How Blu-ray Discs Work

Blu-ray discs placed in a box
Blu-ray discs can hold and playback large quantities of HD video and audio. The Real Tokyo Life / Getty Images

In 1997, a new technology emerged that brought digital sound and video into homes all over the world. It was called DVD, and it revolutionized the movie industry.

The industry is set for yet another revolution with the introduction of Blu-ray Discs (BD) in 2006. With their high storage capacity, Blu-ray discs can hold and play back large quantities of high-definition video and audio, as well as photos, data and other digital content.


In this article, HowStuffWorks explains how the Blu-ray disc works and how it was developed, and we'll see how it stacks up against some other new digital video formats on the horizon.

A current, single-sided, standard DVD can hold 4.7 GB (gigabytes) of information. That's about the size of an average two-hour, standard-definition movie with a few extra features. But a high-definition movie, which has a much clearer image (see How Digital Television Works), takes up about five times more bandwidth and therefore requires a disc with about five times more storage. As TV sets and movie studios make the move to high definition, consumers are going to need playback systems with a lot more storage capacity.

Blu-ray is the next-generation digital video disc. It can record, store and play back high-definition video and digital audio, as well as computer data. The advantage to Blu-ray is the sheer amount of information it can hold:

  • A single-layer Blu-ray disc, which is roughly the same size as a DVD, can hold up to 27 GB of data -- that's more than two hours of high-definition video or about 13 hours of standard video.
  • A double-layer Blu-ray disc can store up to 50 GB, enough to hold about 4.5 hours of high-definition video or more than 20 hours of standard video. And there are even plans in the works to develop a disc with twice that amount of storage.

Source: White Paper: Blu-ray Disc Format

We'll learn more about the differences between Blu-ray discs and DVDs in the next section.


Building a Blu-ray Disc

Source: Blu-ray Disc Association

Blu-ray discs not only have more storage capacity than traditional DVDs, but they also offer a new level of interactivity. Users will be able to connect to the Internet and instantly download subtitles and other interactive movie features. With Blu-ray, you can:

  • record high-definition television (HDTV) without any quality loss
  • instantly skip to any spot on the disc
  • record one program while watching another on the disc
  • create playlists
  • edit or reorder programs recorded on the disc
  • automatically search for an empty space on the disc to avoid recording over a program
  • access the Web to download subtitles and other extra features

Discs store digitally encoded video and audio information in pits -- spiral grooves that run from the center of the disc to its edges. A laser reads the other side of these pits -- the bumps -- to play the movie or program that is stored on the DVD. The more data that is contained on a disc, the smaller and more closely packed the pits must be. The smaller the pits (and therefore the bumps), the more precise the reading laser must be.


Unlike current DVDs, which use a red laser to read and write data, Blu-ray uses a blue laser (which is where the format gets its name). A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only 0.15 microns (µm) (1 micron = 10-6 meters) long -- this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced the track pitch from 0.74 microns to 0.32 microns. The smaller pits, smaller beam and shorter track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information -- about five times the amount of information that can be stored on a DVD.

Each Blu-ray disc is about the same thickness (1.2 millimeters) as a DVD. But the two types of discs store data differently. In a DVD, the data is sandwiched between two polycarbonate layers, each 0.6-mm thick. Having a polycarbonate layer on top of the data can cause a problem called birefringence, in which the substrate layer refracts the laser light into two separate beams. If the beam is split too widely, the disc cannot be read. Also, if the DVD surface is not exactly flat, and is therefore not exactly perpendicular to the beam, it can lead to a problem known as disc tilt, in which the laser beam is distorted. All of these issues lead to a very involved manufacturing process.

Learn how Blu-ray overcomes these obstacles in the next section.


How Blu-ray Reads Data

Source: Blu-ray Disc Association

The Blu-ray disc overcomes DVD-reading issues by placing the data on top of a 1.1-mm-thick polycarbonate layer. Having the data on top prevents birefringence and therefore prevents readability problems. And, with the recording layer sitting closer to the objective lens of the reading mechanism, the problem of disc tilt is virtually eliminated. Because the data is closer to the surface, a hard coating is placed on the outside of the disc to protect it from scratches and fingerprints.

