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I Determine emission spectrum of an LED

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  1. Feb 17, 2016 #1
    I recently purchased some 660nm LEDs. They look kind of orange not deep red. What is the easiest and cheapest way to determine the emission spectrum. Eventually I wanted to try to use the 660nm LEDs to grow some plants.
     
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  3. Feb 17, 2016 #2

    berkeman

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    Check the datasheet for the LEDs -- it may have that information. Or else, maybe use a prism?
     
  4. Feb 18, 2016 #3
    I purchased the 660nm LEDs off amazon. There are no no manufacturer markings on them, but they work. I guess you get what you pay for. How well would a digital camera work in analyzing the spectrum? If a picture was taken of the lit LED or after the light from the LED was put through a prism and refracted. The RGB values of the individual pixels could be observed. It would be interesting to see how accurate this would be. Would the image compression affect the RGB values (TIFF vs JPG)?
     
  5. Feb 18, 2016 #4

    OmCheeto

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    Me too! I've been thinking about it for over a year now. [ref: PF]

    I just did this experiment. The RGB values are of no use, IMHO.

    A couple of weeks ago, my fish tank cast a rainbow on my La-Z-Boy, so I took a picture. I also took pictures of the light coming out of the fish tank.
    oms.fish.tanks.rainbows.jpg
    I just now took a snippet of the red section from the La-Z-Boy image, and the only thing it told me, was that everything was red.

    My camera is quite, um, "inexpensive", and only takes JPG images. But, I think it's a moot point.

    Anyways, here's an interesting article that I just found that may answer a couple of your questions:

    Build a high resolution spectrograph in 15 minutes


    ps. Science!
     
  6. Feb 18, 2016 #5

    Andy Resnick

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    Very well: I took these by simply placing a diffraction grating in the optical path

    _DSC7074s_zpso6o3bzpk.jpg

    That's the spectral output of an incandescent light, and on my camera, the full size the spectrum covers 950 pixels. There are some subtle aspects (the shape of the light is convolved with the rainbow), but if you have a standard spectrum, you can indeed make quantitative measurements. A good standard is the output of a fluorescent bulb:

    _DSC7073s_zps5szkbpbl.jpg

    Each emission wavelength has a clearly identifiable location- overlaying an image of the LED will give a pretty good estimate of the output spectrum. Even so, I have to mention that I'm not sure what the measurement limitations are- can I distinguish between 660 and 650 nm? Unclear, but that's why I have a spectrometer in the lab.

    Edit: OmCheeto's link describes essentially what I did.
     
  7. Feb 18, 2016 #6

    OmCheeto

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    I was going to drop your name, as a known authority on the topic, but I thought it might be seen as a bit presumptuous.
     
  8. Feb 18, 2016 #7

    Andy Resnick

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    Thx- feel free to drop away. I don't know about 'known authority'... maybe 'known blabbermouth' :) Cheers!
     
  9. Feb 20, 2016 #8
    Thanks for the help. I will have to get some diffraction grating, and try this out.
     
  10. Feb 21, 2016 #9

    Tom.G

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    Save your money for a diffraction grating. A CD or DVD works fine.
     
  11. Feb 21, 2016 #10

    Andy Resnick

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  12. Feb 22, 2016 #11

    Tom.G

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    Thanks Andy, that's a good site. They have spectroscope for under $10.
     
  13. Feb 22, 2016 #12

    OmCheeto

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    Beings how today yesterday was Sunday, and the store that sells diffraction grating was closed, I decided to try the CD approach.
    I now can appreciate why there are "optical benches", as my data is somewhat boogered.
    I'll try again in the morning.

    red.neck.spectrometer.png
     
  14. Feb 22, 2016 #13

    davenn

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    Brilliant :smile:

    also good to see you have the obligatory roll of duct tape in there, Om :wink: :smile:


    Dave
     
  15. Feb 22, 2016 #14

    OmCheeto

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    While I'm collecting new data, I thought I'd share some notes, images, comments, retractions, excuses, and theories as to what the heck is going wrong.

