If you have researched lamps for growing plantsYou've probably been bombarded with different numbers and measurements that lamp manufacturers use to market their products. Some of the terms and acronyms you're likely to come across are: Watt, Lumen, LUX, PAR, PPF, PPFD, DLI and photon efficiency.
Although all of these terms are related to lighting, only a selection of them really tell you the most important information for choosing a lighting system for growing plants.
The purpose of this article is to define what PAR lights and ePAR (extended PAR) grolys are, and correct some common misconceptions as well as help plant growers understand which measurements are relevant to grow lighting systems and which are not.
Read on to learn more about what PAR light is.
What does PAR grow light mean?
COUPLE grow lights, are the wavelengths of light within the visible range of 400 to 700 nanometers (nm). All plants need light, but some plants need more light than others.
The amount of light a plant needs to thrive is measured in a unit called PAR (Photosynthetically Active Radiation).
PAR defines which part of the light spectrum the plant uses to create photosynthesis.
Most plant photosynthesis is best fuelled by blue light which is 460nm and red light which is between 650-700nm, also known as chlorophyll-B and chlorophyll-A. However, nature is so cleverly designed that plants can also absorb the green and near infrared rays (NIR 700-730nm).
Plants do not absorb the green wavelengths from the grow light, but instead reflect the light so that the deeper parts of the leaves and plant can use them to produce energy.
It can therefore sometimes seem as if the green light is useless to the plant. Scientists were also convinced of this for many years, but unfortunately this is not a correct assumption.
It has now been proven by several researchers that the green light is less effective for plants than the other colors, but that it contributes to the growth and resistance of the plant in other ways.
The researchers used spinach to prove that green light is less efficient than both blue and red light, as the quantum efficiency under green light is 10% less than red light and 25% less compared to blue light. It can therefore still contribute to healthy growth in most cases.
However, it can still be complicated to find the right balance in PAR grow lights for plants, which is why most professionals recommend using white full spectrum light, as you are then sure that your plants get the correct light.
The plants don't just need red and blue PAR grow lights – Plant growth under the canopy
This process is a very complicated one, which is influenced by many different factors. Green, yellow and NIR light play a different role for the plant, although they are less effective for photosynthesis.
Green and yellow light have the quality that they go deep into the plant and ensure growth under what is called canopy - This means that the light penetrates deep into the leaves and down the entire height of the plant, which is not possible with the blue-red light.
The big problem was that in the early days of research in grow light assumed that plants only need red and blue light. Therefore, many of the first generations of LED grow light panels were flawed.
The errors mainly consisted of all diodes being blue and red with a high amount of wattage, resulting in a lack of growth during canopy - That is, under the "crown" of the plant.
The plants therefore sometimes experienced bleaching and burning at the top, as all the light energy was deposited in the top shoots instead of also reaching deeper into the plant - there was no effective canopy.
But with more modern LED grow lights, which is designed after The PAR principlesLike many of the ones you can buy on this site, these problems are non-existent.
Host plantelys.dk we offer LED grow and grow/PAR grow lights, that are ideal for giving your indoor plants the best conditions to grow and thrive.
Find out more and buy here: LED grow light
Understand the importance of PAR output for your garden
PAR output, is a key factor when it comes to how efficiently your grow lights support plant growth and maximise yields in your garden. It is also a key metric in the benchmark tests we use to evaluate the quality of different grow light systems.
PAR measurements, Widely used across technologies such as HID, CMH, LED and T5 grow lights. Whatever type of grow light you're considering, the PAR output - also known as PAR rating - will determine whether the light matches your growing needs.
To understand why PAR readings are so important, here we explain the difference between the terms COUPLE, PPF and PPFDand what they mean for your garden and plants.
What is PAR?
COUPLE stands for Photosynthetically Active Radiation - in Danish: photosynthetically active radiation. It covers the range of light (400-700 nanometres) that plants can use for photosynthesis.
Unlike ordinary brightness, which measures all visible light, PAR focuses exclusively on the wavelengths that plants actually absorb and utilise as energy. Grow lights emit many different colours and light waves, but only a portion of these are absorbed by the plant's chlorophyll and used in the photosynthesis process. The rest is either reflected or transmitted without contributing to growth.
It's a common misconception that PAR measures the total amount of light from a light source. In reality, it's about how much of the light is actually useful to the plants. Therefore, it is important to distinguish between light that is merely visible to us and the light that is biologically effective for plants.
Why is it important?
When choosing grow lights, it's not enough that the light looks bright or clear - it needs to deliver the right wavelengths in sufficient quantity and intensity. By understanding PAR and related metrics like PPF and PPFD, you can better assess which lamps will give your plants the best growing conditions.
Read more here about light in indoor growing…
How does grolys work
Grow light - also known as grow lights - are artificial light sources designed to mimic the sun's natural light and support photosynthesis in plants. Plants use light as an energy source to convert carbon dioxide and water into sugar and oxygen - a process known as photosynthesis. For this process to happen efficiently, plants need light in specific wavelengths, particularly in the so-called PAR (Photosynthetically Active Radiation) range, which covers the spectrum from approximately 400 to 700 nanometres.
Grow light emit these wavelengths and can be customised to accommodate different stages of the plant life cycle. For example:
Blue light (approx. 400-500 nm) Promotes compact growth and strong leaf formation - ideal for the vegetative phase.
Red light (approx. 600-700 nm) Stimulates flowering and fruiting - important in the generative phase.
Modern grow lights, especially LED-based systems, allow you to customise the light spectrum, intensity and photoperiod (length of light period) according to plant type and growth phase. This allows you to grow effectively indoors all year round - regardless of climate and season.
Grolys work by providing plants with the light they need for photosynthesis, enabling healthy growth and higher yields in a controlled environment.
Can you use LED lights for plants
Yes, you can, LED light, are highly suitable for plant growth as long as they are designed with the right light spectrum. Ordinary LED bulbs for home lighting typically emit a limited spectrum that doesn't necessarily meet the needs of plants. However LED grolys, is specifically designed to emit light within the photosynthetically active range (PAR)that plants use to grow.
Advantages of LED grow lights:
Energy efficiencyLED lights use less power and generate less heat than traditional grow lights like HPS or fluorescent lamps.
Long life: Many LED grow lights last for 30,000-50,000 hours or more.
Targeted spectrum: Manufacturers can design LEDs to deliver exactly the wavelengths that plants need - for example, blue light for growth and red light for flowering.
Low heat generation: LEDs emit less heat, reducing the risk of damaging plants and making it easier to control the climate in the growing area.
Important: If you want to use LED lights for plants, you need to make sure that it is a grow light - not just an ordinary LED lamp. Grow light is labelled with information about PAR, PPF or the specific spectrum, and is designed to support photosynthesis.
What is PAR
PAR is the amount of light that can be used for plants the wavelengths that will be used for photosynthesis. PAR is not the measure of all the light a growing light gives off, but The measurement of wavelengths plants will absorb.
