There are many different kinds of optical media, though most people lump them altogether and just call them all "CDs". All of these media are manufactured in a similar manner and comprise a polycarbonate base, a data layer, a layer of protective lacquer and finally a label substrate that can either be paper-based or just paint / ink. The key differences between each media type lie in the data layer and its composition, and in the specifications of the laser used to write the data.
Optical media use a laser to read from the surface of the data layer, the data being in the form of "pits” in the data layer. Depending on the specific type of media, the color and type of the lasers involved will change. There are two common methods of producing these “pits” on the data layer.
Producing Optical Media:The first method is used in mass producing a particular disc with the software, music or other digital information on it. When the polycarbonate disc is manufactured, it is formed on a master disc that contains a reverse “image” of the pits. The new disc retains these pits. A reflective layer is then formed over the data side of the disc, the protective lacquer is applied over the reflective coating and finally the label is applied by printing, generally using a silk screening method. This is a pressed CD. There are also pressed versions of DVD's.
The read laser on a drive sees these “pits” because the difference in height between a "pit" and the surrounding reflective media cause the light to reflect in different ways. Think of a braille written page, except with the bottom of the indentations shiny so light reflects off of them.
The second method is to use a “Burner” to “burn” the data in the data layer. This process actually starts when the Disc is manufactured. Prior to applying the reflective layer, a photo sensitive dye is applied to the polycarbonate disc. When a specific type of laser light is applied to this dye, it produces a spot that is transparent to “read lasers”. (The untouched surfaces remain opaque to the read laser). Now these spots can be read in the same way "pits" are read on the commercially manufactured discs.
DVD-R and DVD+R's are made with the same technology, a dye. But the RW versions of DVD's are made with a phase change metal alloy (Instead of with a dye and a reflective layer). Basically that means the parts hit by a “Write Laser” reflects light differently than the parts never exposed.
Different types of media:There are basically three common kinds of optical media. CD, DVD, and Blu-Ray, although there may be two more coming down the technology pipeline in the near future - the Holographic Versatile Disc and 5D DVD. Each of these discs are made differently from the others, though the basic structure is the same. The main differences are in the distance the reflective layer is from the read side, the reflective material, the number of layers etc. But the basic technology is the same - bounce a laser off a reflective surface and look for the differences in the reflected light.
Different specifications of Media:There are R discs (which means you can write to the disc until it is filled, or until closed. It cannot be erased), RW discs (you can write, erase and rewrite multiple times), DL discs (dual layer or two data layers), DS discs (Double Sided discs that can be read on both sides) and various combinations. Also there is a '+' and '–' which relates to the handling of the data. In general, most of the newer optical devices can use both, so the differences only become a factor when using older readers or players.
The most common CD media that you will see are CD's, CD-R's and CR-RW's. They are in order, pressed, write once (until full, or closed. They are not erasable.), and rewritable / erasable CD's. There are only single layer single sided CD's available.
The most common DVD media you will see are DVD, DVD-R, DVD-RW, DVD+R and DVD+RW all also in the double sided and / or double layered versions as well. The DVD is pressed commercially with the information. The R versions are write until the disc is filled, or closed (it cannot be erased), and the RW are write and erase capable, up to about 1000 times. The “+” type of disc can be written to faster and supports drag and drop files. Which basically means if you want to use a DVD disc as a spare hard drive, you need to use a DVD+RW disc and drive.
There is one more variation that can throw you if you are unaware of it. CD and DVD's also come in several sizes: The standard 12 cm size, a smaller 8 cm size and another that is about the size of the 8 cm disc, but has the top and bottom clipped off so it becomes the size of a business card.
Types of Media: This list is provided for comparison only.Here is a fairly complete list of all the different kinds of media in all of their variations. Some of them are no longer in production, others still in the development stage.
* Blu-ray Disc (BD): BD-R, BD-RE
* DVD: DVD-R, DVD+R, DVD-R DL, DVD+R DL, DVD-R DS, DVD+R DS, DVD-RW, DVD+RW, DVD-RAM, DVD-D, HVD, EcoDisc
* Compact Disc (CD): Red Book, CD-ROM, CD-R, CD-RW, 5.1 Music Disc, SACD, PhotoCD, CD Video (CDV), Video CD (VCD), SVCD, CD+G, CD-Text, CD-ROM XA, CD-i
* Universal Media Disc (UMD)
* Enhanced Versatile Disc (EVD)
* Forward Versatile Disc (FVD)
* Holographic Versatile Disc (HVD)
* China Blue High-definition Disc (CBHD)
* HD DVD: HD DVD-R, HD DVD-RW, HD DVD-RAM
* High definition Versatile Multilayer Disc (HD VMD)
* VCDHD
* GD-ROM
* MiniDisc (MD) (Hi-MD)
* Laserdisc (LD)
* Video Single Disc (VSD)
* Ultra Density Optical (UDO)
* Stacked Volumetric Optical Disc (SVOD)
* Five dimensional discs (5D DVD)
* Nintendo optical disc (NOD)
