Can we vary the wavelength of an LED light

[Ajender reddy]

I would like to start growing plants in an artificial simulated environment with very low cost, hence i would like to know whether if i could vary the wavelength of light using a normal led light by varying voltage or current

 

[Asymptotic]

What is your definition of "a normal led light"?

 

[Ajender reddy]

Thank you for your response...I mean white light emitting diodes(WLEDs) which is used for general household lighting purpose

 

[Guineafowl]

You can get LED grow lamps very cheaply, as they are mass produced for intensive horticulture.

 

[Ajender reddy]

Ya but i know but i want to understand their working and try myself

 

[NascentOxygen]

I would like to start growing plants in an artificial simulated environment with very low cost, hence i would like to know whether if i could vary the wavelength of light using a normal led light by varying voltage or current

I think I'm safe in saying "not to the extent that a plant would notice".

There's a slight temperature dependence, but nothing useful for you to exploit. https://www.ncbi.nlm.nih.gov/pubmed/1751282

 

[Asymptotic]

LED wavelength is a function of the materials used in their construction. So far as I know, there are two ways to obtain "white" LED lighting.

• red, green, and blue LEDs combined in the right ratio
• blue LEDs as the light source with various phosphors used to re-emit light at other wavelengths.

Varying voltage or current alters brightness, but shouldn't change wavelength very much. Do you see an appreciable change in color output when varying the control of a dimmable LED bulb?

 

[Guineafowl]

Another example: RGB colour-change LEDs have three separate chips - surely if what you ask is possible, they would have found a way to do it with one chip?

 

[Borek]

If memory serves me well LED wavelength is a function of the band gap width, this in turn is a property of material and in general doesn't depend on the voltage applied. But I can be wrong.

 

[Ajender reddy]

Thank you for your responses... if it is not possible then how does horticulture led grow lights work?

 

[Asymptotic]

Thank you for your responses... if it is not possible then how does horticulture led grow lights work?

Don't know how it would be (or if it could be) done with a blue LED/phosphor type of lamp. A tuneable spectrum RGB (red-green-blue) lamp could be achieved by individually adjusting each bank of color LEDs to produce the desired spectrum. For instance, the spectrum would shift to be more green and blue by decreasing green LED output, and/or by turning off a percentage of the green LEDs.

 

[Borek]

Thank you for your responses... if it is not possible then how does horticulture led grow lights work?

My understanding is that white LEDs don't emit light directly, they are just blue LEDs with some kind of a phosphor added (as luminescent lamps do) to change the output spectrum. Change the phosphor composition and you can get any spectrum you need.

Also: I am not convinced you can tune RGB LED to plant needs. While using them you can change the perceived color you don't change the wavelengths, you just trick eyes.

 

[Asymptotic]

Also: I am not convinced you can tune RGB LED to plant needs. While using them you can change the perceived color you don't change the wavelengths, you just trick eyes.

You are right about wavelengths not changing, but it is possible to adjust the radiant flux each wavelength contributes.

After looking through several dozen (sparse in detail) articles regarding variable spectrum LED lamps for greenhouse and hydroponics applications, and an equally detail-free spec sheet for the Osram ZELION® HL300 Grow White "LED Fully Watt Dimmable and Spectra Tunable Horticulture Fixture", found "Solid-State Solar Simulator" in 'IEEE Transactions on Industry Applications' ( Volume: 48, Issue: 4, July-Aug. 2012 ). This is behind the IEEE paywall, but the same authors are cited in a June 1, 2011 PDF article of the same name available from the http://ceme.ece.illinois.edu/files/2014/06/DD00014-000-Solid-State-Light-Simulator.pdf.

They separately controlled six different LED types - neutral white, warm white, green, cyan, blue, and UV - to vary radiant flux levels, and a combination of geometrical layout and lenses to establish an acceptably uniform distribution. It's an interesting read.

