Error-prone photon parameters

Have you ever encountered such a parameter representation of plant lights? The PPFD of a wavelength of 385nm can reach 20 μmol/m²・s. The PPF of a wavelength of 735nm is 4.2 μmol/s. The total PPFD of white light in the wavelength range of 396-710nm is 50 μmol/m²・s. All of the above representations are wrong.

Plant lighting technology has two major parts: photosynthetically active radiation and plant photomorphological response radiation. Bio-optics has different units of measurement for these two parts of light. This article focuses on photosynthetically active radiation. A few years ago, I saw that domestic and foreign companies engaged in plant lighting often made mistakes in parameter definition in public publicity, and even published articles for this (you can search in the official account: Let the bullets fly for a while). Later, the plant light industry gradually returned to the scope of technical logic. Despite this, the definitions of some related parameters are often wrong, so it is necessary to emphasize them.

First of all, we must clarify an important term: PAR. This is a very critical technical term. PAR is the abbreviation of photosynthetically active radiation (Photosynthetically Active Radiation). PAR refers to the radiation in the wavelength range of 400nm to 700nm of the spectrum of the light source. At present, everyone agrees that the photosynthesis of plants is driven by PAR.

The analysis of photosynthetic pigments in plant photosynthesis has also verified this inference. Therefore, as long as we talk about PAR radiation, we must strictly distinguish the wavelength range. In the wavelength range of PAR, the spectrum of the light source can be continuous or discontinuous. PAR can replace the term photosynthetically active radiation. Many people think that PAR is a unit of measurement, but PAR has its unit of measurement. The unit of measurement of photosynthetically active radiation PAR: It is recommended to use photosynthetic photon flux density PPFD, unit: μmol/m²・s It is allowed to use the effective irradiance of this band, unit: w/m².s

Note: After the light quality has been determined, the two units can be converted to each other through the RD factor. For the PAR of plant lamps, there are also parameters such as QE (PPE) and PPF; they all refer to the wavelength range of 400-700nm.

We focus on PPFD. Why do we have to recommend PPFD to represent PAR? According to the Stark-Einstein law, using photons to express the energy of light is conducive to accurate calculation of photochemical reactions, and it is easy to distinguish the effect of photon energy on photosynthesis, while the use of irradiance units cannot distinguish the energy weight of light quality.

What are the photon parameters that are prone to errors:

1. The spectrum is: PAR+FrFr is far-red light (Far Red) and the number of photons of PAR and Fr needs to be calculated separately. Moreover, the photon flux density of Fr is not included in PPFD. A common mistake is that the radiation of Fr is also expressed in PPFD.

2. The spectrum includes ultraviolet (UVA) The spectrum includes 380-399nm of UVA. Usually, this part of radiation can be expressed in photon parameters or irradiance parameters, depending on the content and methods of the research. A common mistake is that the radiation of UVA is also expressed in PPFD.

3. The spectrum includes UVA+PAR+FrPPFD only calculates PAR. For photon parameters other than PAR, such as 360-399nm and 701-750nm, they must be calculated separately and need to be distinguished. This is also the most confusing part of many materials. The reason for calculating separately is that these two parts of radiation energy do not participate in the photosynthesis of plants. From the above analysis, it can be seen that although radiation other than PAR can be expressed by photon flux density, only the PAR band produces photosynthesis. This also verifies the third of the three laws of bio-optics. This understanding needs to be experienced through long-term planting experiments. The representation of PAR also has defects. It does not distinguish which part of the RGB spectrum in PAR weights the photosynthesis efficiency. This goes back to the setting of the RQE weight mentioned earlier.

At present, all experiments have not formed an accurate weight calculation model. For the PAR measured by the sunlight spectrum, the sensitivity to the photosynthesis efficiency is very poor. This is why the plant lights designed according to the PPFD of the sunlight PAR often have high energy consumption and the effect is not necessarily ideal. There is still huge room for continuous improvement in the research of plant planting spectrum technology of bio-optics.

Nevertheless, it is also a fact that the spectral technology derived from planting experimental data still achieves good results in application.