I Is This....Right? Splitting LED colors is not working for me

[Newtons Apple]

Hi, So I've been really getting into color and light and I have a big question that I'm not sure if i'm thinking about correctly..but it could proven...problematic?

Okay so please stay with me here as I build up to the end, I'll be stating things that I believe to be true but if anything alongthe way isn't correct please let me know.

Background:
I have a bunch of LED grow lights, most cheap ones off Amazon. They're labeled as "Full Spectrum" And all of them have a "white" setting.

I know there's a few different ways to produce white in LED. One such method is to use a base Blue LED and put some type of phosphor material on top of it, and it then will produce a white. This method of making white is cheaper than the method that would combine a Red, Blue and Green LED to make a single color white.

With that in mind, many of these cheap grow lights use method 1 of just taking a blue LED and converting it to White.

Okay so.. Plants use a wide range of colors for their functions, Such s blue/violet is used for producing roots, red/orange helps flowering and green is more for chorlphyll production (but most of the green is reflected). Hence why most in the botany field will always say Lights that are "full spectrum" and do better with natural sunlight since it provides these colors.

Now with all of that down... I bought a glass prism that you can shine light through and separate out the colors like so much Pink Floyd. That's where I'm now seeing an issue and I think you know where I'm going wth this. I hold up the prism to LED's and there's no rainbow, no separation of the spectrum.... If I shine the LED red I just see red..which is expected...if I shine blue, I just see blue, again expected.... But when I shine white, I expect to see the rainbow of colors...but it's JUST white.

So what does this mean? Does this mean the white in these LED's isn't really the same white as the sun's ray's? So you an have a white light...that's not actually make up of the rainbow? Even though our eyes see both whites as the same?

And lastly the biggest question I have....aren't these grow lights usless then?? They don't actually provide a full spectrum of white...only the 'empty' white of just blue+phosphor...

 

[DaveC426913]

But when I shine white, I expect to see the rainbow of colors...but it's JUST white.

I ... might be off my rocker but ...

It seems to me it's your prism that's broken.

There is no such thing as white light. It must be made up of a combinaton of colours. If you are not seeing a spectrum - regardless of the source, and regardless of whether its discrete or continuous - that's gotta mean your prism is not working.

And that can't be right. A prism can't not work.

It would require either

1. a prism that does not refract light (has the same index as air), or
2. you are using it wrong (which means you haven't really looked at the cover of PF:DSotM).
   
   Or, to be thoroughly exhaustive:
3. you don't know what a prism is, and are, in fact, holding up a transparent cuboid.

My best guess is that the rainbow is there, but too faintly delineated to make out (whether because of poor viewing conditions or because of poor quality) without careful observation.

Google suggests: "Why a prism might not be working:

• Bright Sunlight: Prisms work by splitting white light into its component colors (the rainbow). You need a strong, focused light source like direct sunlight to see this effect.
• Avoid Dim or Indirect Light: If the light is too weak or diffused, the colors won't be visible."

Can you post pics?

 

[DaveC426913]

Google suggests why your prism might not be working and what you can do:

1. Light Source:

• Bright Sunlight: Prisms work by splitting white light into its component colors (the rainbow). You need a strong, focused light source like direct sunlight to see this effect.
• Avoid Dim or Indirect Light: If the light is too weak or diffused, the colors won't be visible.
  

2. Prism Type and Quality:

• Refractive Prism:
  Ensure you have a basic, refractive prism, also known as a dispersive prism, which is designed to bend light and separate it based on wavelength.
• Check for Damage:
  Make sure the prism isn't cracked, scratched, or dirty, as this can affect its ability to refract light properly.
  

3. Positioning and Angle:

• Sunny Window or Outside: Place the prism by a sunny window or outside on a sunny day.
• Aim Correctly: Aim the prism at a white wall or white paper to see the rainbow more clearly.
• Adjust the Angle: Experiment with different angles to find the optimal position for the rainbow to appear.

 

[Newtons Apple]

Thanks for the replies, let's set my personal experiment aside for a moment...

