There are a lot of various parameters for lighting, such as lumen, PAR, CRI, spectrum and so on. And for grow lights, what should we care about?
What is spectrum for grow light?
Broad spectrum lighting – often referred to as full spectrum lighting, means the complete spectrum of light given by sunlight. This means wavelengths of broad spectrum lighting include the 380nm-780nm range (which we see as color) plus invisible wavelengths too, like infrared and ultraviolet. One advantage of LED grow lights is they can be set up to produce certain wavelengths for specified periods during the day or night. Full spectrum lighting can also provide plants with light they need in different stages to speed up growth rate, enhance root development, improve nutrition and color etc.
Simple matter of photosynthesis: plants can only utilize light that is absorbed. Green plants use the sun's light energy (composed of photons as basic particles, and the wavelength of light that the average human eye can accept is between 380 and 780 nm), assimilating carbon dioxide (CO2) and water (H2O) to produce organic matter and release oxygen. This process is called photosynthesis. Photosynthesis occurs when plants interact with photons between wavelengths of 400 nm (deep violet) to 700 nm (far red). For photosynthesis to occur and chlorophyll to absorb the maximum amount of light for plant growth, plants use both blue and red light most efficiently. Generally, photosynthetic efficiency occurs at the red and blue peaks which means plants absorb these spectrums most when growing. Other spectrums of light, like greens, yellows and oranges, are less useful for photosynthesis due to the amount of chlorophyll B, absorbed largely from blue light, and chlorophyll A absorbed largely from red and blue light.
For growth, blue light is essential to help plants produce healthy stems, increased density, and established roots – all which occur in the early vegetative growth stages. Growth then continues with increased red light absorption, resulting in longer stems, increased leaf and fruit/flowering etc. It’s here that red light plays the dominating role in plant maturity and, therefore, size.
Essentially, we know that controlling grow light spectrum can have a significant impact on areas of growth – like flowering, flavor, color, compactness etc. However, it’s important to recognize that signaling specific growth factors is part of a much larger, complex cycle. Results also vary depending on the environment (indoor or greenhouse), the relative temperature/humidity, crop species, crop nutrition, light intensity, and photoperiod etc.
Here’s specific grow light spectrums and their application in horticulture.
Blue Light Spectrum (400–500 nm)
Blue light spectrum is widely responsible for increasing plant quality – especially in leafy crops. It promotes the stomatal opening – which allows more CO2 to enter the leaves. Blue light drives peak chlorophyll pigment absorption which is needed for photosynthesis. The absorption of chlorophyll a, chlorophyll b and carotenoids is relatively strong, which promotes the assimilation of nitrogen compounds and protein synthesis in plants.
It’s essential for seedlings and young plants during vegetative stages as they establish a healthy root and stem structure – and especially important when stem stretching must be reduced.
Green Light Spectrum (500–600 nm)
Plants appear green because leaves reflect more green light than other visible wavelengths and typically only around 3 to 6 percent of green light is absorbed, and the rest is reflected or transmitted through the leaf. One reason to include green in a plant lighting spectrum is to reduce eye strain. And it can be helpful to detect nutritional problems, physiological disorders and pest issues because in the absence of green light, colors appear different to us, and plants no longer appear green.
Red Light Spectrum (600–700 nm)
Red light is known to be the most effective light spectrum to encourage photosynthesis as it’s highly absorbed by chlorophyll pigments. In other words, it sits in the peaks in chlorophyll absorption. Red light wavelengths (particularly around 660nm) encourage stem, leaf, and general vegetative growth – but most commonly, tall, stretching of leaves and flowers. A balanced pairing with blue light is necessary to counteract any overstretching, like disfigured stem elongation. It’s important to consider that while red is the most responsive light spectrum for plants, its efficacy really steps in when in combination with other PAR wavelengths.
Far-red Light Spectrum (700–800 nm)
Far-red light is generally used in proportion to red light. Due to the structure of photosensitive pigments that absorb red light and far-red light, the effects of red light and far-red light on plants can be mutually converted, which has the effect of dual-light gain.
Emerson Effect: Emerson discovered in 1957 that green plants have their own photosynthesis rate under the irradiation of red light (wavelength 660nm) and infrared (wavelength >680nm) respectively. However, when plants are irradiated by these two kinds of lights at the same time, their photosynthetic rate is much greater than the sum of the photosynthetic rates of the two kinds of lights irradiated separately.
What are PAR, PPF and PPFD for grow light?
Only part of solar radiation is used by plants for photosynthesis. Photosynthetically Active Radiation (PAR) contains the wavelengths between 400 and 700 nanometers and falls just within the visible spectrum (380 - 770nm). PAR directly indicates how much light energy is available for plants to use in photosynthesis，so everything else being equal, higher PAR means more light for the plants. Please note that PAR is a term that expresses optical radiation within the wavelength range of 400-700nm, not a unit of measurement. PPF is the number of photons, measured in micromoles (µmol), within the PAR range (400-700nm) that a grow light produces per second. PPF only measures photons within the PAR range as they are the only photons that will contribute to photosynthesis in plants.
