Seeing Media
Many optical media types use a physical encoding to storage data. When the density of the encoding gets small enough, light can be distorted at some angles by diffraction grating. This causes a different frequency of light to be refracted off of the surface based on the texture of the physical object. Combining this with some high density media formats can result in being able to visibly see elements of the data encoding. In order to do this, a singular point source of light at a carefully positioned angle is needed to cause the diffraction. A video microscope is used on this page to capture the results.
For video in particular, formats that are encoded as CAV, Constant Angular Velocity, will repeat the same horizontal line of a raster scan image in the same angular slice of a disc. This means it is possible to observe the change of video over time by looking at different lines next to each other.
Laserdisc
Laserdiscs are an analog media, the data is encoded in the time between pits on an aluminum layer of the disc. This has a weak diffraction effect but it can still be seen. The image to the right shows, going from left to right, the end of one horizontal line, the horizontal sync pulse, the NTSC color burst, and the luminance for the rest of the line. Each colored stripe is roughly one line of video.
We can look for patterns in the luminance section of the signal to see the change of a line over time. The left image below here shows a repeating pattern that is a nearly comprehensible image.
The ideal subject to look for on a disc to visibly see recognizable subjects is a vertically panning image. This could be a camera being moved up or down, or on screen graphics scrolling on screen. Most movies contain scrolling graphics in the form of credits. As long as the speed of the vertical scroll is within a specific range, you will be able to see legible text in the same way that a slit scan camera works.
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Visible repeating pattern on a CAV Laserdisc
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CAV Laserdisc of The Minds Eye's credit sequence. Because the text scrolls vertically the same line on the CAV angular slice will have the full image pass through it over time. (image vertically mirrored)
CED - Capacitance Electronic Disc
CEDs physically encode analog waveforms in the vertically varied height of a groove that a capacitively coupled stylus reads. These waveforms are standard NTSC video signals like the above laserdiscs and are more pronounced and uniform. CEDs are all CAV encoded by their nature, and similarly we can look for ending credits that scroll to see recognizable text.
Audio CD
Compact discs use a very complicated digital encoding scheme that has a built in checksum/parity. As a result, data on them typically is not recognizable as it is scrambled. However some format layout structures may be seen due to small and heavily repeated data.
The diffraction effect on CDs is also particularly weak. This coupled with the scramble data makes for a good demonstration of the same subject being lit in different ways that can show and not show the diffraction colors.
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Blue Book format CDs contain multiple sessions with lead in and lead out tracks in the middle of the disc. This is the boundary between two sessions on The Offspring's Americana.
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CD Audio tracks sometimes have a "pregap" between them. In the middle of a disc this is typically silent. This makes the pattern visible. This is the boundary between two tracks on The Offspring's Americana.
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A dent in the aluminum layer of a CD with diffraction grating.
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A dent in the aluminum layer of a CD without diffraction grating.
LS-120 SuperDisk
Floppy disks, VHS, Cassettes, Reel to Reel, and other magnetic type formats store data in magnetic flux transitions. This is not visible observable with lighting methods. However, some Super Floppy formats may use Magneto Optical sectoring which uses higher precision optical tracking marks to store higher density magnetic flux in smaller areas. This is the optical marking side of an LS-120 SuperDisk.