Heterodyning light ...

[Externet]

Hello.

Blue plus red emitters are used as grow light.  The resulting hue is the sum of red+blue.  Or is it the difference ?  If the sum; what color is the difference of their wavelengths ? 

 

And what is the intended meaning for 'full spectrum' ?   (Image borrowed from the web)

[Phi for All]

"Full spectrum" is a bit of a lie, it's also called "broad spectrum". Ideally, plants get their light from the sun, and different parts of the spectrum are used at different times. What works best for growth isn't the best for flowering or fruiting, and there are helpful parts of the spectrum that aren't helpful during photosynthesis. The LED growth lights are aiming for light at wavelengths of 400-700nm, the photosynthetically active radiation range. They appear as a sort of fuchsia color rather than the white you'd see if it really was "full spectrum".

I don't know if the industry has changed much, but I used to tell customers to buy LEDs that are high on the Color Rendering Index (CRI) if they want light that's more like natural sunlight. They were more expensive, since they're also used for better visual effects as well (you know exactly how colors on fabric or paint will look outdoors). Lights with a CRI of 95+ should work well with plants (sunlight is 100 on the CRI scale, and I think they have LEDs now with a 99 CRI). I think the key is the intensity of light. You need to bring a LOT of photons to the plants in order to mimic natural conditions.

[swansont]

20 hours ago, Externet said:

Blue plus red emitters are used as grow light.  The resulting hue is the sum of red+blue.  Or is it the difference ?  If the sum; what color is the difference of their wavelengths ? 

When it comes to the response of the eye, optical effects are additive.

https://www.xrite.com/blog/additive-subtractive-color-models

 

There are ways of getting the sum frequency or difference frequency, using nonlinear materials. In general it's called four-wave mixing.

https://en.wikipedia.org/wiki/Four-wave_mixing#Sum_and_Difference_frequency_generation

But this isn't happening with these lights

 

[Sensei]

22 hours ago, Externet said:

Blue plus red emitters are used as grow light. 

..to save the money and energy needed for mostly unused green LEDs..

 

22 hours ago, Externet said:

The resulting hue is the sum of red+blue. 

Illusion..

Like with Newton disc, but with just two colors.

https://en.wikipedia.org/wiki/Newton_disc

Edited May 10, 2022 by Sensei

[John Cuthber]

On 5/9/2022 at 4:55 PM, Phi for All said:

Lights with a CRI of 95+ should work well with plants (sunlight is 100 on the CRI scale, and I think they have LEDs now with a 99 CRI). I think the key is the intensity of light. You need to bring a LOT of photons to the plants in order to mimic natural conditions.

Plats are, in general, green.
They reflect green light so they can't absorb it.

So any green light emitted by your LEDs is wasted on the plants.
As you say, you need a lot of light to get plants to grow well.

But there's no point shining green light on them.

So it makes sense to produce lights that only have the blue and red bits of the spectrum.

People (generally) have three different colour sensors so colour rendering for people is different to efficient illumination of plants.

[StringJunky]

Quote

LET’S BE CLEAR: GREEN LIGHT IS USEFUL
TO PLANTS

By definition, the waveband for photosynthetically active radiation (PAR) is 400 to 700 nm. In the middle of this waveband is green light, which has a wavelength between 500 and 600 nm. If one-third of PAR was not useful in photosynthesis, why would green light be included in the definition? The main reason why green light is purportedly not useful to plants is because it is poorly absorbed by chlorophyll. However, absorption of chlorophyll is usually measured using extracted and purified chlorophyll, in a test tube (in vitro), and not using an intact leaf (in vivo).

There are a few problems making conclusions based on in vitro measurements. First, there are pigments other than chlorophyll that absorb light and make it useful for photosynthesis. These “accessory
pigments” have different absorption spectra, and some of them absorb green light. Second, the absorption of chlorophylls (and other pigments) can depend on the solvent used for extraction. Therefore, in this case, interpretations made from measurements in a test tube do not apply well to whole plants.

Another explanation commonly used to support the green light myth is that plants reflect green light, which is why they appear green. While that’s true, it is usually exaggerated. When light strikes a leaf surface, it can be absorbed (and potentially used for photosynthesis), reflected off the leaf, or transmitted through the leaf. Most plants appear green because their leaves reflect more green light than red or blue light. However, most (e.g., 85%) green light is absorbed, and only small
percentages of green light are reflected or transmitted. The green light that is not absorbed is not lost; it can be reflected to other nearby leaves or transmitted to leaves below.   https://gpnmag.com/article/is-green-light-useful-to-plants/

 

[John Cuthber]

Anyway, lets be a bit more clear.
The action spectrum for photosynthesis has been measured.
https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/29-photosynthesis/action-spectrum.html

There's a big trough for green light- which is, at best, poorly absorbed (even with the help of the carotenoids).
So it makes sense to focus your efforts into making red + blue light.
The most likely fate of a green photon near the bottom of the graph is that it will be wasted as heat.

If you were making coloured light by starting with white light, and filtering it there would be no point (except that taking out the IR might prevent overheating).
But with LED lighting, you can selectively generate the colours that work best.

Also, none of this has anything to do with heterodyning light.

[studiot]

1 hour ago, John Cuthber said:

Also, none of this has anything to do with heterodyning light.

This is a key statement.

Neither the sum nor the difference frequencies/wavelengths resulting from a heterodyining process will be anywhere near the visible spectrum.
This is unlike radio waves where the results may still be in the radio spectrum.
Although even there we can get audible 'heterodyne whistles'.

[studiot]

Adding some figures

 

Visible light lies within the range 400 THz  to 800 THz

A typical red LED has a (peak) frequency of 474 THz and blue has 669 THz

 

This makes the sum frequency 1183 THz  ie > 800THz and the difference frequency  195 THz  ie < 400 THz.

[Externet]

Interesting how the figures nearly coincide... ~400 to 800 THz and ~400 to 800 nm. 

[swansont]

43 minutes ago, Externet said:

Interesting how the figures nearly coincide... ~400 to 800 THz and ~400 to 800 nm. 

The numbers are rounded by a fair bit. 400 nm is actually 750 THz and 800 nm is 375 THz.

[studiot]

From this paper

A flow reactor setup for photochemistry of biphasic gas/liquid reactions
Article in Beilstein Journal of Organic Chemistry · August 2016
DOI: 10.3762/bjoc.12.170

3 authors, including:
Axel Jacobi von Wangelin
University of Hamburg

 

[John Cuthber]

The range of frequencies we can see is (just) less than an octave.
We could possibly just about see 400 THz. 401THz and 801THz.

The closest thing to relevant that I can think of is that both getting the sum of two frequencies, and the photochemistry of photosynthesis rely on 2 photons.

A single  visible photon doesn't have enough energy to split water.