be perceived by various organisms as dimmed light. Plants make use of an absorbance spectrum and read Far Red light as an indicator for nightfall. This mechanism can be utilized in artificial lighting scenarios.
Plants posses a natural photoreceptor called phytochrome, a protein that perceives Far Red light levels in the plant's surroundings. Phytochrome regulates various aspects of the plant's growth such as germination, flower formation and photoperiodism. Photoperiodism designates the regulation of the plant's day and night cycles. Plants display two essential types or conformations of phytochromes: the pr-type (r=red), which has a maximum absorption level of ca. 660nm and the pfr-type (fr=far red) with a maximum absorption level of 730nm.
So what is the connection between phytochromes and grow lights?
It's simple: by utilizing the Far Red spectrum we can influence processes like photoperiodism to optimize the plant's regeneration phase during the night and effect better harvests by optimizing its capacity for photosynthesis. This process is as simple as it is efficient: by leaving Far Red light turned on for 10 to 15 minutes after turning off the regular lights in the evening, the Far Red spectrum will act as an initiator for the natural sleep phase of the plant.
As such the natural day/night cycle of the plant is influenced by the Far Red spectrum in artificially speeding up the process of nightfall. Making the plant "think" night was falling very quickly effects the plant to enter its regeneration phase quicker than it would when using a regular light setup. As a result you can either increase the regeneration time available for the plant without extending the night phase or shorten the plant's night phase without diminishing its regenerative effect. This method is beneficial for artificial grow light setups because it allows to switch from the conventional 12h/12h rhythm of the plant's day/night cycle to a more productive 13,5h/10,5h rhythm. As a result the plant gains additional 1,5 hours for photosynthesis.