What is blue ray or blu ray? Blue ray is the next generation 4.75 inch optical disc format. The first format in this size was the compact disc and it was introduced in 1982. Next, a higher capacity disc appeared (4.7 GB compared to 700 MB for the CD) in the marketplace in 1997 and this was the DVD. The higher capacity of the DVD allowed good quality video to be stored on a small size optical disc.
In 2006, a compact disc sized disc with even greater capacity (25 GB) than a DVD was introduced. This generation of optical disc is called the blue ray disc. This disc allows high definition video to be stored on the disc media, something that is not possible on the lower capacity DVD. In summary, a single layer blu ray disc can store 5.3 times more information than a single layer DVD disc and 35.7 times more information than a CD. Note that both DVDs and blu ray disc media can have two information layers as well. This makes it possible to store 8.5 GB on a dual layer DVD and 50 GB on a dual layer blue ray disc. Blu-ray discs with more information layers are also possible and should be released in 2011.
The Blu-ray Laser
In answering the question what is blue ray, another important difference other than capacity needs to be discussed. Blu ray disc media requires a different laser to read the disc. CDs require a laser with 780 nm wavelength. DVDs require a laser with 640 nm wavelength. For blue ray discs, the wavelength is 405 nm or in the blue region of the light spectrum. This is the principle reason why the discs are referred to as blu-ray. The need for different lasers is why not all players can read all disc formats. Players or drives must be equipped with the laser diodes that can produce laser light at the specified wavelength required by the disc type in order for those formats to be readable or recordable. Why are different lasers required? The smaller the wavelength of the laser, the smaller the beam spot size. Blu ray disc media have very small marks on the discs that represent the digital information and reading these very small marks with a laser with a large spot size would be impossible to do correctly due to interference from adjacent marks. DVDs have larger marks and CDs even larger marks still and this is why higher wavelength lasers with larger spot sizes can be used for these discs.
Finally, a third important answer to the question of what is blue ray is the different formats that are available for this disc type. Like CDs and DVDs there are three categories of blue ray discs. The read-only movie disc is usually known simply as a blu-ray disc or blue ray disc. These are discs that are purchased with movies already on them. All that can be done with the discs is to play them. The second type of blu ray disc is the recordable disc or BD-R. These discs are purchased blank and information is recorded to the disc until it is full. Once the disc is full, it cannot be erased and reused. The third type is the BD-RE or erasable blue ray disc. BD-REs are purchased blank. Information is recorded to the disc, but in this case it can be erased and reused.
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