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Blu-ray Disc Blu-Ray Disc Logo

Blu-ray Disc (BD) was developed for home entertainment high-definition video. Blu-ray Disc uses a violet laser with a wavelength of 405 nm (barely inside the visible limit at 400 nm)[1] that can be focused to a smaller spot than red or infrared lasers. This allows a single-layer Blu-ray disk to hold 25 GiB of data. Up to four layers are possible, giving a capacity of up to 128 GiB.

During its development, Blu-ray competed with a similar format called HD DVD (High-Density Digital Versatile Disc). The main differences between Blu-ray and HD DVD were that HD DVD held 15 GiB (single layer) and that HD DVD had a digital rights management system preventing theatrical movies from being played on computers with HD DVD drives. Theatrical movies were sold on Blu-ray and HD DVD for a short time. However, the Blu-ray format proved more popular, and the HD DVD was withdrawn from the market in 2008.

Unlike CDs and DVDs, Blu-ray disks have no stacking ring. Instead, they have a durable coating that resists scratches. Before this coating was developed, Blu-ray disks came in a plastic cartridge inserted into the drive or player with the disk.

BD-R and BD-RE

Like its earlier counterparts, Blue-ray has recordable and recordable-erasable versions. Recordable Blu-ray disks may have up to four recordable layers and hold up to 128 GiB of data.

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1Most people can see wavelengths shorter than 400 nanometers, but 400 nm is where average sensitivity has dropped significantly and it the official limit of human vision at the violet end of the spectrum.
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