This is according to google:

• RGB LEDs: These use red, green, and blue LEDs combined to create the perception of white light. Passing this through a prism will likely show the separated red, green, and blue components, not a full rainbow.
  
  
• Fluorescent/Phosphor LEDs: These use a phosphor that emits light when struck by the LED's light. The spectrum of the emitted light depends on the phosphor, and passing this through a prism will show the colors specific to that phosphor, not necessarily a full rainbow.

For the 2nd case, using Phosphors, it says it will show the colors specific to the phosphor (along with the original led color light I'd think) And that's what I'm seeing, on my 'white' LED I just see a grayish yellow

 

[Newtons Apple]

It's actually hard to photograph, but this is what I'm using, and you can see the tiny band of yellowish on the far side, but this is from a 'white' led.

 

[DaveE]

I think you need collimated light for the prism to give separation of the colors. The prism will bend colors at different angles, but if there are many incident angles, the end result will be smeared out. You could try putting a narrow slit in front of the LED. This will block all "off-axis" radiation from your light source.

 

[renormalize]

I think you need collimated light for the prism to give separation of the colors. The prism will bend colors at different angles, but if there are many incident angles, the end result will be smeared out. You could try putting a narrow slit in front of the LED. This will block all "off-axis" radiation from your light source.

Yes @Newtons Apple, as @DaveE suggests, the light should be made more parallel by passing it through a slit, and further improvement is possible by using a lens to produce a focused image:

 

[DaveC426913]

Right Of course. A narrow slit is usually required.

 

[Newtons Apple]

Ahh maybe that's it...I did use sort of a focused light, but it wasn't *that* narrow...Thanks guys will give it a try!

 

[SammyS]

Ahh maybe that's it...I did use sort of a focused light, but it wasn't *that* narrow...Thanks guys will give it a try!

It's a good idea, and probably necessary, to project the light onto a white screen of some kind.

By the way, what's the status of that inexpensive spectrometer which you used to show spectra in two of your recent threads?

 

[Newtons Apple]

It's a good idea, and probably necessary, to project the light onto a white screen of some kind.

By the way, what's the status of that inexpensive spectrometer which you used to show spectra in two of your recent threads?

Haha, it's funny you mention that.....I'm working on a big paper (not academic just for fun) And it's hilarious...First I thought that the spectrometer was just busted and inaccurate..so I bought another one more expensive that when I measured the sun a full spectrum as what many suggested I should be seeing.. SO I thought that was it, it was just a fault spectrometer... However a few days ago I was looking at other people's readings of LED lights..and lo and behold I saw the SAME spike in blue light that the original one produced. So Now I really don't know.

My non-scientific intuition is this...
That these grows lights use blue as the base LED color and have a yellowish phosphor on top. So when the "white" light comes out and when I measure it, it's picking up way more blue. Hence the spike in that range.

To my very plebian brain this makes sense. It also would make sense that in this experiment that I'm trying, that for LED created light, there can't be a full rainbow? The way that 'white' is created will never be able to have a full spectrum of color... but apparently this isn't accurate from everyone's response, but I'm not seeing how it would be possible.

 

[Newtons Apple]

Right Of course. A narrow slit is usually required.

Hi,
So the LED that's I'm using for this experiment...they it has a plastic or glass tube around all of the led chips...will this throw off the dispersion when it hits the prism?

 

[DaveC426913]

Hi,
So the LED that's I'm using for this experiment...they it has a plastic or glass tube around all of the led chips...will this throw off the dispersion when it hits the prism?

I don't know what this "tube" you mention is. You could always take a pic of the LED itself.

Normally, I'd think LED lights would want their light dispersed, so I suspect they'd have some sort of dispersal lens. That could be what that tube is. I don't know.

If the light is sufficiently bright, you could still just use a slit (in a darkened room), but if not, you could remove the (hypothesized) dispersal lens to get better directionality.

 

[pbuk]

I have a bunch of LED grow lights, most cheap ones off Amazon

....aren't these grow lights usless then??

Maybe, but that doesn't mean that all grow lights are.

Rather than spend time and money on testing cheap unidentified stuff you could spend a bit more money acquiring products from a known source backed up by full data sheets such as these Samsung devices: https://led.samsung.com/lighting/applications/horticulture-lighting/

(But then you wouldn't have the fun of experimenting of course).

 

[sophiecentaur]

Prisms work by splitting white light into its component colors

To avoid serious egg on face on some occasions, it's much safer (and correct) to talk in terms of wavelengths. A spectrometer / prism / diffraction grating does not have human colour vision. It knows nothing about 'colour'. It just tells you the relative levels of light of different wavelengths.

The way that 'white' is created will never be able to have a full spectrum of color.

Here's another example of confusing use of terms. Colour only comes into it when the light enters your eye. This was all sorted out very soon after Newton's work and is nothing new.

The confusion occurs all over the place and many otherwise bright but badly informed people use these terms inappropriately. Sorry to be so picky but why not use the right terms?

 

[DaveC426913]

To avoid serious egg on face on some occasions, it's much safer (and correct) to talk in terms of wavelengths

Fair enough, although in this case, it really is colours that the OP is concerned with - since he is observing them. Or not, as it were.

 

[sophiecentaur]

Fair enough, although in this case, it really is colours that the OP is concerned with - since he is observing them. Or not, as it were.

Colour is only an illusion. You can get very little reliable evidence of the Science of spectronomy from colour. The fact that you 'see' a colour emerge from a prism doesn't mean that you can be certain about the wavelengths involved. I would say that is "not Physics". We can 'see' yellow (looks like a Sodium doublet) when presented with an appropriate mix of spectral red and spectral green and where there's a gap where the doublet would be.

As with all proper science, we try to eliminate the subjective and concentrate on the objective. Wavelength is objective. Using the word 'colour' may seem to be approachable and friendly but is a distraction. "The observer" should be helped away from natural intuition.

 

[DaveC426913]

Colour is only an illusion. You can get very little reliable evidence of the Science of spectronomy from colour.

Exactly what the OP is struggling with. Because they are looking for colours. They dont have access to a spectrometer, which would plainly show all the wavelengths.

 

[tech99]

You can make an easy spectroscope using a compact disc.
I suspect the geometry of the prism is wrong. Check the ray diagram required to give a spectrum. Also, if you look through the prism at the correct angle, objects should have coloured edges.
In addition, the prism should be glass as plastic ones do not work very well.

 

[sophiecentaur]

You can make an easy spectroscope using a compact disc.

Whatever method you use to cause the light to 'bend' by different amounts for different wavelengths you need to use a narrow slit if you want a better effect than just coloured edges to a patch of light. A narrow slit will not let much light through so the better quality spectrum you want, the dimmer will be the image. To improve on this, you need some proper 'optics' and a bright source OR you need to have very low level lighting in your 'lab'.
You could imagine Sir Isaak spending hours in a darkened room to get his eyes dark adapted to help him understand the topic.

 

[olivermsun]

The fact that you 'see' a colour emerge from a prism doesn't mean that you can be certain about the wavelengths involved. I would say that is "not Physics". We can 'see' yellow (looks like a Sodium doublet) when presented with an appropriate mix of spectral red and spectral green and where there's a gap where the doublet would be.

If you see yellow going into a prism and yellow coming, then that should be distinct from yellow going in and red and green coming out.

 

[DaveC426913]

If you see yellow going into a prism and yellow coming, then that should be distinct from yellow going in and red and green coming out.

True. Spectral yellow is around 580nm.

 

[sophiecentaur]

If you see yellow going into a prism and yellow coming, then that should be distinct from yellow going in and red and green coming out.

What is your point? A spectrograph is nothing like human vision so - surprise surprise- the results are only an approximate match. As with most of Science, our subjective experience doesn't follow the 'laws'. We took centuries to learn to distinguish between the two.

 

[sophiecentaur]

I bought a cheap jeweller's spectroscope off eBay. This link may not last long but give it a try. It will show the difference between different coloured and 'white' sources and cost me less than £30.
It shows the absorption bands of sunlight arriving through the atmosphere too. Worth every penny. LED spectrum is more uniform than sunlight!!

 

[olivermsun]

What is your point? A spectrograph is nothing like human vision so - surprise surprise- the results are only an approximate match. As with most of Science, our subjective experience doesn't follow the 'laws'. We took centuries to learn to distinguish between the two.

My point is that once light has been split by a prism, you do gain information about the wavelengths involved, contrary to what you said above.

 

[sophiecentaur]

once light has been split by a prism, you do gain information

I have to agree with your comment. When I re-read that post of mine it was confusing to say the least. The spectrometer gives you extra information.

 

[DaveE]

https://www.instructables.com/DIY-Low-Cost-Spectrometer/

 

[Newtons Apple]

Whatever method you use to cause the light to 'bend' by different amounts for different wavelengths you need to use a narrow slit if you want a better effect than just coloured edges to a patch of light. A narrow slit will not let much light through so the better quality spectrum you want, the dimmer will be the image. To improve on this, you need some proper 'optics' and a bright source OR you need to have very low level lighting in your 'lab'.
You could imagine Sir Isaak spending hours in a darkened room to get his eyes dark adapted to help him understand the topic.

Thanks! I had to do this multiple times, but I think I finally got a good baseline result.

So this is from a "Full spectrum" LED light. I confirmed that use the Blue Light with Phosphor Coating to produce their white LEDs. I bought some black poster board and cut out a pretty narrow slit on it and got the prism to disperse it. This is a clear as I could get after multiple attempts. It's definitely a rainbow..but it's missing green completely it looks like, and no real orange:

What's now tickling my curiousity...is just how can blue + Phosphor even create *this* much variation in color...

So I suppose this satisfies me. It's not a complete, continous spectrum that you'd see from the sun, so calling it 'full spectrum' may be some marketing over selling I'd think...but it *is* there

 

[DaveC426913]

I bought some black poster board and cut out a pretty narrow slit on it and got the prism to disperse it.

Is that ... tissue paper?

You'd great lot better result with something hard and glossy, reflective white.

 

[DaveE]

just how can blue + Phosphor even create *this* much variation in color

The "phosphor" used could be more than one element/molecule. It may also be a molecule that has many energy levels so that once excited it can have different energy state transitions. You've been seeing this your whole life in old school fluorescent lamps. If you go to the hardware store you may still see many choices: grow lights, cool white, warm, etc. These are mostly all just different phosphors.

There's a table here to demonstrate the wide range of choices.
https://en.wikipedia.org/wiki/Phosphor#:~:text=backlights and lighting.-,White LEDs,of its output in yellow.

 

[sophiecentaur]

how can blue + Phosphor

Afaik many LEDs use UV to start with and a number of different wide band phosphors. The UV photons are high energy (E=hf) so there's a lot of choice of compounds which will re-emit optical bands.

I have a number of friends who do astrospectronomy (very faint sources) and, rather than a slit, they use a well focused image of the star of interest. The optics are much easier to achieve with spherical lenses and collimation is much easier with an off the shelf lens.

 

[Reticulating]

I ... might be off my rocker but ...

It seems to me it's your prism that's broken.

There is no such thing as white light. It must be made up of a combinaton of colours. If you are not seeing a spectrum - regardless of the source, and regardless of whether its discrete or continuous - that's gotta mean your prism is not working.

And that can't be right. A prism can't not work.

It would require either

1. a prism that does not refract light (has the same index as air), or
2. you are using it wrong (which means you haven't really looked at the cover of PF:DSotM).
   
   Or, to be thoroughly exhaustive:
3. you don't know what a prism is, and are, in fact, holding up a transparent cuboid.

My best guess is that the rainbow is there, but too faintly delineated to make out (whether because of poor viewing conditions or because of poor quality) without careful observation.

