Step 4Selecting the wavelength

The slit

To accurately select a very small section of the spectrum, one would have to make an extremely narrow slit. Since this is very hard to do at home, I chose to make two slits instead that are not 100% aligned, so only a very narrow band of light can pass through.

Make a small slit in a piece of metal or cardboard. (I used the grinding wheel attachment of my trusty Dremel 4000)  
Then, with the light source and CD in place, project a spectrum onto the board and make sure that all parts of the spectrum can be aligned with the slit, depending on the CD's angle.
Now glue the board to the base.

Make a slit in a second piece of material, and align it with the first slit, so that only the narrowest band of light can pass through.

Also it's best to shield the light source from the rest of the device using a piece of metal or cardboard, so that only the diffracted light will hit the slit.