 

[Baluncore]

I would like to start growing plants in an artificial simulated environment with very low cost, hence i would like to know whether if i could vary the wavelength of light using a normal led light by varying voltage or current

The short answer is no. I am probably repeating a few things here, but understanding the situation will help.

There is no point in generating energy at wavelengths that are not needed. Plants reflect green light because they do not use it. A blue or violet LED will produce light with a wavelength that can be used by chlorophyll. Different plants have different requirements for their different chlorophylls.
https://en.wikipedia.org/wiki/Chlorophyll#Photosynthesis

We know from quantum theory that the energy of a photon is related to it's frequency by E = h ∙ f, where h is the Plank constant. We also know that frequency is the speed of light divided by wavelength, or f = c / λ. When an electron falls across a bandgap from a higher energy level to a lower energy level it releases a photon with energy proportional to the gap voltage. Here energy is most conveniently expressed in electron volts, eV. The Plank constant is h = 4.135667662×10−15 eV∙s and the speed of light is defined as c = 299,792,458. m/s. We have E = h ∙ f and f = c / λ. Therefore E = h∙c / λ. The product h∙c is 1.23984197×10−6 eV ∙ s ∙ m / s. From all that we get; V = 1239.842 / λnm. Which gives the relationship between LED wavelength and the LED forward voltage, ignoring any series resistance. Obviously, blue LEDs have higher voltages than red LEDs.

A simple light emitting diode will produce light with a specific wavelength. The chemistry of the semiconductor decides the band gap voltage and so the wavelength of the light. The Gaussian energy distribution about the centre wavelength will thermally broaden as temperature increases, but there will not be much change in centre wavelength.

LED growlights are more efficient than RGB LEDs for driving chlorophyll. An RGB combination LED that looks white is only generating blue light useful to plants, the rest of the energy will be reflected and wasted.

White household LEDs now produce a primary internal violet light. That short wavelength light excites a mixed chemical phosphor that produces secondary light with many wavelengths that appears white to a human eye. The concentration of the blended fluorescent compounds decides the colour temperature perceived by our eye. If you could remove the phosphor and use the violet LED alone, the white household LED would be similar to a growlight in efficiency when driving the chlorophyll reactions.

Remember that you have three pigments in your eye that are different to the RGB LED colours. The reflected light from the pigments they illuminate and you then look at are different again, so colour balance can get very confusing when photographed on colour film and displayed on an LED colour monitor.
Judging colours or reading resistor colour codes is now possible under the florescent household LEDs but not under RGB LEDs which usually lead to confusion of red and orange.

If you want to grow plants efficiently, then growLEDs are the way to go. You might do it cheaper by removing the possibly toxic chemical phosphors from the household white light LED product. You might also poison your nervous system more in the process of producing the light, than in consuming the vegetable product you are growing.

 

[Asymptotic]

There is no point in generating energy at wavelengths that are not needed. Plants reflect green light because they do not use it. A blue or violet LED will produce light with a wavelength that can be used by chlorophyll.

I'm on board with what you've said in post 14 but for this.

It appears red and green wavelengths both play active roles in plant growth. Wikipedia has a gloss under the 'Grow Light' topic, and I've found (and am in the process of reading) "Green Light: A Signal to Slow Down or Stop" in the Journal of Experimental Botany, Vol. 58, Issue 12. From the conclusion:

Recent findings of cry-dependent and cry-independent green photoresponses suggest that green, in addition to red, far-red, blue, and UV sensory mechanisms, monitor and adjust plant growth and development. For the most part, the recent findings mesh well with central themes from older studies performed before the advent of molecular-genetic tools and modern techniques. One theme presented throughout this review is that the effects of green light tend to reverse the processes established by red and/or blue light. In this way, green light may be functioning in a manner similar to far-red light, informing the plant of photosynthetically unfavourable conditions. Although seemingly counterintuitive at first, these conclusions make sense in the context of normal plant growth in natural settings. In terms of basic science, together these findings remind us that nature tends not to ignore a conditional environmental input and that inductive biological systems often have antagonistic systems that counter their progression. In this way plants use the full spectrum and the relative ratios of energies within to adjust their form, composition, and physiology to best exploit prevailing conditions.

