Tuesday, March 23, 2010

Looking closely at CDs

I've always been intrigued by the dividing line between the written and unwritten areas on a CD-R. All CD-R formats begin writing in a spiral track starting at the inner radius of the disc and proceeds to the outer radius, so the inner area is written data, while the outer area is still blank. The inner area usually looks lighter in color.

I first placed the CD-R on the microscope stage, bottom-up. This worked at "medium power" 10x, but I could not focus at 40x because the thickness of the CD itself did not permit focusing the 40x objective. Instead, I scratched off a little of the protective coating on the CD top surface, then inspected it from the top.

This is a very old Memorex CD-R. Green-blue dye with a gold top-layer. I lit the CD-R from below with the microscope's built-in light. The image was pretty dark, but by setting the camera exposure to 3 sec, the final image looks great.

Here's a shot of the dividing line between the data area (upper half) of the disk and the blank unused portion of the disc (lower half). The length scale is approximate.

Here is a %100 crop from my camera. The length scale is approximate and computed by dividing the frame height of my camera (13mm) by the microscope objective's magnification (40x), then scaling in GIMP to make the bar an appropriate size for the crop. I checked Wikipedia, and it appears the track spacing for a CD-ROM is 1.6um, so I wasn't too far off.


  1. This is awesome. Can you decode the binary bits by looking at the image?

  2. Cinead, I suppose it would be possible by using machine vision. A CD or DVD drive is essentially just a camera with one pixel of resolution and a spindle. I think I remember reading that some exotic drives have multiple read heads for higher data throughput. With an extremely high resolution camera, it might be possible to achieve higher data rates by reading a whole group of tracks simultaneously

  3. Neat stuff, Ben. I still regret, when I was given the choice between two dissecting microscopes, I chose the one that was comfortable to use over the one with the T-mount. Given how much I actually use it, comfort was not a big issue, and I really miss the ability to take photos.

  4. sir, what is angle between CDs and microscope?
    it is possible to get with 20x?

  5. Amalraj, the CD is held flat on the microscope stage, so the optical axis is perpendicular to the CD surface. This allows the objective to be positioned very close to the pits and lands.