While refining my CNC microscope setup, I thought I'd have a look at some CD surfaces. Unfortunately CDs are not especially useful as calibration artifacts, since the track pitch and channel-bit length can be varied over ~15% ranges and still be in spec (manufacturers played with these parameters to pack more audio onto the disc). Still, it got me interested in the possibility of reading the data using these optical images.
The image below was taken with a Nikon 20x M Plan objective (NA=0.4, 210/0, LWD), a Logitech C270 webcam, and a 3D-printed epi illuminator with microscope-slide beamsplitter, all attached to the carriage of a Lulzbot Taz 4 3D printer:
Some spherical aberration is present. In fact it's a great deal worse if you image through the polycarbonate---a 1.2 mm slab of n=1.55 material is enough to seriously blur the image at this NA. I'd like to find some sort of SA compensator that I could hang on the end of the objective (there is plenty of working distance, ~5 mm). Perhaps just a (spherical) achromat would suffice---I don't want to pay $400 for an Edmund Optics asphere plate. This image is taken from the CD's label side, and there's still ~0.1 mm of polymer acting as a "cover slip". The objective is optimized for an air medium and no cover slip, so this is still suboptimal.
Anyway, here's a section through the image plotted as a waveform.
The eye is open, so slicing detects the following symbols:
0001111100001111100011110001111000001111111000000011000000111111 1100000111000001110000000011111111110000011111110000011110000011 1111110000011110001111000011111000001110000011110000111110000111 0001110001110001110000011111000001111000111000001111100011111100 0001111110000000001110000011100001111000000111111110001110001111 0000011100011100011111110000011110000001110000111111111000011111 0000000001111111000000111100001111000001110000011111111100001110 000011110011110000011100011100001111000000
Decoding the NRZI line code (transition is a "1", no transition is a "0"), the channel bits are:
0010000100010000100100010010001000010000001000000101000001000000 0100001001000010010000000100000000010000100000010000100010000100 0000010000100010010001000100001000010010000100010001000010001001 0010010010010010010000100001000010001001001000010000100100000100 0010000010000000010010000100100010001000001000000010010010010001 0000100100100100100000010000100010000010010001000000001000100001 0000000010000001000001000100010001000010010000100000000100010010 00010001010001000010010010010001000100000
No frame-sync pattern in this short segment (in either direction). I'll have to take several images and stitch them.
A specification for the CD physical format, channel coding, ECC, framing, and metadata is available here: