Why is the lighting measurement method for humans not suitable for plants?

Usually for visible light, the simplest measurement method is to use the human eye as a judgment tool. This practical method is called photometric method. That is, the scientific method of measuring light based on the brightness perceived by the human eye. It is different from the radiation method, which is a science that measures radiant energy (including light) by absolute power. In modern photometry, the radiant power in the visible light spectrum of each wavelength is weighted by a photometric function that models the average spectral sensitivity of human visual brightness perception. In the International System of Units (SI), common luminosity quantities include luminous intensity, luminous flux, illuminance (or illuminance), luminosity and brightness, etc.

To explain with an example, luminous intensity is a measure of unit solid angle power emitted by a point light source in a specific direction and weighted by a luminosity function, similar to radiant intensity, that is, unweighted power intensity. The SI unit of luminous intensity is the candela (cd), which is one of the seven basic SI units. Luminous intensity is similar to radiant intensity.

The above-mentioned photometric method is based on human visual perception of light brightness, and is adjusted by a photometric function. This is sufficient for lighting systems used with humans, but obviously not suitable for plant lighting. When dealing with the interaction between light and plants, there are three reasons to consider measuring light in different ways. First, the way plants "see" light from the human eye is different. According to McCree, there are two peaks in the absorption spectrum distribution of higher plants, which are completely different from the luminosity function of visible light. Since lux is adjusted by the photometric function; it is definitely not a suitable method to determine the amount of light available to plants. Second, when the spectral distribution of light is very narrow, such as light-emitting diodes (LEDs), the calculated illuminance is usually distorted and useless for plants. For example, red light with a narrow bandwidth of 660 nm wavelength will have a relatively low lux. But this is very effective for photosynthesis of plants.

Unlike the human eye, the human eye sees light as a continuous wave and therefore perceives it as continuous energy, while plants see light as discrete photons and interact with them by absorbing photons one after another. Therefore, the particle characteristics of light must be considered, and different measurement methods must be introduced. This is a quantum method in which the number of photons received by the receiver per second and per square meter is used to measure the photon flux density. Since the energy of a photon is very small in terms of the usual energy unit, and the longer the wavelength, the lower the photon energy, in order to have one joule of photon energy, a large number of photons should be involved. For example, the energy of a blue photon with a wavelength of 450 nm is4.4 x 10 -19 J, while 1 J of blue light will require 2.3 billion such photons. The longer the wavelength, the more photons needed to produce 1J of light energy.

Since the illuminance in the photometric method is the total luminous flux density of lux, it is also similar to wm -2, but adjusted by the luminosity function, so it must be converted between μmol/m2/ s (PPFD) and lux. However, this conversion is closely related to the spectral distribution of light p(λ), and there is no analytical expression for this conversion. Different p(λ) and μmol/m2/ s will cause different illuminance, sometimes very different.

Different brands of lamps have different spectral distributions. LEDs of different colors, especially narrowband LEDs, have a very different relationship between PPFD and Lux because of different spectral distributions. For blue light with a center wavelength of 445nm: 1 μmol/m2/ s is approximately equivalent to 7lx. For red light with a center wavelength of 660nm: 1 μmol/m2/ s is approximately equivalent to 10lx. For green light with a center wavelength of 519nm: 1 μmol/m2/ s is approximately equivalent to 105lx.

From these compares, we can see that the farther the wavelength is from the center of the luminosity function, the more photons are required to produce the same number of lux.

Therefore, lux units cannot be used to measure the amount of light absorbed by plants. Under different spectral distributions, the same number of Lux will have completely different effects on the growth and development of plants.

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