Blu-ray Disc

Blu-ray Disc is an optical disc storage format developed as a successor to the DVD, designed mainly to hold high-definition video and large amounts of data. It is the same physical size as a CD or DVD but stores considerably more information. The key to this larger capacity is the laser: where CD players use an infrared laser and DVD players a red one, Blu-ray uses a blue-violet laser with a shorter wavelength. A shorter wavelength can be focused to a smaller spot, so the data pits on the disc can be packed more closely together, raising the amount of data that fits on one surface.

The greater capacity allowed Blu-ray to carry full high-definition movies with high-quality multichannel sound, which was its main consumer purpose when it competed with and outlasted the rival HD DVD format. Blu-ray discs come in read-only pressed form for published content, as well as recordable and rewritable versions that let users write their own data, the rewritable type supporting repeated erasing and re-recording much as rewritable CDs and DVDs do. Dual-layer discs increase capacity further by stacking two recorded layers that the laser can address separately.

A Blu-ray player or drive reads the disc by detecting how the focused laser light reflects from the pattern of pits and lands along the spiral track, converting that pattern back into data. Drives are generally designed to also read the older CD and DVD formats, since they can switch to the appropriate laser and optics, giving backward compatibility with earlier discs.

Frequently asked questions

Why is it called Blu-ray?

The format uses a blue-violet laser to read and write the disc, and that blue light gives the format its name.

How does Blu-ray store more than a DVD?

Its shorter-wavelength laser focuses to a smaller spot, allowing the data pits to be packed more tightly, so much more data fits on a disc of the same size.

Can a Blu-ray drive read CDs and DVDs?

Most Blu-ray drives are backward compatible and include the optics to read the older CD and DVD formats as well as Blu-ray discs.