Compared with traditional DVD, Blu-ray disc not only has more storage capacity, but also provides a new level of interaction. Users can connect to the Internet and immediately download movies subtitles and other interactive characteristics.
What you can do with a Blu-ray disc:
Record high-definition television (HDTV) without any quality loss;
Record one program while watching another on the disc;
Edit or reorder programs recorded on the disc;
Automatically search for an empty space on the disc to avoid recording over a program;
Instantly skip to any spot on the disc;
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.
A disc used to store digital encoded audio and video information in pits, the concrete form is spiral grooves running form the center to the edges on the disc. Then use a laser to read the information stored on it, so movies or other programs stored on it can be played. More data, which included in the disc, smaller and more densely pits must. This small pit (hence the bumps), more accurate reading laser must.
Traditional DVD uses red laser to read and write data, while Blu-ray disc uses blue laser. The blue laser is in a shorter wavelength (405 nanometers) than the red laser (650 nanometers). This smaller beam makes it more accurate reading record information in a pit only 0.15 microns (µm) (1 micron = 10-6 meters) long. This is twice smaller than the pits on a DVD. As well, Blu-ray can reduce the track pitch from 0.74 microns to 0.32 microns. That three aspects, the smaller pits, smaller beam and the shorter track pitch, make the Blu-ray disc hold more than 25(GB) information, which is about five times of the amount of information stored on a DVD.
Each Blu-ray disc is almost in the same thickness (1.2 millimeters) as a DVD. But the type of store data is different from DVD. On a DVD, data is sandwiched between two layers, each is 0.6 mm thick. The birefringence problem may be caused by having a polycarbonate layer on top of the data. In this situation, the substrate layer separates the laser light into two beams. If the beam split apart too widely, discs cannot be read. Similarly, if the disc surface is not smooth enough, there would be not exactly perpendicular to the beam. This can lead to problems called disc tilt, in which the laser beam is distorted. All of these problems are involved in the manufacturing process.