Plants use the Red/Far-Red light ratio to determine how crowded they are, which affects how tall or full they grow
The following excerpt comes from our "Measuring Light" brochure - you can download the PDF below
Plants absorb red light (660-“680nm) and reflect far-red light (720 x 740nm). Plants contain phytochromes, photoreceptors that control physiological and developmental reactions to fluctuations levels of red and far-red light. Some responses that are regulated by phytochromes include germination, stem elongation, flowering, gene expression as well as leaf and chloroplast development.
Plant leaves filter light allowing more far-red light to pass through than red light. This changes the red to far-red ratio below the canopy. Similarly, a low red to far-red ratio is created when plants are close together.
Knowing your red to far-red ratio can help you determine plant spacing and decide when to apply plant growth regulators. Greenhouses with high canopy closure or canopy density may need more applications of plant growth regulators to keep the plants a marketable size.
The Light Scout Red/Far Red meter now includes the Soft-Sided Case (Item 7500), also available separately