Plant Lighting FAQ


What is light?

Light is made up of photons. Photons have a wavelength and frequency, with an energy proportional to their frequency. The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The visual spectrum is only a small fraction of it.

The wavelength of lights, from the higher power UV rays (below 400nm), to the familiar ROYGBIV rainbow colours (400-700nm), to the start of the infrared (above 700nm), is everything we as humans can see. 

Max Planck, the father of 'quantum theory', famously worked out that photons have 'quantized' energy, meaning that there is a discrete, minimum value of energy that photons can have. The equation, E = hf describes the energy of light, where f is the frequency, and h is Planck's constant (6.63 × 10-34 Js).  It is also worth mentioning that f = 1/λ.  That is, frequency is inversely proportional to wavelength.



Why are plants green?

Plants appear green to us because the leaves contain chloroplasts, whose membranes contain chlorophyll A and B, along with other "accessory pigment" molecules.  Chlorophyll A and B are pigments based around a magnesium atom, which reflects green and yellow, while absorbing red, orange, blue and violet. 


How long should I keep grow lights on?

Plants have stages: vegetative and flowering.  There are differences between "short day" and "long day" plants, but the general rule of thumb is, that to ensure a plant stays in a vegetative state, you should give it 18 hours of light per day.  To induce flowering, you should drop it down to 12 hours or less of light per day.



What are the effects of light on plants?

Light is both energy for a plant, and it is information.


There are a few competing measurements of light for plants.  You want to give your plants the energy they need.  Lettuce and herbs don't need a lot, whereas tomatoes and marijuana can use plenty of light.  

The basic measurement is called PAR (photosynthetically active radiation). It is the visible light spectrum (400nm-700nm), and is measured in photosynthetic photon flux, or PPF.  

Generalised to an area, is PPFD, with the D standing for density.  The unit of PPFD measurement is µMol/m²/s.  It describes the number of photons passing through a square meter, per second.


So PAR is the "Quantum Response" box in this picture:

PAR curve

PAR is not an ideal measurement, but it is the standard measurement.  

Typical values required for lettuce might be 100-300 PPFD, whereas for marijuana, it may be 400-800 PPFD.  At about 1200 PPFD, you're reaching the maximum most plants can use, unless you start supplementing CO2.  Noon day sun is about 1500 PPFD.

YPF is a better measurement.


The top right graph describes the absorption and photosynthesis of chlorophyll. As you will read later, this is a bit misleading.  This graph was created by measuring photosynthesis of Chlorophyll in a solvent, rather than in a living plant.

photosynthetic action region





The bottom graph on the left is the "Action spectrum" of an isolated chloroplast (outside the context of a leaf). The graph on the right, the YPF curve, describes the "Action Spectrum" of a leaf on a plant, generalized from various plant species.  (This comes from a 1972 paper, published by KJ McCree, where he measured the photosynthesis rate of 22 plant species.)

So, the YPF curve is a better approximation of the effect of light on plants, as it weights measurements by the effect those wavelengths have on photosynthesis.


Plants also use light for 'photoperiodism'.  You can trick plants into flowering by manipulating their light. You can also alter the focus of what they spend their energy on, whether to focus on root growth, or to synthesize chlorophyll, or to elongate leaves and stems, to change the shape of leaves, or the number of branches, etc. 

This is typically done by manipulating the cryptochrome, and phytochrome molecules.  

Phytochromes are molecules that dance between two inventively named photosystems: Photosystem I and Photosystem II.  The wavelengths of light used by these systems are 680nm and 700nm, (but they'll catch any photon and bounce it around until it's the right wavelength).  

The red-absorbing "Pf" form changes to the far-red absorbing "Pfr" form when it absorbs red light (680 nm) and changes back again when it absorbs infrared light (700 nm).  During the night, it slowly changes it back, and in the morning, this conversion process allows the plant to work out day from night.  So you can prevent a short-day plant from flowering if light is turned on for a few minutes in the middle of the night.  You can also gain an extra hour of growth per day, by shining a plant with infra-red before 'bed-time', as the headstart in Pfr -> Pf conversion will speed up its sense of time passing.