PPF is not the amount of light your plants will actually absorb from the grow lights in your facility – it is the amount of light your fixtures can produce that is capable of being absorbed by the plant. Photosynthetic Photon Flux Density (PPFD) is the amount of light (number of photons) within the PAR range. PPFD is measured in micromoles per square metre per second (µMol/m2/S) which establishes exactly how many PAR photons are landing on a specific area. And it is all about how many of those essential, photosynthetic photons are actually impacting the grow area and measures the light your plants actually receive from the grow lights. Photons are what actually interact with the cells within a plant and affect its growth. As PPFD increases, the number of photons hitting a plant below that light also increases, which provides more light for the plant to use for photosynthesis, promoting more growth, and greater production of desired chemical compounds. As a quantifiable measurement, PPFD is a much better way to determine how much light plants are receiving than lumens. And remember, bright light for humans is not necessarily a bright idea for plants.
What is CRI for grow light?
The total visible spectrum is perceived by us humans as white light, but the "white light" is actually separated into a spectrum of colors from violet to blue, to green, yellow, orange and red made up of different wavelengths. CRI is a numeric indication of a lamp's ability to render individual colors accurately. CRI, or Color Rendering Index, is a quantification of how faithfully a light source reveals the relative color of various objects, as perceived by the human eye, compared to an "ideal" or "natural" light source (a blackbody radiator) of the same color temperature (CCT). The higher the CRI the more natural and vibrant the colors will look. Sunlight and incandescent light bulbs have a CRI of 100 - the highest score possible - as they are effectively blackbody radiators, but most other artificial light sources have a lower CRI.
Just because a light source has a high CRI score does not mean it is better for growing plants, however. A light source can have an excellent CRI but lack wavelengths of light that plants most-efficiently use for photosynthesis. This makes CRI fairly meaningless for evaluating how well a light will grow plants.
The different combinations and the relative intensity of various wavelengths of light determines the CRI of a light source. There are two factors that affect the CRI score: how continuous the spectrum of light is, and how closely the spectral power distribution (SPD, which is the relative intensities of different wavelengths of light) match a perfect "white light radiation" curve of the same correlated color temperature (CCT). Since our lights' spectrum is continuous, it is easy to see subtle variations in color when working under our lights. However, as our spectrum has more blue and red compared to other colors, the blue-ness or red-ness of objects viewed under our lights may be slightly exaggerated.
What is lumen for grow light?
Traditionally, lumens have been the benchmark of a lamps ability to grow plants; meaning the brighter the lamp the better the plant. However, studies have shown that a broader color spectrum lamp will perform much better than a lamp with high lumen output, especially when it comes to plant growth. According to the theory of calculating luminous flux, when other conditions are the same, the more the spectrum overlaps the photopic vision, the higher luminous efficacy can be obtained.
Lumen is a measure of the total quantity of visible light emitted from the grow light. Grow lights producing more lumens look brighter and grow lights producing fewer lumens look dimmer. Lumens are not, however, an absolute measure of the overall power of the grow light. This is because the lumen measurement is weighted according to a model of the human eye's sensitivity to various light wavelengths. Green plants appear green because it is reflected light. "Brightness" being at a maximum in the green spectrum, which is a visual perception, is measured based on photopic vision with peak sensitivity at 555nm. This means that green light, which humans see best, is weighted higher than blue or red light. Use caution when interpreting lumen levels for grow lights. A grow light that has a high lumen output but primarily produces lots of green light will not be very useful to plants, because plants require primarily blue and red light for photosynthesis.
What is keven for grow light?
The K rating is a generalized form of addressing the color output of a Light Bulb. Color temperature is very useful in fields such as astronomy (you can tell how hot a star is by measuring its color) and whenever you're comparing hues of "white" light (as perceived by human eyes). Much like lumen, correlated color temperature is explicitly defined by parameters of the human eye, so it doesn't apply to plants. When talking about "white" grow lights, the color temperature does indicate whether there is more blue or red in the spectrum, but on its own it is not an indicator of how well a light will do in growing plants indoors.
Conclusion
In summary, the light that is useful for photosynthesis is not necessarily bright – but it should be dense. The light given to plants should be measured in spectrum and PPFD which describe the amount of light that is capable of being absorbed by a plant, rather than just the brightness. This provides a more accurate measurement of the amount of light available for photosynthesis, making it far more relevant to growing plants.
Average PPFD is a great statistic to guide facility designs and compare when you buy LED grow lights. PPFD measurements taken at multiple points evenly spaced across the plant canopy are be averaged together to create a single number that represents the average amount of light hitting your canopy. It is also useful for quickly evaluating a grow light’s real-world performance by showing how many of its photons hit a specific area at a fixed height. It is important not to confuse lumen and PPFD. Lumens measure the brightness of light as seen by humans; essentially based on human vision. This has led to the phrase “lumens are for humans”.
