How Blu-ray works

Before cassettes, CDs, DVDs and Blu-rays were invented, music was stored in LP (Long Playing) Records. A LP uses a spiral grove carved into a plate of plastic. This groove consists of many tiny bumps and dips that are run over by a needle. The needle translates the resulting vibrations into electrical impulses. The electrical impulses are carried by wire to an amplification device and put out through speakers in an audible form.

With the advant of digital music, now instead of these groves storing analog signals i.e. variation of bumps and dips, there was a need to store digital data ie 0s and 1s. Hence there was a need to store only same sized holes (pits) or No holes (Land) in grooves - i.e. 0s and 1s.

Hence CDs, DVDs, Blurays store digitally encoded video and audio information in spiral grooves which as pits or flat surface that run from the center of the disc to its edges. A laser reads these pits to read the digital data, and the player, computer or software interprets the data and plays the movie,music or program that is stored on the DVD. The distance between the groves on the disc is called track pitch. The more data that is contained on a disc, the smaller and more closely packed the pits must be. i.e. Less the track pitch, less distance between the pits and smaller the pits. The smaller the pits the more precise the reading laser must be.

DVDs uses a red laser to read and write data. However Bluray uses a blue laser (Hence the name). A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely hence enabling it to read information recorded in smaller pits that are only 0.15 microns (µm)long which is more than twice as small as the pits on a DVD. Blu-ray also has a reduced track pitch from 0.32 microns vs the 0.74 microns of a DVD. The smaller beam smaller pits and shorter track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information - five times the amount of information that can be stored on a DVD.

Each Blu-ray disc is about the same thickness as a DVD. But the two types of discs store data differently. In a DVD, the data is between two polycarbonate layers, each 0.6 mm thick. Having a polycarbonate layer on top of the data can cause a issue called birefringence - i.e. the decomposition of a ray of beam into two beamss. If the beam is split too widely, the disc cannot be read. Also, if the DVD surface is not exactly flat, and hence not exactly perpendicular to the laser beam, it can lead to a problem known as disc tilt. The Blu-ray disc overcomes these 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 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.