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I need your help as I am stuck and I dont know where to start. I have got four blue leds diode that come with dark blue led lights that I really want to get the same led colours for my 3 fans, but I have got no idea what the wavelength for that blue led lights.

Here is the pictures: https://i.sstatic.net/QKP39.png https://i.sstatic.net/wPAaS.jpg

I am not going to take out the LED diode yet until when I have been advice to do so. I think the wavelength is between 450nm to 470nm but I am not too sure. Do you know how I can find out what is the wavelength for that LEDS?

If you know how I can find out what wavelength for that blue leds, I would be very grateful for that useful information.

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    $\begingroup$ LEDs don't usually have the manufacturer's part number on the component itself. If you really really really want exact matching colors, just buy 7 LEDS of the same type and replace the lot. Don't waste hours of your time trying to save a few dollars! $\endgroup$
    – alephzero
    Commented Jul 14, 2018 at 19:49
  • $\begingroup$ @sammygerbil yes I am asking how I can do an experiment to measure the wavelength? I want to know how I can do this as you can see the screenshots I have attached I have only got led diode but I dont know where to start, what things do I need to get to measure the wavelength from the led diode. $\endgroup$
    – Robert Jones
    Commented Jul 14, 2018 at 19:50
  • $\begingroup$ @alephzero I am sorry but the company dont sell the fans anymore so I cant get the led. My only way is to find out the wavelength then I can search on google to find the wavelength to get the leds I want with same colour. $\endgroup$
    – Robert Jones
    Commented Jul 14, 2018 at 19:52
  • $\begingroup$ Sorry I overlooked what you said you wanted the LEDs for. @alephzero's advice makes a lot of sense. ... Blue LEDs range from 450 to 500 nm. High-school methods for measuring the wavelength of light (eg by diffraction from a Blue-Ray Disc) will give you an accuracy of (probably) 5% at best. So your result might be say 470+/-20 nm. You could easily notice the difference in colour between 450 and 490 nm, even between 460 and 480 nm. To get better accuracy you need access to university or industrial equipment. $\endgroup$
    – sammy gerbil
    Commented Jul 14, 2018 at 20:19
  • $\begingroup$ The experiment only makes sense if your goal is to learn about practical physics and have fun, rather than to make an accurate measurement. $\endgroup$
    – sammy gerbil
    Commented Jul 14, 2018 at 20:22

4 Answers 4

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I work at a lab and we have access to a spectrophotometer like this one here https://oceanoptics.com/product/usb2000-custom/. You can get the spectrum of emitted light, its intensity by pointing LEDs towards the fiber optic cable. It is a trivial thing to do if you have access to any of such equipment. But these things are expensive I think.

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  • $\begingroup$ Thank you for the link, but unfortunately I am not going to pay $2k for that type of stuff that I dont want to use. Are there any cheap LED module out there that I can find out the wavelength? $\endgroup$
    – Robert Jones
    Commented Jul 14, 2018 at 20:12
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If you have a decent enough camera, you could try to do a double slit experiment. But you’d have to be able to make fine slits. Or you could get a diffraction grating.

But if all you need is to match the wavelengths then you could perform the double slit experiment under same conditions except for the source LED and chose the ones whose interference pattern overlaps.

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There is the plastic project Starlab spectroscope (instructions). Or you can make your own for free. For example with a piece of a CD-disk as a grating and a cereal box or similar.

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If you just want to compare the wavelengths of your 4 leds, an unused CD or DVD males a good reflection grating. Poke a small hole in a white sheet of paper. With the led and disk at least several inches from the hole on each side, let the beam from the hole hit the disk (at 90 degrees) and look for the first (or second) order reflection on the paper. You may need to work in the dark except for your led.

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