Photosynthetically Active Radiation
What is light?
Light can be described as electromagnetic energy or waves moving from one point to another. Each wave for a certain wavelength looks the same; it has the same beginning and ending, and the same height and width. Some wavelengths are shorter, meaning faster wave formation, while other wavelengths are longer (slower wave formation). The whole range of light wavelengths is called the light spectrum. Wavelengths are measured in nanometers (nm), which is equal to one thousand millionths of a meter (10^9). For example, ultraviolet light lies between 100 and 400 nanometers, while red light can be measured around 680 nanometers.
The particular band of light that growers are especially interested in is PAR, or Photosynthetically Active Radiation. PAR light lies between the range of 400 and 700 nanometers and is also called “visible light”. Plants use PAR light for photosynthesis, or growth.
Photosynthetically Active Radiation (PAR)
PAR stands for Photosynthetically Active Radiation. PAR includes the light ranging from 400 to 700 nanometers that plants are able to use for photosynthesis, or growth. The amount of PAR can be measured with a special PAR meter and is expressed as the number of micromoles (or µmoles) of photons being emitted by the light source.
PAR can be used in determining the strength and overall quality of a grow light. However, it is important to note that the amount of PAR alone is not enough. With a single PAR amount, we still don’t know how much light actually reaches the plant. It is therefore important to also know the position and distance of the light relative to the plant, when determining the strength of the light fixture.
While most plants absorb and use a lot of Blue and Red light, Green gets reflected and this is the reason why most plants look green. This does not mean that plants only need Blue and Red light. Plants use all light wavelengths for different purposes, even light outside of PAR (such as Ultraviolet and Infra-Red light) are useful for plants.