The design of the Blu-ray discs saves on manufacturing costs. Traditional DVDs are built by injection molding the two 0.6-mm discs between which the recording layer is sandwiched. The process must be done very carefully to prevent birefringence.


  1. The two discs are molded.
  2. The recording layer is added to one of the discs.
  3. The two discs are glued together.

Blu-ray discs only do the injection-molding process on a single 1.1-mm disc, which reduces cost. That savings balances out the cost of adding the protective layer, so the end price is no more than the price of a regular DVD.

A BD-ROM disc researcher holds a disc up to the light.
Photo courtesy Blu-ray Disc Association

Blu-ray also has a higher data transfer rate -- 36 Mbps (megabits per second) -- than today's DVDs, which transfer at 10 Mbps. A Blu-ray disc can record 25 GB of material in just over an hour and a half.

We'll look at some of Blu-ray's competitors in the next section.


Blu-ray Competitors

Samsung BD-P1000 Blu-ray player
Photo courtesy photo courtesy Samsung

Will Blu-ray replace previous DVDs? Its manufacturers hope so. In the meantime, JVC has developed a Blu-ray/DVD combo disc with an approximate 33.5-GB capacity, allowing for the release of video in both formats on a single disc. But Blu-ray is not alone in the marketplace. A few other formats are competing for a share of the DVD market.

The other big player is HD-DVD, also called AOD (Advanced Optical Disc), which was developed by electronics giants Toshiba and NEC. HD-DVD was actually in the works before regular DVD, but it didn't begin real development until 2003.


The advantage to HD-DVD is that it uses the same basic format as the traditional DVD and can therefore be manufactured with the same equipment, saving on costs. HD-DVD matches the storage capacity of Blu-ray. A rewritable, single-layer HD-DVD can hold 15 GB of data, a double-layer disc can hold 30 GB, and a triple-layer disc can hold 45 GB (that's compared to 27 GB and 50 GB for Blu-ray). The read-only versions hold slightly less data. Also, HD-DVD offers the interactive capabilities of Blu-ray, with HDi.  For more information on HD-DVD, check out How HD-DVD Works.

Blu-ray and HD-DVD are the two major competitors in the market, but there are other contenders, as well. Warner Bros. Pictures has developed its own system, called HD-DVD-9. This system uses a higher compression rate to put more information (about two hours of high-definition video) on a standard DVD. Taiwan has created the Forward Versatile Disc (FVD), an upgraded version of today's DVDs that allows for more data storage capacity (5.4 GB on a single-sided disc and 9.8 GB on a double-sided disc). And China has introduced the Enhanced Video Disc (EVD), another high-definition video disc.

There are also professional versions of the blue laser technology. Sony has developed XDCAM and ProData (Professional Disc for Data). The former is designed for use by broadcasters and AV studios. The latter is primarily for commercial data storage (for example, backing up servers).

It seems that the future holds a whole lot more than 25 to 54 GB on a single disc. According to T3: Pioneer goes beyond Blu-Ray, Pioneer is developing an optical disc that will blow away the hard disc in most of our PCs in terms storage capacity, holding 500 GB of data. How so? Pioneer's lasers are ultraviolet, which have an even shorter wavelength than the blue.

For more information on Blu-ray and related topics, check out the links on the next page.



Frequently Answered Questions About Blu Ray

What is Blu-ray Disc?
Blu-ray Disc is a high-capacity optical disc format designed for the storage of high-definition video, audio, and computer data. A Blu-ray disc can hold up to 256GB of data.
Are Blu-ray discs obsolete?
No, Blu-ray discs are far from being obsolete.
What is the difference between DVD & Blu-ray?
The difference between DVD and Blu-ray is that Blu-ray has a higher resolution than DVD.

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