Main image - A segment of a DVD-R bottom layer with the photo-reactive dye removed
See also - http://publiclab.org/n/11552 Does leaving the dye on a DVD-R grating make any difference?
All DVD-Rs are made with two layers of moulded polycarbonate about 0.58mm thick, glued together. There is a guide groove for the burning laser moulded into the upper (inside) surface of the lower layer (the 'label' side being the top side of the DVD-R). This spiral guide groove has a sufficiently fine and regular pitch of 0.74 microns, to act as a diffraction grating. The aim of processing a DVD-R to convert it into a usable transmissive diffraction grating for a spectrometer is to end up with an optically transparent piece of the lower polycarbonate layer with the spiral groove moulded into one side. The DVD-R or any diffraction film must be handled very carefully (use rubber gloves) to avoid scratches, dirt and finger oils contaminating the optical surface.
The process starts with splitting the two polycarbonate layers apart. DVD-R splitting seems best done by first separating the entire disc into two halves, rather than cutting out a segment and splitting that. CDs and DVDs are injection moulded from polycarbonate and in theory, if the temperature of the liquid plastic and the mould are properly controlled, the moulded end product should be pretty tough and shatter resistant. In practice this is often not the case because the manufacturers don't control temperatures that carefully and don't reject product with high internal cooling stresses that are produced when the moulding machines are run up or down for production. I found for example that trying to cut up a DVD-R before splitting it resulted in a lot of shattering and cracking, although that should not happen on a properly made DVD.
A sharp craft knife can be applied to the edge and wiggled (very carefully) until it gains purchase between the two layers and starts them peeling apart. Once separation is started it can be completed by gently prising at the edges of the discs with the fingers. The aim is to get the lower part of the disc to peel away with as little of the aluminium left on it as possible, but the process is hit and miss. Once split like this it is relatively easy, using a pair of stout shears or scissors, to cut the plastic up into a suitably sized rectangle for use in a spectrometer.
The problem I found with the initial split is that I tended to end up with a lot of aluminium and purple tinted dye on the bottom half of the disc. I tried to remove the dye by soaking a piece of the DVD-R in a fairly strong household cleaner (Cillit Bang - which contains Sulfamic and Phosphoric acid and some detergent) designed to remove limescale.
NOTE - Further experiments show that the dye is easily soluble in plain water. Just soak a portion of the lower layer of the DVD-R in warm water (adding a little liquid soap helps) and, after only ten seconds or so gently moving the fragment back and forth, the dye starts to dissolve and wash away. Use purified distilled water if you are worried about contaminating the surface with minerals from the water.
The dye dissolves easily, but any aluminium silvering left on the bottom disc is quite hard to shift. On my first attempt I ended up with a clear piece of plastic that did not diffract, I must have used the wrong layer of the DVD-R (I have a big stack of cheap DVD-Rs that have no 'label' surface – the top looks much like the bottom). I have since repeated the experiment, and with the dye removed, I have a clear section of polycarbonate that still diffracts. The colour filtering due to the dye must affect the response of a finished spectrometer if the dye is left in place. In comparison holographic film grating, or a properly processed piece of DVD-R material are colourless.
If you get stuck with a lot of aluminium left on the bottom half, then getting rid of it is harder than dissolving the dye. I used some drain cleaner granules (basically caustic soda a.k.a. lye or sodium hydroxide) mixed with water. After a good long soak the aluminium flaked off in small pieces. Be very careful with caustic soda, it can cause nasty burns.
If anyone comes up with better solvents or methods for cleaning a DVD-R of aluminium and dye please let me know. I have seen it suggested that these coatings, or at least the aluminium, can be peeled off by rubbing down sections of adhesive packing tape and then pulling it off, but I haven't been able to make this work.
BTW – Providing you leave a part of the centre of the DVD-R in place on any piece you cut, you can tell which surface the grooves are on. Most DVD-Rs have a moulded ring or circular bump near the centre hole. The bump is on the outside surface and the grooves are on the inside.
As it says in various places on the Public Lab site, the grooves in the DVD-R surface must be oriented as close to parallel with the spectrometer slit as possible. I found that it was relatively easy to cut a geometric segment or pie slice out of the DVD-R material using the edge and the centre hole as reference. However judging the geometry correctly to trim that slice down to a rectangle and maintain the orientation of the grooves was much harder.
NOTE - The kit for the Public Lab Spectrometer 3.0 now includes a small binder, or butterfly, clip to clamp the DVD-R diffraction grating to the angled grating support. This allows for easy replacement or adjustment of the grating. Gratings with and without dye can easily be tried out and If the output from the web cam shows that the grating is not parallel to the slit, the grating can often be nudged into the correct orientation without removing the clip.
Image 2 - Bottom half of a split DVD-R (supplied with the PL kit). The purple / blue tint from the photo sensitive dye is clearly visible and the distribution of the dye remaining on the surface seems to vary a lot. A rectangle of the 1,000 lpmm holographic film can be seen for comparison, lying in the cut away section of the DVD-R. It appears to be colourless.