Edit: BTW, I came across the above cite from an article on the HortScience website titled, "Plant Productivity in Response to LED Lighting", which also may be on interest to the OP.

 

[Ajender reddy]

The short answer is no. I am probably repeating a few things here, but understanding the situation will help.

There is no point in generating energy at wavelengths that are not needed. Plants reflect green light because they do not use it. A blue or violet LED will produce light with a wavelength that can be used by chlorophyll. Different plants have different requirements for their different chlorophylls.
https://en.wikipedia.org/wiki/Chlorophyll#Photosynthesis

We know from quantum theory that the energy of a photon is related to it's frequency by E = h ∙ f, where h is the Plank constant. We also know that frequency is the speed of light divided by wavelength, or f = c / λ. When an electron falls across a bandgap from a higher energy level to a lower energy level it releases a photon with energy proportional to the gap voltage. Here energy is most conveniently expressed in electron volts, eV. The Plank constant is h = 4.135667662×10−15 eV∙s and the speed of light is defined as c = 299,792,458. m/s. We have E = h ∙ f and f = c / λ. Therefore E = h∙c / λ. The product h∙c is 1.23984197×10−6 eV ∙ s ∙ m / s. From all that we get; V = 1239.842 / λnm. Which gives the relationship between LED wavelength and the LED forward voltage, ignoring any series resistance. Obviously, blue LEDs have higher voltages than red LEDs.

A simple light emitting diode will produce light with a specific wavelength. The chemistry of the semiconductor decides the band gap voltage and so the wavelength of the light. The Gaussian energy distribution about the centre wavelength will thermally broaden as temperature increases, but there will not be much change in centre wavelength.

LED growlights are more efficient than RGB LEDs for driving chlorophyll. An RGB combination LED that looks white is only generating blue light useful to plants, the rest of the energy will be reflected and wasted.

White household LEDs now produce a primary internal violet light. That short wavelength light excites a mixed chemical phosphor that produces secondary light with many wavelengths that appears white to a human eye. The concentration of the blended fluorescent compounds decides the colour temperature perceived by our eye. If you could remove the phosphor and use the violet LED alone, the white household LED would be similar to a growlight in efficiency when driving the chlorophyll reactions.

Remember that you have three pigments in your eye that are different to the RGB LED colours. The reflected light from the pigments they illuminate and you then look at are different again, so colour balance can get very confusing when photographed on colour film and displayed on an LED colour monitor.
Judging colours or reading resistor colour codes is now possible under the florescent household LEDs but not under RGB LEDs which usually lead to confusion of red and orange.

If you want to grow plants efficiently, then growLEDs are the way to go. You might do it cheaper by removing the possibly toxic chemical phosphors from the household white light LED product. You might also poison your nervous system more in the process of producing the light, than in consuming the vegetable product you are growing.

Thank you everyone... after reading a series of articles i noticed that different plants need different proportions of RGB and it is also different in growth stages of plants... so may be i need to use three different red,green,blue LEDs for plants and try to produce the light as per characteristics of the plant i choose or is it better to use a voilet light as you have suggested?

 

[OmCheeto]

Thank you everyone... after reading a series of articles i noticed that different plants need different proportions of RGB and it is also different in growth stages of plants... so may be i need to use three different red,green,blue LEDs for plants and try to produce the light as per characteristics of the plant i choose or is it better to use a voilet light as you have suggested?

According to the University of Michigan: "...appropriate color ratios, lighting periods, and lighting intervals have yet to be determined." [ref: © Copyright 2011, Michigan State University. All Rights Reserved.]

It's my guess, that this is an "emerging" technology/science, and we will be learning much about it, in the years to come.

