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Why we need to phase out the term "lux" in horticulture

The measurement unit "lux" is long accepted by the lighting professionals as a unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square meter. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. However the unit “lux” is not appropriate to measure horticultural light output as it only takes into consideration the human eye sensitivity of colors.

The International Commission on Illumination - also known as the CIE from its French title, the Commission Internationale de l´Eclairage - is devoted to worldwide cooperation and the exchange of information on all matters relating to the science and art of light and lighting, color and vision, photobiology and image technology. It has developed a color RGB matrix called CIE 1931 that describes the visible colors of human eye.

What is CIE 1931?

The CIE 1931 color spaces are the first defined quantitative links between physical pure colors (i.e. wavelengths) in the electromagnetic visible spectrum, and physiological perceived colors in human color vision. The mathematical relationships that define these color spaces are essential tools for color management. They allow one to translate different physical responses to visible radiation in color inks, illuminated displays, and recording devices such as digital cameras into a universal human color vision response. CIE 1931 RGB color space and CIE 1931 XYZ color space were created by the International Commission on Illumination (CIE) in 1931. The CIE XYZ color space was derived from a series of experiments done in the late 1920s by William David Wright and John Guild. Their experimental results were combined into the specification of the CIE RGB color space, from which the CIE XYZ color space was derived.

What is human visible spectrum?

A typical human eye perceives colors from 390nm (Violet) to about 700nm (Deep Red).

Beyond these wavelengths human eye cannot perceive the electromagnetic radiation.

The eye behaves differently in high or low light conditions: in daylight, for brightness levels above 3 cd/m2 the vision is mainly done by the center of the retina, we can see colors and the maximum sensitivity is at 555 nm (in the green region). This type of vision is called photopic vision.

Therefore, a light source radiating 1 W of green light will appear much brighter than another source radiating the same amount of power of red light because the eye is more sensitive in the green region. 

In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light

What is plant “visible spectrum”?

Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities (energy transformation). This chemical energy is stored in carbohydrate, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis. All light is stimulating photosynthesis but some specific spectrum is more efficient than others.

Unlike human eye sensitivity to green color at wavelength of 555nm, plants are sensitive to other wavelengths.

The correct unit of measure light with respect of the plant sensitivity is PPFD.

PPFD (Photosynthetic Photon Flux Density) represents a field measurement and is defined as, the number of photons emitted per M 2 per Second. Units of measure: µMol/M2S

What is Chlorophyll?

Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll serves two primary functions. The function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light and transfer that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems.

What are Cryptochromes and Phytochromes?

In plants, cryptochromes mediate phototropism, or directional growth toward a light source, in response to blue light. Cryptochromes receptors cause plants to respond to blue light via photomorphogenesis. Cryptochromes help control seed and seedling development, as well as the switch from the vegetative to the flowering stage of development.

Phytochrome is a photoreceptor, a pigment that plants, and some bacteria and fungi, use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms.

Due to the different sensitivity of perceiving the light of human beings and plants, professionals must separate the "lux" unit, dedicated to the human light perception, with the PPFD, dedicated to plant light perception.  

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