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Grow Lighting Basics

What is a grow light?

Grow lights are a specialised lighting fixture designed to deliver the required color or spectrum of light and the optimum light intensity for healthy and vigorous growth.

The measure of light which causes plant growth is called photosynthetically active radiation or PAR.

What is photosynthetically active radiation? What is PAR?

Plants use the range of the light spectrum known as photosynthetically active radiation. Photosynthetically active radiation (PAR) is light in the wavelength range of 400 nm (blue) to 700 nm (red). Plants use all of the PAR spectrum at different stages of growth.


 Photosynthetically active radiation is part of the total electromagnetic radiation used by plants for photosynthesis

What does photosynthetic photon flux mean? What does PPF mean?

Light consists of small packets of energy called photons. The number of photons emitted by a light source in the PAR range is known as the photosynthetic photon flux or PPF. A unit of measure for a large number of photons of light is called a micromole (μmol). PPF is measured in µmols/sec. PAR photons are converted by plants into chemical energy in a process called photosynthesis.

PPF or Photosynthetic Photon Flux

What is photosynthesis?

Photosynthesis is the process used by plants to convert photons of light and carbon dioxide into chemical energy (glucose) used for growth and development. All that is needed for this process is photons of light in the PAR range, carbon dioxide (CO₂), nutrients, and water.

Photosynthesis reaction uses PAR photos, CO2, water and nutrients to fuel plant growth

What is PPFD?

The amount of PAR photons reaching your plant canopy is measured in micromoles per square metre per second. This measurement is known as photosynthetic photon flux density or PPFD and the units are µmols/m²/sec. More PAR photons to plants (PPFD) = more plant growth

Photosynthetic Photon Flux Density of PPFD

How much PAR light do I need for my grow area?

To optimise your grow it is recommended that your plants receive 500-1000 µmols of PAR light for every m² (PPFD). Less than this and growth rates will be low. Higher PAR intensity for your plants than this and the increase in growth rates do not justify the extra energy cost. The plants require additional CO₂ to utilise very high PAR intensity.
PAR intensity vs growth rate with and without CO2

What is McCree’s action spectrum and relative quantum efficiency?

In the 1970s a scientist called McCree determined the wavelengths of light a plant can absorb. He assessed the relative quantum efficiency of different wavelengths of light on photosynthesis. In other words he recorded the rate of growth of plants exposed to each wavelength or colour of light. McCree discovered that plants use all parts of the PAR spectrum but at different rates.

The quantum efficiency for red is close to 100% efficient whereas blue and green were less efficient, ranging from 65% to 75%. McCrees action spectrum identifies what colors of the visible spectrum are best for photosynthesis. McCrees action spectrum is often used as a basis for assessing the suitability and effectiveness of a grow lights spectrum output.

McCrees action spectrum graph

What is a full spectrum grow light?

A full spectrum grow light contains all colours of light from the photosynthetically active radiation or PAR range. The light frequencies or colours range from blue to red. A full spectrum grow light is suitable for growing your plants from seedlings to flower.

MIGRO ARAY LED grow lights output a full spectrum grow light and is a ‘warm white’ colour to the human eye, comfortable to use and ideal for inspecting your plants health.

What is a PAR map?

The grow area is divided into a grid and the grow light is hung at its optimum height. A measurement is taken with a quantum meter of the PPFD in each square on the grid. The average and total PPFD across the grow area can be calculated in this way.

The information can then be represented as a PAR intensity map to show the PAR levels across the grow area. Each PPFD measurement is then translated into a PAR intensity map so that you can visually see how well the light covers the target grow area.


What is grow light coverage?

A single and concentrated light source will provide intense PPFD levels directly under the light. This will encourage the plants to compete and all grow towards the light, blocking each other out.

Grow light coverage from a spread out light source provides photon intensity from multiple directions (like the moving sun) so plants do not grow towards one point but evenly across the canopy. This results in even coverage, reduces hot spots and stress on the plants.


How close should a grow light be to plants?

Having to maintain a big grow light distance between your Grow lights and plant canopy limits the height your plants can grow and prevents you growing in low spaces. HPS, MH and boxed LED grow lights must be positioned from 16" to 24" or 40cm to 60cm above the plants to avoid hot spots and to provide even coverage When the light source is not as concentrated the plants can grow close to the light source. When the light source is spread like with LED bar lights you can grow up to 30cm or 12in below the light sources without hot spots or plant damage.

What maintenance does my grow light need?

Todays LED grow lights are sealed and require no maintenance other than cleaning the heatsinks. The lights will continue to output 80% of their initial level after 50,000 hours (10 years at 15 hour per day) in compliance with the international test standards, LM-80 and TM21. Traditional HPS OR MH bulbs and the reflectors must be changed at least every year to maintain 80% of their initial output.



1 thought on “Grow Lighting Basics

Alemayehu Hailu

Average PPFD needed in 20×12 Meter room size ?

March 29, 2022 at 13:49pm

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