Meanwhile, cryptochrome molecules, which absorbs UV and blue light, are present not only in plants, but in animals, and regulate the circadian rhythm.  They're why you shouldn't stare at your phone before going to bed!  In plants, cryptochromes mediate the directional growth toward a light source, in response to blue light.  This is known as phototropism.

Cryptochromes do a lot of things, but the short story, is that increasing the blue content results in more compact plants, because the plant will focus more on root growth and branching, than on leaf elongation and stem growth.

So a good vegging light will have a decent amount of blue in it.

And a good flowering light will have a decent amount of red in it.

If you'd like to read a scientific paper on the effects, try this link.


What is the spectrum of the Sun? 

The total amount of power hitting the ground, from the sun, is around 1120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared, 42 to 43 percent visible, and 3 to 5 percent ultraviolet.

So plants are only using about half of the energy that reaches them.  Most of the infrared is not absorbed.  Infrared is experienced as heat.

On a typical sunny day in an ideal location, the wavelengths of light hitting us are diminishing as the afternoon approaches the evening. (source)

As you can see, the infrared part of the spectrum increases later in the afternoon as the sun sets.  Sunlight has a Red to Infrared ratio of about 1.2 : 1.


What is full spectrum light?

LEDs produce only a specific wavelength. 

But by shining a blue wavelength onto a phosphor coating, you get a spectrum of light.

Full spectrum LEDs are described as warm-white (3000K) or cool-white (6500K), or natural-white, or pure-white, or sky-blue, etc., and have color temperatures, measured in Kelvin (K). These approximate the colour of light emitted by a "black body", which in the early 1900s, was done by heating up a ceramic chamber, to various temperatures in degrees Kelvin, and viewing the spectrum of light cast out of a pinhole. A great gif from wikipedia on the subject shows the approximate curve which these colors represent. 


The basic wisdom is that warm red spectrums are recommended for flowering, and cool blue spectrums are recommended for vegetative growth. 

Here is a quote from a scientific paper, regarding warm and cool white:

Overall, the low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants. Initial growth under cool white LEDs should promote the growth of short, sturdy hypocotyls. Subsequent transition to warm white LEDs should promote leaf expansion. Finally, after canopy closure, cool white LEDs should be used again to prevent excessive stem elongation.


There are also "full spectrum" LEDs that emit mostly blue and red.  These are the "Chinese full spectrum" LEDs.  We generally don't recommend them, unless you need a single 3W LED per plant.  Otherwise, it will almost always be more efficient to use specifically blue or red LEDs.  They are particularly wasteful because of something called the Stokes shift, which describes the energy wasted as heat by converting blue photons to red photons.

Full spectrum light is generally good for plants.  Because it's what the Sun puts out, you really can't go wrong.  You can target red and blue, but that will miss all the accessory pigments in between. There is also a limit to how much red a plant can take, which is why many horticultural reference designs use mostly full spectrum light.  See the myth busting on Spectrum, below!


Myth busting: Wattage

The biggest myth with grow lights is the advertised wattage.  A 3W LED is rarely 3W.  It is just vernacular in the industry, to refer to any LED that has a maximum rated current of about 700mA.

According to the formula, Power = Voltage * Amperage.  A red LED has a typical voltage of 2.6V, so a safely driven LED, at 600mA will result in a wattage of 1.56W.  Meanwhile, a blue or white LED has a typical voltage of 3.6V, and thus a wattage of 2.16W.

So when a grow light advertises itself as 120W, check that it is 120 actual watts.  Typically, the seller will just multiply the number of LEDs by 3W, resulting in a value much higher than the actual power being used.


