often encountered in micro-publishing the classic and most well known example
being the BBC Domesday project which is contained on two 12″ CD-ROMS.
So that gets over that a disk is a disk and not what you put on it. Why optical disks
at all ?
Optical disk technology has been around in laboratories for some 15 or more years,
and things like the TV disk have been commercially available for 8 years, so it’s not
exactly new, it’s actually a well tried technology. Like all things which work, it works
very simply. It’s only required the improvement in the manufacturing technology of
the component parts to reduce the price to make it viable for mass acceptance. The
heart of optical disk technology is the strange properties of the Laser (Light
Amplification by the Stimulated Emission of Radiation, and don’t let anyone spell
it with a Z or tell you it means something different). The Radiation referred to is
light, and not anything radio-active. In this instance, the Laser is not a large
cumbersome machine out of Star Wars, but a small electronic component about
5mm in diameter by about 5mm long. Laser light is known as coherent light and has
different properties from the light emitted by – say a torch. For optical disks the
two properties of most interest are the ability to focus the light to a pin-point (in
fact a pin-point is the proverbial blunt instrument compared to the focal point of
the Laser beam used by an optical disk drive) coupled with the fact that the beam
can be controlled (turned on and off) very fast electronically.
Ok, to write to a WORM, the Laser is switched on and off very fast in the required
binary code and focused on the mirror smooth surface of the optical disk. The disk
is of course rotating, and a mechanism moves the Laser light mechanically (either
using a mirror or by moving the Laser) so that the Laser light never falls on the same
place twice. It’s following a course rather like a gramophone record except that it
starts at the inside of the disk and works outwards towards the edge, and it’s not a
single spiral, rather a number of concentric rings (called tracks). Every time the
Laser comes on, it burns a minute hole or pit in the surface of the disk. Where the
Laser is turned off, there isn’t a pit, the disk surface is unaffected and remains mirror
smooth. So the pits could represent the ‘1’s and the absence of pits represent ‘0’s.
Simple as that !
The clever bit is reading it back and is the same for either a WORM or a CD-ROM.
This also uses the Laser but with the power turned down so it doesn’t burn the
surface of the disk. The Laser follows the tracks previously written, and where the
Laser light falls in a pit, the light which is reflected goes off at odd angles. Where
the Laser light falls on the mirror surface, the light is reflected directly back towards
the Laser. It would go all the way back to that Laser if it weren’t for a special mirror
in the way called a beam splitter. This diverts some of the reflected light into a light
sensor. The light sensor sees flashes of light where the Laser plays on the unpitted
surface of the disk, and no light where the Laser plays on a pit. The light sensor