What are the other ways to pass particular wavelength of light without using optical band pass filters? I'm working on an experiment that have visible spectrum light source (RGB LED at e.g. 495–570 nm wavelength), holder with sample, narrow bandpass filter (eg: CWL:520nm, FWHM:5nm) and a photodiode to detect the signal. As light source is RGB LED I can easily change it's wavelength but simultaneously I also have to change bandpass filter (different filter for different wavelength).
Now problem is every time I have to replace filter according to light source wavelength which is not right way and these filters are costly too.
Is there any alternative for these bandpass filter?   
 A: You could use a grating. The different wavelengths of light would leave the grating under different angles. By changing the position of the diode you can thus measure different wavelengths.
You are effectively creating a spectrometer.
A: Firstly an RGB led is not really a variable wavelength source. It has 3 broad color LEDs and varies the brightness of each to fool your eye into seeing a continually varying color.
If you are only using 520nm in your detector you would do better with a single brighter source at 520nm. If your wavelength requirements aren't strict you can get 532nm laser diodes very easily,  520nm diode lasers are available but rarer/more expensive.  Most green LEDs peak at 568nm.
A: One can get tunable bandpass filters where the filter frequencies are set by the tilt. I think I've used ones in this product line before: 
https://www.semrock.com/Data/Sites/1/semrockpdfs/whitepaper_versachrometunablebandpassfilter.pdf .
They are pretty neat.
But, as usual with these things, the right solution will depend on details that only you know, like acceptable bandwidths and losses and prices, so you shouldn't expect a very useful answer in a medium like this one.