Myth busting: Lumens

Another misleading advertising trick is when manufacturers advertise their LEDs for plants using lumens.  Lumens are a measurement catered to the human visual spectrum. Lumens can be used for comparison between white lights of the same spectrum, however, in order to compare the efficiency of white lights.


Myth busting: Spectrum

Another myth is regarding green light.  There is a wide-spread belief that plants do not use green light. Many grow lights focus on blue and red, and leave out green.  The reason for this is electrical efficiency.  Blues and red LEDs are just more efficient, because of a dip in efficiency in semiconductor physics around 550nm.  As you learned earlier, blue and red are also the more important wavelengths for the "action spectrum".

But red, having the shortest wavelength of the visual spectrum, will saturate leaf surfaces.  So there is a maximum amount of red that a plant will take.  Yellow and Green have a bit more energy than Red, and so they are able to penetrate deeper into the leaf, or canopy, and excite chloroplasts lower down. 

So in low lighting conditions, red light is best (in conjunction with at least 1-10% blue), but under high lighting conditions, there is a saturation level for red light, so it is best to have some green in the light.

As mentioned, green is not an efficient colour to produce, so instead, white LEDs are used, which are just blue LEDs heating up a phosphor coating.  They typically have as much green output as a green LED. 


What other cool stuff should I know?


The Emerson effect

Emerson discovered that adding infrared to a deep red light caused greater photosynthesis than either deep-red (670nm) or far-red (700nm+) by themselves. This has in hindsight been understood as a fairly obvious consequence of the molecular dance between the Photosystem I and Photosystem II photoreceptor structures, explained earlier.  It's why you should use red with warm white.  The Warm white provides the 700nm.  It's a myth that 730nm is useful for this effect.  730nm is useful for 'putting plants to sleep', but not for the Emerson effect. 


The inverse square law

Every time you double the distance between your plant and its light source, the power it receives is quartered.  One of the benefits of LEDs is that they are relatively cool, so you can have them very close to the plants.  



Do you have any cool links to read further?

Here is a link to a comprehensive guide to plant lighting on Reddit, by the scientist, SuperAngryGuy.

A good website to compare grow lights is here.

The LED company popularised by GrowMau5 is ChilLED Tech, which has some great educational pages.



Product ideas from other sources


We examine some of the ideas from various sources below.



Heliospectra's products use the following wavelength choices with the following approximate descriptions for their products:

E60 and LX60 - their generic models

 450 nm, 660 nm, and 5700K white

 450 nm, 660 nm, 735nm, and 5700K white

RX30 - their scientific model, allowing wifi control of each wavelength

 380, 400, 420, 450, 520, 630, 660, 735nm, and 5700K white

Their research paper has some interesting points on the absorption of green light.  Apparently 50%+ of green light is actually absorbed, but it is used deeper in the leaf (the under-side), for CO2 fixation.  It is used for photosynthesis, but doesn't contribute much to growth, it seems.  This brings up an interesting question about the effects of lighting plants from below. 


Our Thoughts: They have taken two approaches here.  The first is to focus on the Chlorophyll peaks, and then use full spectrum lights to boost the remaining wavelengths.  The second approach, which is obviously focused at a scientific crowd, is to provide many different wavelengths, which can be adjusted for research purposes. I like the idea of wifi, and I like the ability to dim individual wavelengths. The simple approach of their more generic models shows that you can generally "get by" so long as you approximate the PAR graph from wikipedia.




Illumitex has trademarked Surexi, which is another range of light spectrums for various effects on plants, based on NASA's work.