Not willing to wait for a conclusion, I decided to purchase a 5 meter long string of LED grow lights, with a lamp color ratio of 4 red: 1 blue, as trying to make sense of biology, with zero knowledge of biology, was making my head hurt.

My suggestion would be to do your own experiments, with different plants, under differing lighting conditions.
That's what I'm doing.
So far, none of my plants have died.
And the lemon tree... Ehr mehr gerd... It now has WAY too many blossoms.

ps. Obligatory graph:

and some photos, of how excited I am about science:

note the date and time stamp

 

[OmCheeto]

ps. @Andy Resnick , any ideas on how to interpolate camera bloom to light intensity?

note black "galactic shaped" outlines.

#ScienceIsHardWhenYoureAsStupidAsOm:(

 

[Baluncore]

And the lemon tree... Ehr mehr gerd... It now has WAY too many blossoms.

An over-active lemon tree is a sign of too much nitrogen in the growth medium and a lack of wind. Has someone been introducing too much uric acid into the mix?

 

[OmCheeto]

An over-active lemon tree is a sign of too much nitrogen in the growth medium and a lack of wind. Has someone been introducing too much uric acid into the mix?

Well, it's not very windy in my living room, and I water my tree with my fish tank water, so I'll blame the fish.

hmmmm...
Google is not helping at all with this.

Gout? What the hell is Gout?
Sounds like something my great-great-grandmother would have suffered from...

 

[Ajender reddy]

According to the University of Michigan: "...appropriate color ratios, lighting periods, and lighting intervals have yet to be determined." [ref: © Copyright 2011, Michigan State University. All Rights Reserved.]

It's my guess, that this is an "emerging" technology/science, and we will be learning much about it, in the years to come.

Not willing to wait for a conclusion, I decided to purchase a 5 meter long string of LED grow lights, with a lamp color ratio of 4 red: 1 blue, as trying to make sense of biology, with zero knowledge of biology, was making my head hurt.

My suggestion would be to do your own experiments, with different plants, under differing lighting conditions.
That's what I'm doing.
So far, none of my plants have died.
And the lemon tree... Ehr mehr gerd... It now has WAY too many blossoms.

ps. Obligatory graph:

View attachment 216706

and some photos, of how excited I am about science:

View attachment 216709
note the date and time stamp

View attachment 216710

Thank you @Om Cheeto i also aim to same kind of experiments including fish and grow lights...you told that you have conducted some kind experiments with diffetent plants can you share something more regarding this experience

 

[Andy Resnick]

ps. @Andy Resnick , any ideas on how to interpolate camera bloom to light intensity?

#ScienceIsHardWhenYoureAsStupidAsOm:(

Not entirely sure what you mean, but if there's blooming/clipping, you've lost the ability to correlate with received intensity.

Like the hashtag!

 

[OmCheeto]

Thank you @Om Cheeto i also aim to same kind of experiments including fish and grow lights...you told that you have conducted some kind experiments with diffetent plants can you share something more regarding this experience

If you look at the time stamp in my two images, you'll see that I've only been doing this for about 6 weeks. So there's not really much to share so far, other than my lemon tree sprouting 50 blossoms after I put it under the light. And without a control tree, we don't really know if it was the lights that caused it. The tree was on my front porch on October 29th, and was brought inside on November 1st. So it may have been the increase in temperature that made the tree think; "Wow! It got warm all of a sudden. It must be Spring! Time to make babies."

The other plants I'm growing are:
3 year old sage plant
2 x 1 year old Thai hot pepper plant
3 year old Thai hot pepper plant (looks dead, but teeny tiny leaves are starting to grow)

I was going to say that the "Thai hots" are not a good indicator, as I thought they were annual plants, but one website says

Most people are not aware that peppers are actually perennials! The reason they are usually grown as annuals is because the winter temperatures in most places will kill them. They are semi-tropical plants, and just cannot handle cold weather. However, it is possible to keep a pepper plant alive for several years, via a technique known as "overwintering". [ref]​

I did not know that.

