Hi, I have discovered that CDROMs split in a certain way make excellent (at the $0 level) diffraction gratings with lots of amateur uses, like color separation in projects involving optics and analyzing various materials and light sources. Recipe to make a very simple and useful optical (light) spectrum analyzer: -------------------------------------------------------------------------- (in about 15 minutes): What you need: 1 CDROM (must be silver aluminium type, not CDR or CDRW). 2 opaque, empty 35mm film cans 1 Xacto knife some duct tape (black) Serves one per pair of film cans, up to 10 per CDROM. Procedure: 1. Using an X-acto knife cut two almost parallel radial lines from the inner hole to the outer rim of the CD, about 8 mm (1/3") apart, then snap the CDROM between your fingers to obtain a sliver. Cut and snap again to obtain a shorter sliver that fits forced into the inner diameter an empty 35mm film can (black or aluminium please). Take the part of the sliver that used to be at the outer rim of the CD. This is the diffraction grating. The more parallel the sliver to an actual radius of the deceased CDROM the better (take your time to cut precisely). 2. Using the same X-acto knife cut a slot in the first opaque film canister. The slot should be 5-6 mm (1/4") wide and go all the way from the upper (open end) rim to the lower rim of the canister, and straight. 3. Using the knife, cut a straight slit in the remaining (other) empty file canister, at about its center (of the closed end), so the slit is about 1/3 of the diameter of the canister and centered. This is the slit (optical aperture). Take some abrasive paper, make a strip of it that fits in the slit, and work the slit edges until they are as straight and clean as possible but do not widen the slit (check against light). My slit was of about 0.2mm (in plastic canister). 4. Insert the sliver of diffraction grating into the lengthwise cut film can, shiny (not label) face towards you, and press it down so it lies at the bottom of the can as level as possible, and aligned with the lengthwise cut (slot) so you can see all of it through the slot. Some hot stuff helps to keep it in place. 5. Assemble the film canister with the slit with the one with the grating, open end to open end, using black duct tape. Make sure that the slit is perpendicular (at 90 degrees) with the sliver of CDROM lying at the bottom of the other film can. You can improve the angle by looking at the image of the slit on the grating while assembling. 6. Test your optical spectral analyzer instrument by pointing the slit towards a light source (desk type low power fluorescent lamps work best, they have a few very sharp spectral lines). Look through the slot at the CDROM sliver and adjust your angle (keep the instrument steady with the slit towards the lamp and move your head) until you see the spectral lines. The first order lines appear at about 20 degrees from the axis of the instrument and the second order are also very well visible (and better spaced out) at about 40 degrees. The separation achieved is imho amazing for such a simple instrument and I was able to see some very 'blue' lines from an UV lamp (do not try this) so the polycarbonate passes UV apparently. Also IR (this is guaranteed, the original CDROM readout laser uses near IR). 7. Improvements: Increase the amount of light as much as possible using a lens. The lens should focus a light source directly on the slit (outside it). Adding a small piece of translucent paper directly on top of the slit (outside) also improves the image. The slit can be replaced with one cut in a thin aluminium sheet for much better precision. Install an eyepiece to be able to view the spectral lines more comfortably. Install a calibrated scale. I have tried a color TV camera, and the colors look great. Although the CCD color filter probably cheats big time about the actual colors in despite of locked white balance. 8. Uses: Use as color filter with lamps, LEDs, 'homebrew' color sensitive detectors and many more uses. Teach kids about light spectra (do you have any idea how much a decent prism costs ? Let alone one that covers UV to near IR). 9. Question: What is the pitch of the tracks on a normal CD ? I am unable to find this information (it is of about 2 um but how much exactly?). sorry for the long post (but others posted longer and worse ones ), Peter PS; An now you have an excuse to split those spare CDROMs at every occasion ;-) -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.