simply converts the flashes of light into an electrical signal and the rest of the
electronics in the disk box amplify and shape the signal into a digital signal, exactly
the same as the digital signal which wrote to the disk in the first place.
Notice how a WORM can only write once. Once an area has been pitted by writing
to it there is no way of smoothing out the pits to allow the same area to be written
Optical disks – Pros and Cons.
That’s the way it works, what are the advantages ? The biggest advantage relates to
the small size the Laser can be focused down to. Something like 16,500 pits to the
inch, that’s also 16,500 tracks to the inch across the disk, and as the usable surface
of a 5.25" disk is close to an inch and a half, that’s one hell of a lot of pits, representing
a very large amount of information. If the tracks on a disk were unwound and joined
together, the resulting single track would stretch between 5 and 30 miles depending
on the size of the disk !! So the overriding advantage of optical disks is the
information storage capacity, it’s vast ! Far greater than the other competing
mediums around, Any other advantages, well CD-ROMs are fairly robust and hard
to damage, although sandpaper is effective, and WORMS are pretty strong,
although not as robust as CD-ROMs, as they are made of a glass sandwich —
WORMS are usually totally enclosed in a tough plastic case and the disk can not be
removed (unlike many advertising pictures which show the disks without their
There must be some snags. CD-ROMs and WORMS are not as fast toread or write
as the hard disks on a computer, but come a close second. The optical drives used
to spin the disks and control the laser are more expensive than computer hard disks,
but the trade off between capacity and cost doesn’t so much favour the optical disk,
it screams it – double the price for 10 to 20 times the capacity. WORMs are Write
Once media. Ok, that’s bad news if a WORM was to be used as a hard disk in a
computer, but then a hard disk is not usually replaceable like the WORM and
certainly not secure against someone altering the information on the disk. Security
on a WORM is far greater than a hard disk. Once information is written it’s virtually
impossible to change it, and definitely impossible to change it invisibly.
So all in all, optical disks are used in an Imaging system because of their high
capacity, the difficulty of altering data once written, the fact that optical disks are
replaceable media and the technology is such that the data integrity is guaranteed
for at least 10 years, and this period is getting longer.