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The magnetic floppy disk is sometimes used as an (infrared) light filter as shown on some pictures below. That led me to think about the actual magnetic disk and if inconsistencies or damaged could be detected using visual inspection (for example a good backlight and high definition camera). If for example a floppy disk was placed on top of a bright light and visually scanned or inspected would it be possible to find (physical) damage or predict for example bad sectors or damage to the floppy?

Also would using a specific kind of light from the visible spectrum increase or decrease those chances? I think IR (non-visible) would be blocked on the disk material and UV (also non-visible) would be damaging the magnetic disk as discussed here: What is the effect of direct exposure of the magnetic disk of a floppy to ultraviolet (UV) light?.

light filter using magnetic floppy disk light filter using magnetic floppy disk

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  • I mean, certainly I've had instances where I couldn't read from a floppy disk, and when I took it out and looked at it, there was damage that was visually obvious (hole, indentation, crease, etc)
    – Nate Eldredge
    Jan 26, 2022 at 22:50
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    IIRC the magnetic properties of surface can be detected by electron-scopy (not normal camera but more like electron beam scanner similar to electron microscopy or old silver based tube cameras) the image in your other question was most likely obtained by it. I think there might be also some indirect principle out there with CCD camera at back end covered by dust on some R&D lab somewhere but again with some additional field generators in front of it ... where medium is not light but charge or beam of ions
    – Spektre
    Jan 27, 2022 at 10:59

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Visual inspection, without special lighting, can reveal the most common types of damage to floppy disks: mold, and damage to the medium (up to delamination). You can see examples on this page on disk cleaning and in the 8-bit Guy’s description of restoring some Commodore PCs. Floppy drives and disks operate at scales where most physical problems are visible to the naked eye or with only slight magnification.

I don’t think visual inspection would help detect non-physical defects; regardless of the light source use, you wouldn’t be able to see magnetisation problems, as far as I’m aware.

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There are (para?)magnetic fluids that can disclose a magnetic domain structure on a floppy disk, like Kyread. These are useful in the coarse bit arrangements on old floppy disks, and magnetic tape, and (with the aid of microscopy) can even read out some kinds of data. The weakness of this approach is that it operates in the absence of polarized light, and is insensitive to polarity.

Some polarized-light rotation techniques are also available, employing Faraday effect, which DO sense polarity.

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