The energy produced by the sun reaches the earth as electromagnetic radiation which travels in packets of energy called photons. Each photon has a characteristic energy that determines the frequency of vibration or oscillation. The distance that a photon moves during the oscillation is referred to as a wavelength and is measured in nanometers. Electromagnetic radiation spans a broad range of wavelengths - from 100 nm to about 1 mm (1,000,000 nm). A very small part of this spectrum is visible to the human eye i.e. between 380 nm and 780 nm. Electromagnetic radiation that falls in this range of wavelengths is called visible light.

Photosynthesis
The photo biochemical process that uses light as a catalyst to drive the synthesis of glucose from carbon dioxide and water is called photosynthesis. Photosynthesis is the ultimate source of metabolic energy for all plants. The photo biochemical process can be divided into two phases: light dependent reactions (light reactions) and light-independent reactions (dark reactions). In the light reactions, energy from sunlight is harvested to drive the synthesis of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), while releasing oxygen as a waste product. In the dark reactions (so named because they do not use light), carbon dioxide is modified by the addition of hydrogen to form glucose and ultimately other carbohydrates, proteins and fats. The assembly of carbon atoms in organic molecules requires the energy-releasing cleavage of high energy bonds of ATPs and NADPHs. In this phase, the ATP loses one of its three phosphates to become ADP (Adenosine diphosphate) and the NADPH loses one electron to become NADP+. The two phases of photosynthesis are interlinked and complimentary. The energy-depleted ADPs and 12 NADPs are restored in light reactions to their high energy forms (ATP and NADPH).

The spectrum of light strongly affects the primary metabolic processes (growth and development of leaves, stems, roots, and floral organs) and production of secondary metabolites (flavonoids, terpenes, cannabinoids). Photosynthetic chlorophylls and carotenoids, and other accessory pigments, absorb more efficiently blue and red light. However, all plants have a species-specific light preference, and photoreceptors that influence their anatomical, physiological, morphological and biochemical properties can span multiple wavelengths. For horticultural lighting, one of the major challenges is to develop a light spectrum that enables efficient activation of different photoreceptors for optimal results in all aspects of plant growth, including photosynthesis, photomorphogenesis, photoperiodism and phototropism.