You can check out their website for a better view, but here is the summary:

F1 - General purpose, high efficiency

Blue (400-499nm) 22.8%
Green (500-599nm) 0.3%
Red (600-699nm) 76.8%
Far Red (700-780nm) 0.1%


F3 - Best for germination through flowering

Blue (400-499nm) 11.0%
Green (500-599nm) 7.7%
Red (600-699nm) 81.0%
Far Red (700-780nm) 0.3%


X5 - Best for human visualisation 

Blue (400-499nm) 18%
Green (500-599nm) 17%
Red (600-699nm) 64%
Far Red (700-780nm) 0%


F6 - Best for vegetative growth

Blue (400-499nm) 49.5%
Green (500-599nm) 0.5%
Red (600-699nm) 49.9%
Far Red (700-780nm) 0.1%


F7 - Best for seedlings

Blue (400-499nm) 74.5%
Green (500-599nm) 0.6%
Red (600-699nm) 24.8%
Far Red (700-780nm) 0.1%


They then have specific wavelengths, which supplement deficiencies in other lighting systems. 


Our Thoughts: Illumitex has a wider range of spectra available, for various effects, instead of Heliospectra's more general purpose design, or their multi-spectrum design. 

Presumably a grower would invest in the whole suite of products, and then have different areas for plants in their progressive stages of life.  When a plant graduates from seedling to vegetative growth, or from vegetative growth to flowering, it could be picked up and placed under the next light. 

This is an interesting design difference, catering to different sized areas.  Illumitex probably saves a few watts by separating their spectra like this - but at the cost of requiring more human intervention, when light requirements change.

Also interesting is the low priority placed on green light.  Though Heliospectra has suggested it is absorbed, and is used, it seems you can get by without much at all.


Super Grow LED

A full spectrum approach - using nothing but a single LED wavelength, with phosphor coating, (as is the case with all the temperature style LEDs "5700K", etc.)

As you can see, their Spectrum King product takes a somewhat different stance compared to the red-blue heavy competition.  Interesting is the heavy proportion of green/yellow 550-600nm light, which many sources do not entertain as being important for plant growth.


Our Thoughts: Using these lights will probably guarantee a good result, and they claim to have good results, after 10 years of selling these products.  Because they are supplying a full spectrum, and by virtue of using LEDs, it will be more efficient than non-LED solutions.  But they are not particularly taking advantage of the absorption peaks. I would wager that lower wattage LEDs that are better distributed to the peaks, might be more efficient than the Spectrum King. 



 Black Dog LED

A long time player, Black Dog LED offers good information on their site, including temperature advice for optimal "Leaf Surface Temperature" - something to consider, since LEDs convert most of their electricity to heat.  Their products use a spectrum called the Phyto-Genesis spectrum, which they've trademarked.

They employ 15 different LED wavelengths to target the peaks of not only Chlorophyll, but also accessory pigments, like Phycocyanin and Phycoerythrin.  They use only 5W LEDs, which they say reaches lower leaves better than 3W or 1W LEDs.  Their products also have a switch to change between vegetative and flowering mode.

The spectral measurements of their PhytoMax products approximates the relative photosynthetic efficiency curve.

black dog led


Our Thoughts: This looks like a good product.  By using 15 different wavelength LEDs to target various peaks, they are offering an approximation of a full spectrum, but still taking advantage of the science to make their product more efficient. Offering the ability to switch between veg and flowering modes is convenient, and using 5W instead of 3W is an interesting change.  Many blogs recommend 3W as the 'sweet spot' of efficiency and heat.  But with a well-cooled product, increased canopy penetration may be a worthwhile benefit.


 Apollo Horticulture

Apollo is also a pioneer in grow lights.  They seem to have taken a pragmatic approach, covering the red and blue spectra, and then boosting the rest with full spectrum white, with the addition of far-red for blooming stage.

  • Spectrum of Light : Vegetated Stage - 430-475nm, 620-630nm, 660nm and White
  • Spectrum of Light: Growth & Bloom - 430-475nm, 620-630nm, 660nm, 730nm and White
  • Their COB products also have green added.
  • Light Spectrum: 440nm, 465nm, 510nm, 635nm, 660nm

    Our Thoughts: This is a sensible design, based on the red and blue peaks, boosting the remaining spectrum with white light.