Google suggests: "Why a prism might not be working:

• Bright Sunlight: Prisms work by splitting white light into its component colors (the rainbow). You need a strong, focused light source like direct sunlight to see this effect.
• Avoid Dim or Indirect Light: If the light is too weak or diffused, the colors won't be visible."

This is incorrect.
To say White light doesn't exist you can say RGB light doesn't exist. In so much as you'd rank senses by the rods they require, none of the rods in general have unique area without sensitive overlap, so they are all at least 2 cones, colors need 0, 1, 2, 3 or 4 cones.

Moving past that, the reason it is incorrect is the user clearly says they expect the light to have a lot of inputs, but they are seeing a few outputs. This means the White light is not broad spectrum, White != White just as Red != Red.

You can have White light made from 3 lasers (or 3 LEDs aka phone screen) or from an incandescent tungsten filament which gives off a continuous spectrum
so use the Sun or an old lightbulb to see a broad spectrum from a prism, and use RGB lasers to ... see RGB lasers.

 

[DaveC426913]

This is incorrect.
To say White light doesn't exist you can say RGB light doesn't exist.

It feels like you are taking my sentence out-of context for the sake of argumentation.

If you read my post, you see that it is plain I mean there is no light whose frequency corresponds to "white".

As I go on to say, white light "must be made up of a combination of colours frequencies."


And indeed, you say the same thing:

You can have White light made from 3 lasers ...

Operative term being "made from".

 

[sophiecentaur]

You can have White light made from 3 lasers (or 3 LEDs aka phone screen) or from an incandescent tungsten filament which gives off a continuous spectrum

Three lasers is a sledge hammer to crack a walnut. Lasers are fun and can be very useful but don't reach for a laser when a perectly good LED or filament will do the job. You'll get a brighter image for the same input power as a laser.

Operative term being "made from".

A better way to put it would be 'can produce a light that looks white'.

To say White light doesn't exist you can say RGB light doesn't exist. In so much as you'd rank senses by the rods they require, none of the rods in general have unique area without sensitive overlap, so they are all at least 2 cones, colors need 0, 1, 2, 3 or 4 cones.

You seem to be confusing the functions of rods and cones. The rods are sensitive to all visible frequencies and (for most of us) the perception of colour involves three types of cone.

To analyse the spectrum of light, you need to have a small (or narrow) source. Without that you will get a blurred image with reds one side and blues on the other side. In the middle, the reds greens and blues from the original source which will produce a white, except at each end. The width of the 'slot' will determine the resolution of the spectrum produced.

 

[DaveE]

You'll get a brighter image for the same input power as a laser.

Perhaps semantic nitpicking, but...
This isn't true. Nothing beats the brightness of a laser. The higher beam quality allows them to be focused to a smaller spot size. Lasers always win in the "energy into a small bucket" contests.

But, your general point is valid I think (with the exception of big projection systems). Especially if you care about cost.

 

[sophiecentaur]

Nothing beats the brightness of a laser.

A laser has its applications, of course but 'Lasing' is not a particularly efficient technique and not an efficient way if you don't actually need a narrow beam, a well defined frequency and good coherence. People often suggest using a laser because they sort of assume it will be somehow 'better' for an everyday experiment. I can't think of such an application - except for, perhaps a cheap Laser Pointer which gives a fair beam shape and lasts well. I have a green laser which is only borderline legal and it's good for pointing to astronomical objects. But the battery doesn't last long so, as a pointer it's brill but not as a source of green light, IMO. Try to imagine a 4K TV screen full of lasers????

 

[BVirtual]

Congrats on getting a better experimental set up. That is what an experimentalist does. It's struggle, and you are well on your way to mastering it with your perseverance. Also, they team up as more minds make for shorter time frames for a working solution. One person can not know it all. Thus, websites like this one.