    Notes:
    1. My camera is old and cheap. The only adjustment available is "focus", which consists of markings: "Flower"(1 ft), "Mountain"(∞), and a "4 feet" line, which I drew on with a sharpie pen one day.
    2. When overloaded, my camera turns colors white.
    Images:
    1. In the course of the experiment, I rounded up 7 unique light sources from around the house, of which, I collected spectral images:
    cd.spectrum.experiments.png

    Comments: Looking at the first raw images, I didn't think this experiment would work.
    what.my.camera.saw.png
    But, I would discover that the fact that my camera had 2560 x-axis pixel resolution, would yield ≈1 nm information, based on:
    white LED bulb range
    x-axis____ limits of visible range
    1418______22 pt red
    1702______64 pt blue
    284_______difference​
    ----
    google:
    A typical human eye will respond to wavelengths from about 390 to 700 nm.
    700-390= 310 nm​

    Retractions: I may suffer from multiple personality syndrome, as, well....
    Om; "...everything was red"
    Everything was NOT "exclusively" red.​
    Om; "The RGB values are of no use, IMHO"
    Thank god I included the "IMHO"........​
    RGB values are somewhat useful, to certain points.
    Wavelength to Colour Relationship
    A simple tool to convert a wavelength in nm to an RGB or hexadecimal colour.
    Fortunately, Lotic7's problem fell outside of the usefulness of this tool.
    The tool freezes the RGB settings at 255,0,0 from 650 nm to 700 nm.
    And the 700 to 780 nm measurements are done with diminishing "red" values, so this is a useless tool for studying monochromatic light

    analyzing.spectrum.by.rgb.number.rev.1.png
    Excuses:
    My younger brother showed up on Saturday, and was not impressed with my cereal box and cd, that I was going to build a spectrometer from. Things went downhill from there.
    My sister is flying in from out of town tomorrow. She will be here for 7 days. Do not expect much, after today.

    Theories:

    1. My red LED really warmed up. Could there be some black body radiation effect, affecting the emitted wavelengths?
    2. When I looked at one of my LEDs(they are all clear lensed), I wondered if human eyes can be oversaturated, kind of like my camera, and give an off color interpretation?




    ps. RGB values courtesy of @lpetrich 's most awesome :bow: "Image Measurer" software, Version 1.0 (1) [ref]
     
    Last edited: Feb 22, 2016
  16. Feb 22, 2016 #15

    OmCheeto

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    I hate science.........

    science.laughs.at.me.png

    I swear to god, it laughs at me.......

    But I did capture a missing mercury vapor yellow band there.

    Off to recalibrate my devices.

    ps. I added a lens.
     
  17. Feb 22, 2016 #16

    davenn

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    outstanding effort considering the setup !!
    am impressed


    Dave
     
  18. Feb 22, 2016 #17

    OmCheeto

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    Neither science, nor my setup, are treating me kindly.

    @Lotic7 , invest in the diffraction grating........

    argh!

    Off to take my 3rd set of photos............

    ps. did I mention, that science, is stupid?
     
  19. Feb 22, 2016 #18

    Andy Resnick

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    Nice! Here's one I put together of the planets (JPG):

    planetss_zpsmiybtclx.jpg

    I did this a long time ago, before I had a tracking mount- I can now leave the shutter open longer, getting a better signal-to-noise ratio. In any case, plotting the (B, G, R) values shows the color differences. Here's Venus (the top planet):

    venus_zpsawflr8lv.jpg

    Note the pixel position of the red and green peak. For comparison, here's Mars (the next one down), which is redder in hue:

    mars_zpsdqdza6pv.jpg

    It's noiser, the peaks have shifted to larger pixel values and the red channel is generally brighter, reflecting the redder hue. Here's Jupiter and Saturn:

    jupiter_zpsdjbbliai.jpg

    saturn_zpswchf21nj.jpg

    And the bottom one is a star, Porrima, bright and conveniently located:

    porrima_zpsaraum4l6.jpg

    The two dim ones are moons of Jupiter, which I can definitely capture next time.
     
