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

[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.