Blu-ray is one of two formats that use a blue-violet laser instead of a red laser to read or write optical information on a disc. The laser, very near the ultra-violet range, has a much smaller wavelength of 405 nanometers compared to the ruby red, 650 nanometer wavelength of lasers used for DVDs. (CDs use a near-infrared laser of 780 nanometers. Remember the rainbow colors--red is at the low end of the visible spectrum; violet is at the high end of the visible spectrum.)
The smaller wavelengths allows smaller pits and smaller tracks on a disc so that more information can be molded into the disc.
Blu-ray data discs came out about three years ago, before the video version.
Sony planned to use
MPEG-2
compression, the same as that on DVDs, for high definition video; and that meant packing as much information on a disc as was possible. To do that, Sony moved the laser as close to the disc as possible and moved the data information to the very bottom of the disc. The risk of damage was minimized by the use of a cartridge; but when Panasonic's
DVD-
RAM was hindered by its cartridge, Sony decided to get rid of the cartridge and coat the bottom of the disc with a thin layer of hard coating using microsilica and flourine.
Toshiba and
NEC, on the other hand, figured that using the more efficient
MPEG-4 encoding or
VC-1 encoding would allow high definition video on a disc made just like regular DVDs, with the data stored in the middle of a 2-piece plastic "sandwich." Rather than develop special lenses and cartridges to squeeze out 25GB of storage, the alternate
HD DVD could get the same quality of video with 15GB and use the same assembly equipment as that used for DVDs.
Sony's initial releases on Blu-ray discs used
MPEG-2 on 25GB discs and looked inferior to HD DVD releases using VC-1 on double layer 30GB discs. Now Sony has also switched to the more efficient encoding, and double layer 50GB Blu-ray discs are also beginning to appear. Any difference in video quality between Blu-ray and HD DVD is now more a result of differences in mastering than in the formats themselves.
