1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Determine emission spectrum of an LED

  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.
  2. jcsd
  3. Feb 17, 2016 #2


    User Avatar

    Staff: Mentor

    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


    User Avatar
    Gold Member

    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.
    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

    User Avatar
    Science Advisor
    Education Advisor

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


    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:


    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


    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor
    Education Advisor

    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


    User Avatar
    Science Advisor

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

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

  12. Feb 22, 2016 #11


    User Avatar
    Science Advisor

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


    User Avatar
    Gold Member

    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.

  14. Feb 22, 2016 #13


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Brilliant :smile:

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

  15. Feb 22, 2016 #14


    User Avatar
    Gold Member

    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.

    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.
    1. In the course of the experiment, I rounded up 7 unique light sources from around the house, of which, I collected spectral images:

    Comments: Looking at the first raw images, I didn't think this experiment would work.
    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
    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

    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.


    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


    User Avatar
    Gold Member

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


    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


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    outstanding effort considering the setup !!
    am impressed

  18. Feb 22, 2016 #17


    User Avatar
    Gold Member

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

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


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

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

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor

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


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


    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:



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


    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Determine emission spectrum of an LED
  1. Emission spectrum (Replies: 16)

  2. Emission Spectrum (Replies: 17)