In practical terms, plants need full spectrum light for energy in order to live, grow, and flower. Without the right light, plants die!
Plants also respond to the quantity of light (brightness measured in lumens and photons per area in u mols), the quality of light (CRI / spectrum), and the duration of light (period) to guide the plant through the stages of its life (germinate, grow, flower, hibernate / die).
Green plants use light to produce energy to grow, flower, and reproduce. The “green” in green plants is from the chlorophyll that is required for photosynthesis. We seen the color green with our eyes as that is the color that is actually reflected by the leaves of the plant.
Some plants may require unique light colors, but most plants require full spectrum light for photosynthesis. This means visible light (red, orange,yellow, green, blue, indigo, and violet.) and a little radiation from the ultra-blue (UV) range and the far-red (IR) range.
If your plants are only getting blue and red as is typical for old style LED arrays or if they are only getting shades of red as is typical for HPS lights, you are short-changing them!
Full spectrum light means all colors between the wavelengths from 400 nanometers to 700 nanometers.
Studies have demonstrated that all wavelengths are required, not just red or blue. See Learn About Light for details).
Visible light is also measured as temperature using the Kelvin scale. For example, a High Pressure Sodium (HPS) light is typically a 2200 K red light. The Kelvin scale for lights ranges from deep red at 1500 K to bright daylight at around 6500 K. Many times an artificial light with a specific Kelvin rating means that most of the light from that source is a specific color even though the human eye may perceive it as mostly white light.
In the world of older incandescent lamps and High Pressure Sodium (HPS) lights, growers used to compare lamps and light sources based upon electrical consumption or watts. With the addition of significantly more efficient lighting systems like COB LED lights, comparisons should be made using watts to show electrical efficiency, Lumens / umoles to show brightness, and CRI to show how accurate the light source is compared to natural sunlight. Independent tests done by multiple labs have demonstrated that Chip On Board LED systems are the most efficient (with all costs considered), rank consistently among the brightest light sources, and have the highest CRI ratings.
So, why is this important to the indoor garden, greenhouse, or grow tent?
It is because plants need the full spectrum for photosynthesis (energy production) and they also respond to the quantity, quality, and spectrum of the light in a process called photomorphogensis.
Another consideration is the period of time that the plant is exposed to light. This is called Photoperiodism. Some plants only flower if the photo period changes from “long” to “short”. Other plants just don’t care.
For a very detailed explanation, please see Wikipedia at https://en.wikipedia.org/wiki/grow_light, and at https://en.wikipedia.org/wiki/Photosynthetically_active_radiationfor an excellent article on what is referred to as "PAR" ratings, brightness measurements in Lumens, and photosynthetic photon flux as measured in u moles. Thanks to Wikipedia for the above information!