The Spectral Composition Analysis of LED Grow Light

Compared with the simple spectrum of traditional plant lamps, led grow lights have a wide variety of spectrum, which also are more effectively meet the needs of crop growth.

Spectral design is one of key factors in the design and application of led grow light. Because the matching degree between spectrum and crop can reflect whether this led grow light is practical and beneficial. By understanding this factor, growers can also judge the magnitude of value of this led grow light , weigh its cost performance , finally decide if it is worth buying.

In terms of its visible property, the spectral applications of LED grow light can be divided into visual application and non-visual application.Spatial lighting belongs to visual application, while crops illumination belongs to non-visual application. The spectral composition analysis mentioned in this paper is mainly to study its non-visual applications. In spectral design, the proportion of their non-visual application also should maximized as much as possible.

The first thing we need to know is how light is used by crops.Plants are experts at capturing light energy and using it to make sugars through a process called photosynthesis. This process begins with the absorption of light by specialized organic molecules, called pigments, that are found in the chloroplasts of plant cells.

In photosynthesis, the sun’s energy is converted to chemical energy by photosynthetic organisms. However, the various wavelengths in sunlight are not all used equally in photosynthesis. Pigments are selective in their absorption of light.

Most photosynthetic organisms have a variety of different pigments, So they absorb different colors of light, which leads to the fact that the spectrum appropriate for them is mostly different.In the diagram below, you can see the absorption spectra of seven pigments in photosynthesis: chlorophyll A, chlorophyll B, and Carotenoids,Phytochrome Red,Phytochrome Far-Red,Cryptochromes,Phototropins.

red light : regulate growth and development

many commercial LED fixtures ,developed for plant growth applications, normally emit a large proportion of red light red light is highly effective at regulating growth and development. So the spectrum of most grow lights will have a relatively large weight of red light.This is even more true with the led grow lights mainly used for the vegetative growth process. so if the focus of the crops you plant is on the vegetative growth period,such as lettuce, basil and other leafy greens,When examining the selected led grow light spectrum, the specific gravity of red light deserves special attention.

Though red light has obvious growth promoting effect, a single light source of red colour can also bring "growth disaster". research show that plants grown indoors under only red LEDs, have a stretched, elongated appearance; the leaves are thin and large and plants become tall. In most cases, plants grown under only red light do not have desirable growth characteristics.

when a small amount of blue light is added, the results are different. the extension growth of plants grown indoors is inhibited. The plants will show the growth characteristics of  smaller leaves and shorter stems, which  plants are more robust and healthy than growth state they are under a single red light source .

It is worth noting that in addition to blue light, the effect of red light is also affected by far-red light. This effect is achieved by the ratio of red to far-red light (R:FR).  A change in this ratio would result in a change in the effect of photosynthesis. When the ratio of far-red light is higher than red light, the plants feel as if they are not being illuminated (actually they are illuminated ), and they continue to grow upwards in order to get more light. The plants become thin , slender and tall. 

Red light is not without impact on the blooming of plants. its effect,merely, is not as direct as blue light. It works in conjunction with far-red light to induce flowering. at the flowering stage of cannabis plants , a proper proportion of red light and far red light is a good guarantee of quality and quantity of cannabis buds.

blue light :  combine with red light to grow healthy and robust

Blue light also has pronounced effects on plant growth. Another obvious effect of blue light is its effect on flowering. moreover, blue light also can regulates the opening of stomata on the plants. stomates are the tiny openings on leaves that control both water loss and the uptake of carbon dioxide. 

A spectrum with no blue light at all is not worth considering unless indoor crops can get blue light from somewhere else.

we have mentioned it above, but now we will reiterate it again .both red light and blue light have effect on regulating the growth of plants. red light can promote extension growth of plants. However, a single red light source is not enough to bring growers the ideal growth effect. This lighting strategy will only make the crops falsely high shape,thin leaves and throw them into an unhealthy growth state.On the other hand, blue light has an inhibitory effect on extension growth of plants.Plants that grow under blue light usually have shorter stem spacing and smaller, thicker, greener leaves. 

The two kinds of light seem to have opposite effects on the extension growth of plants. One uses plant growth substances
to grow taller, extend and enlarge leaves. The other is to limit the crops to grow taller.the plant growth substances are used to make the stems stocky and thicken the leaves to discourage them from being so big or thin. However, when the quantificational growth substance is  regulated by different lights, the coordination between the two can bring different and better growth effect that is more in line with the expected effect than using either one alone. 

In addition, blue light can affect the production of color-related compounds in crops.In the two groups of mixed light, experiments by controlling for one variable，blue light, showed that the plants lacking blue light were lighter in color or had a significant change in color.While this may not be true for all crops, it appears to be true for at least some crops.

