Optical storage is defined by IBM as any storage medium that uses a laser to read and write the information. The use of lasers means that more information can be packed into a smaller space than tape could manage (at the time)! Better quality and longer media time are natural results. A laser burns information into the surface of the media, and then the reading laser, which is less powerful, can decipher these burnt areas into usable data. The surface is usually some sort of metal or dye sandwiched between protective layers of plastic that burns easily, producing ‘pits’ or less reflective areas for the laser to read.
This is why fingerprints and scratches can pose such a problem for reading data; even though you aren’t damaging the actual data storage, like you would be if you scratched a hard drive disk, fingerprints prevent the laser from being able to read the data. Scratch up the plastic layer above the dye, and the data’s as good as destroyed.
Destroying data can be even more complete than that, even. Shredding the disc in a capable paper shredder (ONLY IF IT SAYS IT CAN SHRED DISCS) destroys the data, as does microwaving the disc. Don’t microwave the disc unless you plan on trashing the microwave soon, though. Most discs contain some amount of metal, and that can wear the microwave out faster. Fun!
CDs
“Burning a CD” replaced “making a mix tape” when both CDs and downloadable music were available to teenagers, and for good reason. The amount of content may be roughly the same, but the quality is significantly higher.
Most CDs are CD-Rs – disks that can only be written on once but can be read until the end of time. A CD-ROM is just a CD-R that’s been used! The average CD-R has room for about an album’s worth of music, and maybe a hidden track or two, about 75-80 minutes depending on the manufacturer of the disc. Alternatively, if you’d like to store data instead of high-quality audio, you’ll get about 700 MB of data onto a single disc.
To burn a CD, you’d need an optical drive that’s capable of also lasering information into the disc, which wasn’t always the standard. The laser will burn the information into the metal-dye mix behind the plastic coating the outside of the disc, which permanently changes how reflective those sections are. This makes it possible to visually tell what has and hasn’t been used on a disc yet, and CD-Rs can be burnt in multiple sessions! Data is typically burnt from the center outwards.
But everybody knows about CD-Rs. What about CD-RWs, their much fussier brethren?
CD-RW
The primary difference between a CD-R and a CD-RW is the dye used in the layers that the optical drives can read. CD-RWs are burnt less deeply than CD-Rs, but as a result, they take a more sensitive reader. Early disc readers sometimes can’t read more modern CD-RWs as a result!
To reuse the disc, one has to blank it first (the same drive that can write a CD-RW in the first place should also be able to blank it), which takes time. After it’s been wiped, new data can be put onto the disc again. CD-RWs wear out quicker than other memory media as a result of their medium. That wafer-thin dye layer can only handle being rearranged so many times before it loses the ability to actually hold the data. It’s pretty unlikely that the average user could hit that re-write limit, but it’s more possible than, say, a hard drive, which has a re-write life about 100 times longer than the re-write life of a CD-RW.
DVDs
DVDs store significantly more data than CDs do, even though they take up about the same space. Where a CD can hold about 700 MB, a DVD can hold up to 4.7 GB. This is enough for most movies, but if the movie is especially long or has a lot of other extra features, it has to be double layered, which can store up to 9 GB. Why can it hold so much more in the same space?
The long answer is that there are a number of small differences that ultimately lead to a DVD having more burnable space, including a closer ‘laser spiral’ (the track a laser burns, like the grooves in a vinyl record), as well as smaller readable pockets. It all adds up into more data storage, but a more expensive product as well.
DVD +R DL
That double-layering mentioned earlier isn’t present on every disc. Sometime in the later 2000s, double layer discs hit the market at about the same price as single layer discs (although that changed over time). The first layer that the laser can read is made of a semi-transparent dye, so the laser can penetrate it to reach the other layer.
Most modern DVD drives can read dual layer, but if your computer is especially old, it would be wise to check its specs first – DVD readers programmed before their release might not understand the second layer, and readers that can read them might not be able to write to them. DLs are a great invention, it’s just a struggle to find good disc readers when everything is switching to digital.
Compatibility
CD players aren’t usually also able to play DVDs. CDs came first, and the reader would have to be forwards compatible. Obviously, this would have taken a time machine to actually assemble. Picture expecting a record player to read a CD! The gap between the two is almost that large. Nowadays, the manufacturing standard seems to be a DVD player with CD compatibility tacked on. You should double check before you buy a disc reader to be sure it can do everything you want it to, but it’s less common to see CD-Only tech when a DVD reader is only slightly more expensive to create, and can work backwards.
DVDs also carve out pits (or burn marks) into the shiny material of the disk. Just like CDs, a DVD can only be written on once, although DVD-RWs do exist (and struggle like CD-RWs do).
FlexPlay Self-Destructing Entertainment
Remember FlexPlay self-destructing entertainment? The disc that was meant to simulate a rental and could have generated literal tons of trash per family, per year? The self-destructing medium that the disc was coated in turned very dark red to thwart the disc reader’s lasers! The pits aren’t directly on the surface of the DVD, they’re under a couple of layers of plastic. All FlexPlay had to do was sandwich an additional layer of dye between the plastic and the metal/dye that’s being inscribed upon. When that dye obscures the data below it, it’s as good as gone! The laser can no longer get through to the information and read it. Even Blu-Ray tech was thwarted by the dye.
Blu-Ray
Blu-Ray discs have higher visual quality than DVDs because they hold even more information. The blue-ray technology enables the pits to be even closer together, so more optical data can be crammed into the same space. Blue light has a shorter wavelength than red light, which shrinks the necessary pit size! A single-layer Blu-Ray disc can hold up to 25 GB of information! Blu-Ray discs are most commonly used for entertainment media rather than storage. Disc readers have to be specifically compatible with that blue laser technology, rather than just programmed for it. An ordinary DVD player may be able to play a CD, but it wouldn’t be able to fully read a pit in a Blu-Ray disc before that pit’s passed the reader.
Right now, the state of the art is Blu-Ray: most good Blu-Ray readers are backwards compatible with DVDs and CDs. However, many companies still sell ordinary DVDs alongside their Blu-ray releases due to cost. If you have a DVD player, you can probably hold off on upgrading, at least for a little while longer.
Sources:
https://www.britannica.com/technology/optical-storage
http://www.osta.org/technology/cdqa13.htm
https://www.techrepublic.com/article/all-about-cd-r-and-cd-rw/
https://www.scientificamerican.com/article/whats-a-dvd-and-how-does/
https://kodakdigitizing.com/blogs/news/cd-vs-dvd-how-are-they-different
http://recordhead.biz/difference-blu-ray-dvd/
https://www.dell.com/support/kbdoc/en-us/000147805/guide-to-optical-disk-drives-and-optical-discs