In any event, I believe your original question was answered:

i would like to know whether if i could vary the wavelength of light using a normal led light by varying voltage or current

Answer: no

And in answer to your second question:

how does horticulture led grow lights work?

Answer: Quite well, IMHO. They mimic the appropriate spectral range of sunlight.

As to your third question:

...is it better to use a voilet light as you have suggested?

I disagree with baluncore's statement:

If you could remove the phosphor and use the violet LED alone, the white household LED would be similar to a growlight in efficiency when driving the chlorophyll reactions.

Violet light, according to my records, has a wavelength between 380 and 450 nm, and only covers the absorption range of Chlorophyll a.
But, as I mentioned earlier, I have zero knowledge of biology, so I may be misinterpreting his statement.

I'm also confused about the "bandwidth" statement, that at least a couple of people have mentioned.
Both my red and blue LED lamps appear to be somewhat "broadband". I will have to do further research on why that is.

Here's an experiment I did last year:

Not knowing what my unknown "bright white" light was, I did some more searching.

Same day, different unknown "bright white" bulb:

From todays experiment, I've decide the first unknown bulb was a white LED, and the second, was a CFL(compact coiled fluorescent).

Not entirely sure what you mean, but if there's blooming/clipping, you've lost the ability to correlate with received intensity.

That's ok. Lying in bed this morning, I decided it was a silly question, and decided on a course of action. Science!

 

[Ajender reddy]

If you look at the time stamp in my two images, you'll see that I've only been doing this for about 6 weeks. So there's not really much to share so far, other than my lemon tree sprouting 50 blossoms after I put it under the light. And without a control tree, we don't really know if it was the lights that caused it. The tree was on my front porch on October 29th, and was brought inside on November 1st. So it may have been the increase in temperature that made the tree think; "Wow! It got warm all of a sudden. It must be Spring! Time to make babies."

The other plants I'm growing are:
3 year old sage plant
2 x 1 year old Thai hot pepper plant
3 year old Thai hot pepper plant (looks dead, but teeny tiny leaves are starting to grow)

I was going to say that the "Thai hots" are not a good indicator, as I thought they were annual plants, but one website says

Most people are not aware that peppers are actually perennials! The reason they are usually grown as annuals is because the winter temperatures in most places will kill them. They are semi-tropical plants, and just cannot handle cold weather. However, it is possible to keep a pepper plant alive for several years, via a technique known as "overwintering". [ref]​

I did not know that.

In any event, I believe your original question was answered:
Answer: no

And in answer to your second question:
Answer: Quite well, IMHO. They mimic the appropriate spectral range of sunlight.

As to your third question:
I disagree with baluncore's statement:
Violet light, according to my records, has a wavelength between 380 and 450 nm, and only covers the absorption range of Chlorophyll a.
But, as I mentioned earlier, I have zero knowledge of biology, so I may be misinterpreting his statement.

I'm also confused about the "bandwidth" statement, that at least a couple of people have mentioned.
Both my red and blue LED lamps appear to be somewhat "broadband". I will have to do further research on why that is.

Here's an experiment I did last year:
View attachment 216794

Not knowing what my unknown "bright white" light was, I did some more searching.

Same day, different unknown "bright white" bulb:

View attachment 216796

From todays experiment, I've decide the first unknown bulb was a white LED, and the second, was a CFL(compact coiled fluorescent).

View attachment 216800That's ok. Lying in bed this morning, I decided it was a silly question, and decided on a course of action. Science!

@Om Cheeto...Thank you so much...is it true that we can increase plant productivity using grow lights than natural light

 

[OmCheeto]

@Om Cheeto...Thank you so much...is it true that we can increase plant productivity using grow lights than natural light

Well, if you live in a place like I do, that gets insufficient natural light during the fall, winter, and spring for growing anything other than mushrooms, then grow lights can increase your harvest 3 times. If natural light is available, it's somewhat silly to use grow lights, as sunlight is free. Grow lights, and the electricity to run them, are not.