Is there any cheap method by which to create tune-able optical band pass filter?

I am working with a chemical process which will fluoresce when excited by a specific wavelength of light. As shown in the image below, the FAM" reaction will fluoresce at ~520nm when excited by a light source of ~490nm. I want to build a cheap device that will go over a well full of samples and shine the various excitation frequencies while monitoring for the emission frequencies. The idea is that this device will then be able to determine which reaction (out of those listed on the left) took place.

It is my understanding using an RGB LED and RGB color detector is not possible because "mixing" RGB LEDs does not actually alter the wavelength of the color - it only appears that way to our eyes. I am exploring thin-film filters like this but was wondering if any other options exist.

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  • $\begingroup$ Are you trying to reinvent a wheel called "monochromator"? Why? The kind of instrument that you are talking about is commercially available. The requirement that it has to be "cheap" is usually a bad sign for whatever application you have in mind. $\endgroup$ – CuriousOne Jan 6 '15 at 8:14
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    $\begingroup$ We don't really focus on "cheap" here - so don't expect people to go comparison shopping, and certainly not to hit a price target - but I think it may be reasonable to ask whether there's a way that doesn't involve highly specialized lab equipment. $\endgroup$ – David Z Jan 6 '15 at 8:15
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    $\begingroup$ This question is interesting but I think it should be a bit tighter. You ask a how to build a tunable bandpass filter, but then you mention a few things about LEDs. If the question is just how to build a bandpass filter, just ask that. If the question is more generally how to achieve your diagnostic, at least change the title. $\endgroup$ – DanielSank Jan 6 '15 at 8:17
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    $\begingroup$ Seriously, what you want is a multiwell fluorescent plate reader like this biotek.com/products/microplate_detection/…, right? Used working instruments can be had for a couple thousand dollars. This stuff is cheap these days because every biotech company has like half a dozen of them. $\endgroup$ – CuriousOne Jan 6 '15 at 8:33
  • $\begingroup$ @DanielSank the question is how to build a tunable bandpass filter using the most cost efficient way possible. I included back-round information to try and explain why I want one/show what sort of precision is required. I am open to other excitation sources beyond white light + filter tho. $\endgroup$ – EasyOhm Jan 8 '15 at 6:22


  • A white light plus a commercial monochromator (perhaps expensive, but you can buy it used)
  • A white light plus a home-made monochromator (can be as simple as a diffraction grating on a rotatable platform)
  • A white light plus a liquid-crystal tunable filter (perhaps expensive, but you can buy it used)
  • A white light plus twelve different band-pass filters that you switch between by hand or (more expensively) with a computer-controlled filter wheel
  • Twelve different lasers or LEDs that you switch between by hand. (Probably the cheapest option.)


  • Your best bet is a commercial CCD spectrometer. Maybe you can find a used one in your price range.
  • You may be satisfied by a super-cheap DIY spectrometer such as one of these -- http://publiclab.org/wiki/spectrometer

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