The effect of blue light on the flowering of crops is extremely complex, and its realization requires preconditions. Blue light plays an important role in the regulation of flowering in some long-day plants. 

In some long-day plants, the blue light photoreceptor cryptochrome 2 (cry2) is the main body that recognizes long-day signal. Under certain conditions, the blue light photoreceptor cryptochrome 2 (cry2) in plants is regulated by the blue light signal, inhibiting the expression of a substance that happens to be an inhibitor of a substance that promotes flowering. With fewer inhibitors, flowering promoting substances are allowed to accumulate to the extent that they affect flowering and ultimately promote plants flowering. (from American Society of Plant Biologists)

far-red light :shade-avoidance response and flowering induction

There are two known and relatively certain effects of far-red light on crops. one is its shade-avoidance response. another is flowering induction.

shade-avoidance response,sometimes it is also described as promoting extended growth. however , this does not mean that biomass increases synchronously. Too much far-red light makes the plant feel as if it's not being illuminated by photosynthetic light, so the plant can't wait to enlarge leaves and extend branches to capture more light suitable for photosynthesis.

Shade avoidance generally occurs in the lower canopy or in the lower branches and leaves of the plant.In the case of crops being directly exposed to the sun,generally top canopy get more red light than the plant life grow under the canopy or the lower branches and leaves of lower position of plants. while the shaded lower plants or branches and leaves obtain more far-red light than red light as the far-red light is more penetrative.

Therefore, for the shade-avoidance response, the current hypothesis of researchers is to rationally utilize far-red light to promote the extended growth of crops while ensuring the synchronous increase of biomass, so as to achieve the expected growth shape.

In fact, crops are very sensitive to the proportion between red light and far-red light. The ratio can give your crops very specific guidance, such as inducing them to bloom. When it comes to crops' perception of these two kinds of light, it has to be mentioned the medium that crops perceive both kinds of light,that is photoreceptor - phytochrome. there are both active phytochrome (absorb red) and inactive phytochrome (absorb red).Your plant switches between active and inactive phytochrome depending on the ratio of red and far-red light it perceives.

For some long-day crops, the duration of daylight is not enough to induce flowering when the season changes. The method of adjusting the ratio of red light and far-red light is used to promote the conversion of phytochrome, so that the accumulation of some phytochrome can induce flowering, so as to achieve flowering induction.

UV light : secrete more useful substances

Although UV light cannot be caught by the eye (since it has been out of the range of visible light),As long as the dosage is strictly controlled, it can still bring no small benefit to crop cultivation.

The use of UV light in plant illumination mainly refers to the UV-A and UV-B. UV-C,because of its extremely destructive power, generally is not used for crop lighting, but mainly used for disinfection. No matter which range of UV light is used in lamp, be remember that don't have your workers directly exposed to UV light.

The UV-A contained in led grow light will not cause damage to your crops as long as the dose of it is under strictly control.After all, the sunlight hitting the ground also has a large number of UV-A. crops have adapted themselves to analogous light environment in the long history of evolution. For cannabis cultivation, the introduction of an appropriate amount of UV-A into led grow lights will be of great help to improve the content of THC and CBD.

UV-B,which is much more more energetic and destructive than UV-A,also has its redeeming features.research has found that cannabis exposed to an appropriate amount of UV-B has more developed trichome on its buds and secrete more cannabinoids and terpenes etc. these "natural sunscreen" produced by cannabis ,mainly for protecting against UV-B light ,is exactly the producing purpose that we want to achieve.

The use of UVA and UVB in led grow lights should be case-by-case; not every crop spectrum has to be added and not every growth stage.

green light : is it necessary?

The effect of green light on plants may be greater than you think. What does green light do? To answer this question, we first need a better understanding of green light and how crops use it.

Some studies have found that adding a certain amount of green light to the spectrum has a positive effect on the increase of plant biomass. The reason why we see plants are green is that most of the green light is reflected by the leaves of plants. but there is still a small number of green light penetrating the top canopy of plants to the lower leaves for photosynthesis. In addition ,phycoerythrin can absorb green light well to drive photosynthesis.

Beside that,The green light also pushes plants to photosynthesize from the side .We all know that green light is not liked by plants.they prefer the red light and blue light, with an absorption peak in each wavelength range of them. the green light, for this reason,prompts  plants to respond - extending the stems and branches to increase the ability  to capture light.

Green light responses vary among different plant species. So it's important to know more about your plants and The effect of green light on it before deciding whether to include it in the spectrum.