What is Blue Ray? 

What is blue ray or Blu-ray? Blue ray is the newest 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.

What is Blue Ray Capacity

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 Blu-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, which always only has one data storage layer. 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 as can be found in the BD-R format. The amount of information layers and capacity depends on the Blu-ray disc format and these formats are discussed later on this page.

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. An illustration of this is shown in the image below. 

The laser wavelength for reading a CD at 780 nm and in the far red of the light spectrum, a DVD at 650 nm and in the near red, and a Blu-ray disc at 405 nm and in the blue/violet wavelength region.

CDs require a laser with 780 nm wavelength. DVDs require a laser with 650 nm wavelength. For blue ray discs, the wavelength is 405 nm, which is actually more in the violet range of the light spectrum than in the blue region. Despite this fact, probably for marketing purposes, the disc is called Blu-ray instead of Violet-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. This means more than one laser diode in the drive, if multiple formats need to be read.

Why are different lasers required? The smaller the wavelength of the laser, the smaller the beam spot size (that reads the disc) can be made. 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. As shown in the image, the laser light spot size used to read a CD would read many spots at the same time on a Blu-ray disc and the information would not be interpreted properly.

The relative size of data pits or spots of a CD, DVD, and Blu-ray discs. The laser beam spot is smallest on Blu-rays and largest on CDs.

Blu-ray Formats

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 Blu-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 rewritable Blu-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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