Talk in micromoles, not lumens
The point of this article is to explain why these parameters are incorrect for agronomic lighting, to explain why, in my opinion, they were used and to introduce the correct parameters and terminology when working with plant lighting.
Similarly to plants, the human eye sees light in wavelengths between 400 to 700 nanometers (PAR). However, very differently from plants, the human eye is most sensitive to a wavelength of 555 nanometers.
This means that if we use a light source producing this wavelength (555nm) with 100w power consumption, it will appear much brighter to the human eye than a light source with the same power consumption at a different wavelength. In other words, the larger the 555 nanometer part of the spectrum in the light source, the brighter the light will seem to humans.
This is important - it will seem brighter to humans, but not to plants
As a result, when measuring light, whether it be lumens (light output) or lux (the measurement at the illuminated surface), we check how many 555 nanometer photons are in the light and how much of the light is in the surrounding spectrum, and this affects the measurement. This is meant to simulate how the measured light will be perceived by humans.
CRI is the measure of light quality. It will measure what percentage of the broad full sunlight spectrum is represented by our light source. It will be reflected in how realistic different color shades of objects will look under the light source. The higher the CRI, the more colors in the light source spectrum and the object's colors will appear more realistic. However CRI is a measure of 8 pastel colors which appear in the spectrum. The more they appear, the higher the rating. There is no consideration to the ratio or dominance of wavelengths in the spectrum or of the continuity of the spectrum, just that the colors appear in the light source.
CCT is a measure which defines shades of white light by the color temperature using Kelvin units. The lower the Kelvin temperature, the yellower/oranger the light produced. This is caused by the ratio of blue and red in a spectrum which tends towards the red end. 3000 Kelvin will appear yellowish-orange and 6000 Kelvin will appear bluish-white. If you follow this then you basically understand (depending on lighting technology) what the difference is between the spectrum of a 3000K light and a 6000K light.
In the past, lighting was a limiting factor in growth protocols. We had to rely on technologies with limited capabilities such as HPS and metal halide, which used similar electric power and spectrum output per technology. We hardly had any choice in the spectrum and a limited range of outputs. The options were mostly 250, 400, 600 or 1000 watts for all the manufacturers.
When we came to choose a lighting solution, in the majority of cases, the choice was between 2 technologies according to growth stages (usually metal halide for vegetative stages and HPS for flowering) and 4 different electric power consumptions according to the size of the facility, working temperature and air conditioning systems. Lumens were relevant since, anyway, I compared HPS to HPS and I only had 4 choices of electric consumption. Lux were relevant since I was limited in technology and there was no option of working with expensive measuring tools and complicated parameters. CCT was relevant because it allowed us to differentiate between technologies without an in depth look at the spectrum which we couldn't alter anyway. CRI was relevant because even though I had limited possibilities I was still looking for a way to compare and this showed if there are more colors in the light. More colors=fuller spectrum=better.
These were the relevant tools for comparison and selection, but they were very limited and didn't look at the plant but at who was purchasing the light and gave a simple way to compare.
Light planning was more technology oriented than plant oriented.
That was then, happily the situation has changed.
Light drives plant photosynthesis in the visible spectrum between 400-700 nanometers (the PAR range). There is no one light wavelength with specific sensitivity. Wavelengths activate the plant mechanisms. Some areas in the spectrum activate photosynthesis, some activate flowering, and some activate mechanisms that affect the plant architecture and synergy between mechanisms. When perceiving light intensity, the plant isn't specifically interested in how many 555 nanometers photons there are in the light, this wavelength is no more important than any other in the PAR range
In addition, plants work with light differently from humans. Light is energy, fuel for the plant's engines. We measure it not just as intensity, but as intensity per time. PPFD measures number of photons per second and DLI measures number of photons per day. When we measure light for plants we try to understand how much fuel we need to put into the system each day in order to see how the engine increases the plant biomass.
Instead of CCT and CRI we examine the wavelength power distribution (SPD) of the spectrum in the 400-700 nanometer range (PAR range) + what I call OVERPAR (FR-IR) and UNDERPAR (UV). We need to learn to examine SPD; analysis of this picture together with light intensities will help us understand the rate of plant growth against energy costs. What I find more exciting is that by controlling light wavelengths and spectrum dynamics we can influence the morphology, traits and secondary metabolic processes in the plant.
Lighting is no longer a limiting factor. We can now plan for unprecedented intensities with precise control of a rich, continuous spectrum. This allows us to create dynamic accurate lighting protocols. In addition to being a tool in creating growth and biomass, lighting has become the tool in designing the final product. With time and development of growth protocols we are learning to create effects that will dictate leaf size, peel thickness, internodal spacing, color, taste, scent, active ingredient concentrations and more.
I've made this table which I hope will simplify the transition from past terminology related to humans to what we know is now relevant to plants:
In the most straightforward way, with the new technology available today for artificial lighting, measuring light for plants using measurements such as lux and CRI is no longer relevant. It is the equivalent of guesswork and will not give you confidence in your choice.
Invest what is necessary to study and measure the correct parameters, those that are relevant to the plant and you will be able to select the solutions that most precisely serve your purposes.
As technology advances so do our solutions and capabilities. Our responsibility is to advance ourselves at least at the same rate so that we can make the most of it
I wish us all a common language with our plants and much joy in growth.
Bruce Bugbee has made a video that very clearly illustrates this >> Click here to watch