Reading with light
Optical discs store information in a pattern that a laser can read. In a typical pressed disc, a spiral track contains tiny pits and flat areas called lands. A laser beam shines onto the surface and the reflected light changes depending on whether it hits a pit or a land. A sensor turns these changes into a stream of bits. Because nothing touches the disc surface, the reading process does not wear it in the way a needle or a magnetic head does.
From LaserDisc to compact disc
The idea of using light to read recorded video was developed in the 1960s and 1970s, and it led to the LaserDisc, a large analog video disc sold to consumers and used in some training and reference applications. It was a niche product, but it showed that optical media could work in the real world.
The compact disc followed. Philips and Sony cooperated on a digital audio format, and compact discs reached the public in the early 1980s. A CD stored music as digital samples, which meant that copying and playing it did not add the hiss and wear typical of tape and vinyl. The standard disc held roughly 74 to 80 minutes of audio, and later versions of recordable discs were commonly sold at 700 megabytes for data. People often mention a story that the playing time was chosen to fit a particular symphony. The details are debated, so treat it as a popular tale rather than settled history.
CD-ROM
Because a CD is already digital, it was a short step to use the same physical discs for computer data. The CD-ROM, which became available in the mid-1980s, offered a large amount of read-only storage that was cheap to mass-produce. In the early days, this was a major jump in capacity compared with floppy disks and many hard drives.
The first uses were large reference works, such as encyclopedias, dictionaries and databases. In the 1990s, CD-ROM became a standard way to distribute software, games and multimedia. Programs that mixed text, images, audio and video, often called multimedia titles, were practical for the first time because the disc could hold all those files. Many personal computers came with a CD-ROM drive as a standard feature by the middle of that decade.
Game makers made heavy use of the format, since CD-ROM could hold long music tracks and video clips, and some video game consoles adopted disc drives in place of cartridges. That shift is described in the story of game consoles and the cartridge era.
DVD and Blu-ray
The DVD arrived in the middle of the 1990s with a similar size and appearance to a CD, but with finer pits, tighter track spacing and a shorter-wavelength laser. A single-layer DVD holds about 4.7 gigabytes, many times a CD. It was designed for feature films, with compression making a full movie fit on one disc, and it also served as a data medium.
Blu-ray discs, introduced in the 2000s, used a blue-violet laser with an even shorter wavelength, allowing still smaller pits. A single-layer Blu-ray holds about 25 gigabytes, with dual-layer discs holding more. A format contest between Blu-ray and a rival high-definition format, HD DVD, ended in the late 2000s with Blu-ray as the survivor.
Recordable and rewritable discs
Pressed discs are produced in factories, which is efficient for large runs. For individuals and small organizations, recordable versions arrived: write-once discs, where a laser burns dye or alters a layer, and rewritable discs, which use a material that can change state repeatedly. Home users could make music mixes, back up files or share photos.
These discs were convenient but their durability was less certain than many people assumed. Recordable discs depend on dyes and layers that can degrade over time, and quality varied widely among brands and storage conditions. Anyone relying on them for long-term storage has reason to check the discs occasionally and keep other copies, a concern linked to digital decay.
Why discs lost ground
Several changes pushed optical discs aside:
- Broadband internet made downloading and streaming practical for software, music and video.
- USB flash drives and memory cards offered portable storage with no special drive.
- Laptops became thinner, and many makers dropped built-in disc drives.
- Hard drives and later solid-state drives made large local libraries easy to keep.
By the 2010s, many new computers shipped without optical drives. Discs have not disappeared entirely. They persist in some video game, film and software releases, in libraries, and in archives that rely on long-lived formats. Specialized forms of optical storage continue to be developed for archival use.
What they contributed
Optical discs were important because they made distribution cheap. A factory could press a disc for a small cost, and the buyer had a large amount of data in a form that could be read by many machines. That helped bring multimedia into the home and made the personal computer a platform for entertainment, education and reference as well as work. Their history also reminds us that physical media tend to be replaced when networks and small solid-state devices make the physical object unnecessary.