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Difference between PAR, PPFD, PFD, Photosynthesis, Chlorophyll Absorption Spectrum

The Science of LED Grow Lights

Understanding PAR, PPFD, PFD and how plants grow using different parts of the spectrum for photosynthesis.

The right understanding of light and its energy can make you a better LED horticulturalist. Light is essential in the process of photosynthesis and the right light at the right height can make all the difference to plant growth.
Nature obviously provides the sun and the seasons to help regulate the growth of plants. Study by scientists has shown that only part of this light energy is required for the success of the horticulturalist. They have also recognized that speed and density of this spectrum of light can impact on the characteristic of the growth of the plant. Expert knowledge on the electromagnetic spectrum, the solar spectrum and the impact of photons can increase your levels of success. Here we guide you through all you need to know about the science behind LED grow lights, which in turn will help you make the right choice of equipment for you.

Terms you should know

The science of light is full of technical terminology that can make it seem impenetrable. Before we begin exploring what light you need to grow better plants, let’s define what each term means.

Solar Radiation

The energy that comes from the sun is called solar radiation. This solar radiation includes the solar spectrum that covers ultraviolent, visible and infrared lights. Plants, as with all life on earth, use the energy from the sun to grow and to thrive.

Electromagnetic Radiation (EMR)

This is the electromagnetic radiation (EMR) spectrum and is how we understand all light energy. The visible light we see all around us is just a small proportion of EMR. The complete wavelength of light includes radio waves, microwaves, ultra-violent, infra-red, and more. Indeed, the amount of visible light within the EMR spectrum is small in comparison to the whole.
To measure EMR we use wavelength and frequency. Wavelength and the highs and lows of energy emission and frequency is how rapidly this wave travels. The wavelength is measured in micrometres and the frequency in hertz. Blue light travels in shorter waves, for instance, therefore this light is scattered more when it hits the atmosphere – hence the sky is blue. Understanding EMR helps the LED horticulturalist work out know what part of the spectrum the plant needs to grow effectively. The wave will likely be stated in nanometers (nm) and sometimes the frequency in hertz (Hz). Plants in theory broadly use the light in the wavelengths between 400nm and 700nm – this is known as PAR. However, some engineers of LED grow lights understand that there is more useful light than this and extend the range to 380nm to 780nm, through UVA and far-red for healthy, tight plants.


PAR response spectrum
PAR stands for the Photosynthetically Active Radiation. This is the spectral range of solar radiation, from 400 to 700nm, that results in the bulk of photosynthesis in plants. In short, this is the part of the sun’s light that helps your plants to grow.


PPF stands for Photosynthetic Photon Flux. This measures the photons within the range that is useful to photosynthesis in plants. It is measured in joules per second and the higher the number of joules the more efficient your light is at scattering helpful molecules over your plant. Therefore, understanding PPF helps you to choose an efficient LED grow light.


PPFD stands for Photosynthetic Photon Flux Density. This measures the amount of PAR that the plant receives and therefore the number of photosynthetically active photons. Not all light that is scattered hits the plant. The scattering of light is denser at the source and becomes less so as it reaches the plant. Understanding PPFD helps the LED horticulturalist understand the vertical and horizontal positioning of the lights they use.


PFD stands for Photon Flux Density, note that the first ‘P’ is removed which stands for Photosynthetic because measuring in PFD will cover spectrum beyond PAR. In this case we consider 380 – 780 nm. When you see lights being measured in PFD they clearly have a broader full (expensive) spectrum that creeps beyond PAR into the 300’s nm which is UVA and above 700 into far-red. Full explanation of these advantageous for your yield are outlined at and also numerous scientific resources.


Chlorophyll is a complex molecule that is essential at different points in the photosynthesis process. Chlorophyll needs to absorb the right level of energy from EMR to successfully facilitate the growth of the plant. For instance, it requires the energy to convert CO2 and water into oxygen and glucose that are essential processes for thriving life in the plant world. More on .
Understanding what light is useful to the production of chlorophyll is therefore essential to successful plant growth. The best LED grow light manufacturers will know, for example, that green light is not essential to plant growth – therefore this is removed from the spectrum of the best products.

How does this help you choose your LED Grow Light?

All this is helpful information for the LED horticulturalist. However, what does it mean? How does it help you to select your LED grow light? Here are some important summary points. First, the EMR for your grow light should go beyond PAR. The spectrum from 400 to 700nm misses out of nearly a 100nm of useful energy either end of that. Check out our LED light reviews to see which ones cover the whole of the useful spectrum of light energy. There is a lot of opportunity to select an LED light that helps the plant more than those that are limited to the PAR spectrum. If you select a light that includes UVA and far red within the spectrum then the plant becomes more resistant to diseases and pest. The use of far red light means plants become denser, with less spindly and more squat in growth. This is because the plant believes it is growing out in the open and not in the shade. Remember the plant does not understand context – it doesn’t know the light isn’t the sun. It will respond to the quality of the light it receives. Second, check out the PPFD and the average at certain heights. Does the set-up of the LED light match the optimum density of PPFD / PFD for the best growth of the plant? Some cheap LED lights can cause erratic flow of PPFD / PFD due to the driver, which is not good for the consistent growth of plants. Your LED light should publish the level of PPFD / PFD at vertical and horizontal positions. This should give you the average PAR received by the plant and the ideal position for the LED. Finally, choose high quality LED grow lights that understand the role the light spectrum plays in photosynthesis, plant growth and disease resistance. Remember the plant doesn’t know it is growing indoors, it only knows how to respond to the energy provided. This energy comes from the power of the light you provide. Thank you to for their help on this article.