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Action spectrum vs absorption spectrum: What differentiates them?

Action spectrum vs absorption spectrum are two elements that many do not know how to differentiate. And, for the correct growth of plants, this is definitely a key point, which is important to understand when developing the cultivation method.

Although both are related to light, they are distinct processes that complement each other. If we put a little common sense into it, we can notice that their names give us clues as to what they may mean.


On the one hand, the spectrum of action is related to the action of light on the photosynthetic process. While on the other hand, the absorption spectrum reveals to us which colors of light are most absorbed by plant pigments.

It is important to remember that not all the pigments contained in plants contribute to the process of photosynthesis, therefore, thanks to the information offered by the absorption spectrum we can know the action spectrum in certain plants.

Now, since the other can be derived from one, many tend to be confused or even considered equal. Therefore, we have been in charge of making a differentiation between these two spectrums and telling you how you can use them to your advantage to choose growth lights in your cannabis crops.

Difference between action spectrum vs absorption spectrum

Although they resemble the relationship with light, there are differences between action spectrum vs absorption spectrum. Let's see, in detail, the characteristics of each of them so that you learn to distinguish them:

What is the action spectrum?

As we had already told you, the action spectrum is the one that makes known the effectiveness of a specific color of sunlight (wavelengths) to promote photosynthesis.

A marijuana-plant-receiving-sunlight

A validated result of the action spectrum, is what we know today as photosynthetically active radiation or PAR range; which is what establishes that the wavelengths (color of light) most used by plants to perform photosynthesis, are those that are contained in the range of visible light, specifically between 400 and 700 nanometers (nm).

That is, plants need, compulsorily, the colors that integrate visible light (violet, blue, green, yellow, orange and red) to carry out photosynthesis correctly; using red and blue light to a greater extent.

Hence the importance of knowing the action spectrum of our crops, in order to stimulate their photosynthesis and that they can become more productive and healthy.

Therefore, to find out the rate of photosynthesis (result of the action spectrum) it is necessary to measure the:

  • Amount of oxygen production(O2)
  • Reducción de Nicotinamide Adenina Dinucleotide Phosphate (NADP+)
  • Fixation of carbon dioxide(CO2)

What is the absorption spectrum?

The absorption spectrum, on the other hand, reveals the absorption of the different colors of sunlight by the pigments of a plant.

The pigments frequently analyzed are those that are related to the photosynthetic process, such as:

  • Chlorophyll A.
  • Chlorophyll B.
  • Carotenoids.

Chlorophyll A has an absorption spectrum between the wavelengths of violet, blue, red and orange light. While chlorophyll B absorbs wavelengths of blue, green, orange and red light. In contrast, carotenoids have a spectrum of violet, blue and green light absorption.

A-cannabis-plant-dark green

Although plants usually absorb all of these colors of light, violet, orange, and green light are known to have less absorption than red and blue lights. The latter two being absorbed and processed by the pigments, in greater quantities. Thus converting light energy into chemical energy to achieve photosynthesis.

If you want to discover, on your own, the absorption spectrum of a specific plant, you can do so by placing the pigment solution you want to analyze on a spectrometer, and waiting for the team to yield the results you want.

Simply put, what we have is that both (action spectrum vs absorption spectrum) possess remarkable differences from each other; and these range from the information they can show us, to the elements they analyze and the ways in which they can be measured. While their similarities are in: the incidence they can have with light and that the reflected data can help us to positively improve our crops.

LED grow lights for cannabis: the best technology to take advantage of the action spectrum

LED grow lights, although they can be used for any type of plant, are the best technology when growing cannabis in enclosed spaces. And this is because with its high yield you can take advantage of its action spectrum, thus stimulating the growth and proper development of this type of plants.

Cannabis, like other species, uses the frequencies of the PAR range to carry out the process of photosynthesis. But, currently, it is known that the light receptors of these plants have the ability to benefit from longer wavelengths than those established by the PAR range.

This puts the LED grow lights in a positive scenario. Since its technology provides a full spectrum of light, very similar to that of sunlight; fully covering the needs and demands of our cannabis plants.

Let's know the benefits that the different colors of light bring to our crop:

  • PAR range (violet, blue, green, yellow, orange and red light)

Blue light and red light are the lengths most absorbed by the cannabis plant. With them, photosynthesis is stimulated to the maximum, allowing their growth levels to increase in a short period of time.

During the early stages, blue light contributes to strengthening roots and stems, so that the plant can establish itself correctly. Once the first stages have been passed, blue light will now function as a growth regulating agent; this will allow it not to grow so much vertically but to expand horizontally, increasing its leafiness.

Red light, on the other hand, intervenes in the flowering process of cannabis and reinforces the growth process. Exposure to this light improves the quality of the crop, as well as expands the production and concentration of meta-topoline and terpenes.

Because red and blue light are considered the most beneficial for plants; many of the creators of LED equipment have limited their lights to producing only these two wavelengths. Therefore, we can find, abundantly, simple LED lights or violets, which lack a full spectrum of light.

Well, although violet, green, yellow and orange light are absorbed in smaller quantities and have a less important role in the photosynthetic process than the others, they are still necessary for the quality growth of cannabis plants. Since, these colors contribute to the healthy development of the crop and the metabolization of nutrients.

  • Ultraviolet light

Although, for the most part, they indicate that ultraviolet light becomes harmful to plants, in the case of cannabis, the production of resin in the flowering process has to be favored. Likewise, they come to intervene in the production of cannabinoids and terpenes.

This positive incidence between ultraviolet light and cannabis is thanks to the existence of its protective molecules (trichomes), which protect the plant from any negative effect that this type of light may cause.

A marijuana-plant-with-ultraviolet-light

  • Distant red light

Last but not least, you have distant red light; that although it does not have greater prominence in the process of photosynthesis and is little absorbed by pigments, it comes to help in the process of flowering and germination of cannabis plants.

As a conclusion we have that, cannabis plants need for their cultivation, mandatorily, a full spectrum of light, like that offered by high-end LED lights. These, in turn, guarantee the quality and quantity of our crops, since they cover the demands of the plants from germination to harvest.

What about the rest of the lights for growth?

Although LED grow lights are the best technology to take advantage of the action spectrum, traditional growth lights have not stopped being used, because they are effective – although to a lesser extent – to undertake your cultivation.

Its disadvantages are based, mainly, on:

  • CMH grow lights: The spectrum granted by these bulbs is one of the most complete, as they provide a good intensity of red and blue light. Its deficit is that the green and yellow light is in greater proportion than what the plants really need, that is, greater than ten percent (10%).
  • Fluorescent grow lights: This type of luminaire has the same disadvantage as the previous lights. In addition to the fact that its power can play against you, if the cultivation space is large; so you will need to acquire more lights of this type.
  • HPS Grow Lights: These lights, in particular, produce enough red light but sparse blue light; making them deficient during the early stages of vegetative growth.
  • MH Grow Lights: Unlike HPS, MH lights emit enough blue light but little red light. Which considerably affects the flowering stage of crops.


For these reasons, traditional growth lights are obscured by the adjustability offered by LED lights, because with them, the production of crops is optimally increased.

In that sense, we can precisely notice how important it’s important to know the differentiation of action spectrum vs absorption spectrum; therefore, it is in them where the emission of light that the plants receive from the crops is found and through which we can understand which ones work best for their stages of growth and development.

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