  20. Feb 22, 2016 #19
    I don't just "like" this I love it
     
  21. Feb 24, 2016 #20
    Some fairly bright ideas. (Bad pun, bad. :sorry:) But perhaps you are overthinking it?

    Go down to Home Depot to the paint department and use their spectrograph. Tell them you want to match some paint to this light color. (Not a bad idea for the grow room, BTW.)

    Of course the machine is designed to match paint, not LEDs, so it may not work. But if it doesn't you are only out some gas.

    All hail OmCheeto. :bow:His setup shows a deep understanding of the problem. With a little more bubble gum and some bailing wire I now expect him to build a 747.
     
  22. Feb 24, 2016 #21

    OmCheeto

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    I decided that the flexibility of the cardboard box was one major problem.
    I placed the lamp inside the box each time, and I'm sure it shifted the slit in the end, thus throwing off measurements.
    On my 4th data collection run, my red LED finally burned out.
    So for my 5th data collection run, I decided simply to take some photos of the light reflected by my LEDs, and see if I couldn't extract useful information from the RGB levels.
    The data there was also screwy. I used my red laser pointer as a reference, and my camera thinks there is blue light coming out of it.

    Code (Text):

    R    G    B     Notes
    203  0    46    red laser
    196  0    51    red laser

    183  0    25    red led
    165  0    9     red led
     
    I believe it was at that point, that I decided that I should start over from scratch.
    And perhaps get a new camera, as this one is obviously just making things up. o0)

    I will probably do future optics experiments on the kitchen counter, as that little table is a bit wobbly.
     
  23. Feb 24, 2016 #22

    Andy Resnick

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    I see this issue as well, when I illuminate with a strong monochromatic source. I think it's a combination of Bayer filter artifact and intensity, but I don't know for sure. Here's an example, a diffraction pattern from a 532 laser:

    DSC00379_zps66ypu8jv.jpg

    And the color line trace from the central peak outwards:

    Profiles%20of%20DSC00379_zpspmma3gaa.jpg

    The red and blue channels get excited when the intensity is high.
     
  24. Feb 24, 2016 #23

    OmCheeto

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    I think my Bayer filter is broken. :oldgrumpy:

    Here's a thumbnail of my red laser reflected off of a white surface.
    I knew it would oversaturate my camera, so I duct taped my burned out led to the end to scatter the light.
    Even the dimmest regions show blue.
    red.scattered.laser.thumbnail.png

    hmmmm.... There's a total eclipse next year, and I should probably get a new camera for that. I think I'll go shopping in the morning. :oldsmile:
     
  25. Feb 25, 2016 #24

    Tom.G

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    If the camera has "automatic white balance", turn it off!
     
  26. Feb 25, 2016 #25

    Daz

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    I too have seen this occur. It happens with CCD detectors where each pixel is essentially a potential well with a finite density of states. If the incident light intensity is high, the individual colour pixels saturate and charge starts spilling over into adjacent pixels. It’s called white-out and reducing the intensity should resolve it.

    Even in the absence of saturation, a monochromatic input will still lead to some signal in all three colour channels because the red, green and blue pixel filters aren’t perfect. Here’s a plot of quantum-efficiency taken from the data sheet of a common CCD image sensor. Note that none of the filters completely block wavelengths outside of its pass-band.

    CCD_QE.jpg
    I guess this might explain OmCheeto’s observation. Also, the signal processing in consumer cameras usually includes white-balance correction which adjusts the R-G-B signals to try to compensate for different lighting conditions. So interpreting those values to determine wavelength should be done with great caution.

    Coming back to the original post about LED colour, I’ve noticed (through error, rather than design) that if I overdrive an LED the emission blue-shifts and broadens. I once accidentally connected an LED display directly to the supply without a current-limiting resistor. The normally red display lit up brightly with an almost whitish-orange hue. Then started to smoke! Maybe that’s why the OP observed orange emission from a red LED?
     
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