As the disc is rotating under a focused laser beam, two levels of reflectivity are measured (high or low) depending on whether there is a "pit" (seen in black) or a "land" (seen in blue) under the spot. One state can be associated with bit "1" and the other with bit "0". A bitstream can therefore be written as a succession of pits of variable length and spacing.
However, in order to realize a sufficiently high level of reliability, a bitstream of data is first transformed into a "channel bitstream" before being written. This "channel bitstream" comprises an error-correcting code (ECC) and a modulation code. The ECC makes it possible to recover the correct information in the presence of errors that may occur during read-out of the disc whereas the modulation code transforms an arbitrary sequence of bits into one that possess "desirable" properties2.
The data density being about 12.5 Gbit per square inch, there is at most 600 bits in this portion of the disc. So don't expect to catch a glimpse of the movie by decoding it.
Why in the world would someone image a Blu-ray?
Mainly just because it can be done. However, if like me, you are troubleshooting an atomic force microscope and need a sample to scan, the surface of a Blu-ray may be a good option because it is cheap, widely available, easy to image and has recognizable features. Also, since the distance between the tracks is known, you can use it to calibrate your scanner in the X and Y directions.
Note that first, a transparent protective layer of 0.1 mm had to be removed in order to access the surface on which data is written. ↩
For example, the modulation code guarantees that sequences written on disc contain neither very short nor very long runs of successive 0's or 1's. This is desirable because very short runs lead to small signal amplitude and are therefore more prone to random errors in the bit-detection module. On the other hand, very long runs make it impossible for the reading system to adjust its internal "bit clock" as it needs to detect a transition to do so. In this case, "clock-drift" may occur which will lead to an inaccurate recovery of the number of bits contained in a given temporal signal. Read section 4 of the Blu-ray specification for more details. ↩