From your spectrum picture a suggestion was made to use white paper, which I agree. Also, I noticed a lot of stray light in the picture, which a fully dark room would not have. A dark room would increase your eyeball's ability to see the green, that I seem to see. I would not expect each frequency to have the same intensity, so color brightness will vary, and orange might never be seen.

I did the same experiment you did. With many LED sources, expecting to find 3 lines, red, green and blue. With single color LEDs that is what I did find. With full spectrum, natural, and sunlight LEDs I found a spread of colors like you. I was amazed in I was expected 3 lines, RGB. So, reading up, they use coatings inside the LED or just outside it. These coatings are designed to create 'natural sunlight' LIKE spectrum. That is, not 3 lines, but a "good" spread, and not similar in intensities to the Sun. Unless it is explicitly labelled as a "grow" light, and then I saw red and blue were greater in intensity than the other colors.

I have yet to set up my military grade prisms, removed from a tank periscope, that I picked up at a garage sale for real cheap. My plan is spread the spectrum twice, and aimed at a flat plastic white sheet (whiteboard quality) over 5 feet away. While at the optimal prism angles I will get less than 1/4th the full spectrum, I will have to repeat with different incident angles to get pictures of the full spectrum. I hope to get integer resolution. Or at 10 feet away get half integer, if the intensity of the original light is bright enough. I have reason to think it will be. And then compare my DIY results with a $300 USD mini spectrometer. Expectations I will need to have several spectrometers, to get into the far IR and far UV, not just visible light. As well as a terahertz to microwave spectrum device. And X-Ray and Gamma Ray as well. What fun!!!

 

[sophiecentaur]

The higher beam quality allows them to be focused to a smaller spot size.

Absolutely but the laser uses more DC power in for a given power of emitted light. the efficiency of the stimulated emission process is not high. But I guess that three small spots of light from lasers could be used to obtain a brighter result than when using leds.

 

[Charles Link]

Thanks! I had to do this multiple times, but I think I finally got a good baseline result.

So this is from a "Full spectrum" LED light. I confirmed that use the Blue Light with Phosphor Coating to produce their white LEDs. I bought some black poster board and cut out a pretty narrow slit on it and got the prism to disperse it. This is a clear as I could get after multiple attempts. It's definitely a rainbow..but it's missing green completely it looks like, and no real orange:

View attachment 359662
What's now tickling my curiousity...is just how can blue + Phosphor even create *this* much variation in color...

So I suppose this satisfies me. It's not a complete, continous spectrum that you'd see from the sun, so calling it 'full spectrum' may be some marketing over selling I'd think...but it *is* there

You really need to use about 3 (converging) lenses: First focus the light onto the slit. Next collimate the light onto the prism by putting the slit in the focal plane of the second lens. Finally, with the light that emerges from the prism, (it may be a beam of white light that is a half inch across or more), get the far-field pattern by putting a white screen in the focal plane of the 3rd lens. This 3rd lens can be placed very near the prism. This is how the optics of any good spectrometer are typically designed, whether it uses a prism or a diffraction grating for the dispersive element. (They may use spherical or paraboloidal mirrors, but they essentially do the same thing as a lens).

 

[DaveE]

Absolutely but the laser uses more DC power in for a given power of emitted light. the efficiency of the stimulated emission process is not high. But I guess that three small spots of light from lasers could be used to obtain a brighter result than when using leds.

The thing about a physics website is if you say brightness, we are likely to assume you are talking about brightness*. This isn't so true at an art gallery, recording studio, or movie theatre.

Yes, we know, lasers don't win efficiency prizes. Power, efficiency, and brightness are different things.

* Or luminance et.al. Although the laser engineers I worked with never used those terms. Everything was W/m^2 or J/m^2 with them. They, like me, weren't a fan of candelas, lumens, etc. That was more for people that make light bulbs.

 

[sophiecentaur]

Power, efficiency, and brightness are different things.

You are right but I know what I mean when I talk about a bright spot of light or a dim spot of light. I was too colloquial, perhaps.

They, like me, weren't a fan of candelas, lumens, etc.

Too right. When they appear, I have to look at the book all over again.