You may guess from the word "optical" that it has to do with light. You'd be exactly right! Laser light, in fact.

Optical disks come in several varieties which are made in somewhat different ways for different purposes.

How optical disks are similar

• Formed of layers

• Data in a spiral groove on starting from the center of the disk

• Digital data (1's and 0's)

• 1's and 0's are formed by how the disk absorbs or reflects light from a tiny laser.

The different types of optical disks use different materials and methods to absorb and reflect the light.

How It Works (a simple version)

An optical disc is made mainly of polycarbonate (a plastic). The data is stored on a layer inside the polycarbonate. A metal layer reflects the laser light back to a sensor.

To read the data on a disk, laser light shines through the polycarbonate and hits the data layer. How the laser light is reflected or absorbed is read as a 1 or a 0 by the computer.

In a CD the data layer is near the top of the disc, the label side.

In a DVD the data layer is in the middle of the disc. A DVD can actually have data in two layers. It can access the data from 1 side or from both sides. This is how a double-sided, double-layered DVD can hold 4 times the data that a single-sided, single-layered DVD can.

Materials

The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical disks.

Read Only:

The most common type of optical disk is the CD-ROM, which stands for Compact Disc - Read Only Memory. It looks just like an audio CD but the recording format is quite different. CD-ROM discs are used for computer software.

DVD used to stand for Digital Video Device or Digital Versatile Device, but now it doesn't really stand for anything at all! DVDs are used for recording movies.
 
The CDs and DVDs that are commercially produced are of the Write Once Read Many (WORM) variety. They can't be changed once they are created.

The data layer is physically molded into the polycarbonate. Pits (depressions) and lands (surfaces) form the digital data. A metal coating (usually aluminum) reflects the laser light back to the sensor. Oxygen can seep into the disk, especially in high temperatures and high humidity. This corrodes the aluminum, making it too dull to reflect the laser correctly.

CD-ROM and DVD-ROM disks should be readable for many, many years (100? 200?), but only if you treat them with respect.

Write Once:

The optical disks that you can record on your own computer are CD-R, DVD-R, and DVD+R discs, called writable or recordable disks.

The metal and data layers are separate.  The metal layer can be gold, silver, or a silver alloy.

Go for the Gold:  Gold layers are best because gold does not corrode. Naturally, the best is more expensive. Sulfur dioxide can seep in and corrode silver over time.

The data layer is an organic dye that the writing laser changes. Once the laser modifies the dye, it cannot be changed again. Write Once! Ultraviolet light and heat can degrade the organic dye.

Manufacturers say that these disks have a shelf-life of 5 - 10 years before they are used for recording. There is no testing yet about how long the data will last after you record it. Humph!

A writable disk is useful as a backup medium when you need long-term storage of your data. It is less efficient for data that changes often since you must make a new recording each time you save your data. Pricing of the disks will be important to your decision to use writable disks.

Rewrite:

An option for backup storage of changing data is rewritable disks, CD-RW, DVD-RW, DVD+RW, DVD+RAM.

The data layer for these disks uses a phase-changing metal alloy film. This film can be melted by the laser's heat to level out the marks made by the laser and then lasered again to record new data.

In theory you can erase and write on these disks as many as 1000 times, for CD-RW, and even 100,000 times for the DVD-RW types.

Advantages of Optical Disks

1. Physical: An optical disk is much sturdier than tape or a floppy disk. It is physically harder to break or melt or warp.

2. Delicacy: It is not sensitive to being touched, though it can get too dirty or scratched to be read. It can be cleaned!

3. Magnetic: It is entirely unaffected by magnetic fields.

4. Capacity: Optical disks hold much more data than floppy disks.

Plus, the non-data side of the disk can have a pretty label!

For software providers, an optical disk is a great way to store the software and data that they want to distribute or sell.

Disadvantages of Optical Disks

1. Cost: The main disadvantage has been cost.
   The cost of a CD-RW drive has dropped drastically and quickly. In 1995 such a drive was around $3000. In the summer of 1997 CD-RW drives were down to just under $1000. In March 2003 a CD-RW that will read at 40X speed, write on CD-R media at 40X speed,  and write on re-writable media at 12X,  could be bought for under $100 US!!

   The cost of disks can add up, too. Recordable disks (one time only) cost about $.30 US each (March 2003). Re-writable disks cost about $.64 to $.85 each.

   So for commercial use, the read/write drives are quite cost effective. For personal use, they are available, but may not be quite yet cheap enough to use for data storage for everyone.
    
   

2. Duplication: It is not as easy to copy an optical disk as it is a floppy disk. You need the software and hardware for writing disks! (This is an advantage as far as commercial software providers are concerned!) This is balanced by the fact that it is not as necessary to have extra copies since the disk is so much sturdier than other media.

Care of Optical Disks (CDs, DVDs)

Your CDs and DVDs are not going to last forever. They certainly store data longer than floppy disks! Mis-handling your optical disk can quickly make your data unreadable. Even fingerprints can do damage over time.

Data loss comes from:

• Physical damage - breaking, melting, scratching...

• Blocking of laser light by dirt, paint, ink, glue...

• Corrosion of the reflecting layer

Here are some do's and don'ts for keeping your CDs and DVDs healthy.

• Cleaning:

  • Keep it clean!

  • Handle by the edges or center hole.

  • Put it back in its case as soon as you are finished with it. No laying around on the desktop!!

  • Remove dirt and smudges with a clean cotton cloth by wiping from the center to the outer edge, NOT by wiping around the disk. Wiping in a circle can create a curved scratch, which can confuse the laser.

  • For stubborn dirt, use isopropyl alcohol or methanol or CD/DVD cleaning detergent.

• Labeling:

  • Don't use an adhesive label. The adhesive can corrupt your data in just a few months!

  • Don't write on or scratch the data side of the disk - ever!

  • Don't scratch the label side.

  • Don't write on the label side with a pencil or pen (scratches!)

  • Don't write on the label side with a fine-point marker or with any solvent-based marker. Use markers for CDs. (Solvent may dissolve the protective layer.)

• Storage:

  • Store optical disks upright on edge, like a book, in a plastic case designed specifically for them. Not flat for long periods!

  • Store in a cool, dark environment where the air is clean and dry. NO SMOKE! Low humidity.

• How you treat it:

  • Keep away from high heat and high humidity which accelerate corrosion.

  • Keep out of sunlight or other sources of ultraviolet light.

  • Keep away from smoke or other air pollution.

  • Don't bend it!

  • Don't use a disk as a coaster or a frisbee or a bookmarker!

Recording

• Check disk for flaws and dirt BEFORE recording on it.

• Only open a recordable disk just before you plan to record on it.

• After recording, make sure the disk works as you expect: Read data; run programs.

Complete recommendations from NIST as part of the "Digital Preservation Program" sponsored by the US National Institute of Standards and Technology, designed to give libraries and museums guidance on how to preserve digital data.
(in pdf format. Requires Adobe Acrobat Reader)