Blu-Ray technology was developed in February 2002 to be DVD’s successor by a consortium made by companies that include Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson. HD-DVD, on the other hand, was created by Toshiba and recently got support from Microsoft, HP and Intel.

Both Blu-Ray and HD-DVD discs have the same physical size of DVD discs (and CDs), with a diameter of 12 cm (120 mm, around 4 ¾”).

The key to both Blu-Ray and HD-DVD technologies is the use of a blue-purple laser to read and write data from the disc. This laser has a smaller wavelength compared to the red laser used by DVD players, allowing a higher recording density.

During the recording of a Blu-Ray or HD-DVD disc a high-power blue-purple laser is used to burn pits on the disc surface. The areas than weren’t burned are known as lands.

During the reading of a Blue-Ray or HD-DVD disc a blue-purple laser with lower intensity is thrown on the disc surface. When the light reaches a pit it is reflected back with a lower intensity compared when it hit a land. As the pit height corresponds to ¼ of the laser wavelength, the reflected light reaches the photosensor found on the Blu-Ray or HD-DVD player half wavelength dephased (¼ on its way in + ¼ on its way back = ½). This is how the Blu-Ray or HD-DVD player can recognize if an area is a pit or a land: the amount of light reflected by a land is higher than the amount reflected by a pit.

You could think that each spot where the light is reflected with a higher intensity (land) equals to a “1” bit while each spot where the light is reflected with a lower intensity (pit) equals to a “0” bit. However Blu-Ray and HD-DVD players use the transition between pits and lands or lands and pits to represent a “1” bit and the absence of such transitions to represent a “0” bit.

Each bit read this way is called an optical bit. They still do not represent directly a data “0” or “1”. A process known as modulation is still required to convert optical bits into data bits. HD-DVD uses a modulation technique called ETM (Eight to Twelve Modulation), which converts each group of eight data bits (one byte) into 12 optical bits. Blue-Ray uses a modulation technique called 17PP that converts a variable number of optical bits in data bits based on rules defined by the modulation algorithm.

Just like what happens on CDs and DVDs, on Blu-Ray and HD-DVD discs pits and lands are organized into a single spiral track that starts at the center of the disc and ends at the border of the disc. The smaller the distance between the spiral tracks, more data can be stored on the disc. On Blu-Ray discs this distance is smaller than on HD-DVD discs (0.32 µm vs. 0.40 µm, respectively).

Also, on Blu-Ray discs each pit measures 0.13 µm, while on HD-DVD discs they measure 0.20 µm.

But why do HD-DVD discs use bigger pits compared to Blu-Ray if both use a laser with the same wavelength? The answer is the distance between the recording layer and the lacquer layer. On Blu-Ray discs the recording layer is only 0.1 mm distant from the lacquer layer, which allows a higher recording density (HD-DVD discs use the same distance as DVDs, 0.6 mm). On the other hand Blu-Ray discs must be handled more carefully, as any scratch on its surface can easily reach the recording layer, compromising the recorded data. That is why some Blu-Ray discs come inside a caddy in order to avoid physical damage.

As we can see, technologically speaking Blu-Ray and HD-DVD are very similar, the main difference between the two being the storage capacity.

A single-layer Blu-Ray disc can hold up to 25 GB, while a single-layer HD-DVD disc can hold up to "only" 15 GB. A dual-layer Blu-Ray disc can store up to 54 GB, while a dual-layer HD-DVD can store only up to 30 GB.

On the table below we summarized the main technical specs of Blu-Ray and HD-DVD discs.

Feature	Blu-Ray	HD-DVD
Capacity	25 GB (single layer) 54 GB (dual layer)	15 GB (single layer) 30 GB (dual layer)
Laser	Blue-violet (0.40 µm)	Blue-violet (0.40 µm)
Pit width	0.13 µm	0.20 µm
Distance between tracks	0.32 µm	0.40 µm
Distance between the recording layer and the lacquer layer	0.1 mm	0.6 mm
Supported codecs	MPEG-2 MPEG-4 AVC VC-1	MPEG-2 MPEG-4 AVC VC-1

On the table below you can see the main entertainment-related companies (i.e. movies and games) that support each technology.

Blu-Ray	HD-DVD
20th Century Fox Buena Vista Home Entertainment Electronic Arts MGM Studios Paramount Pictures Sony Pictures Entertainment The Walt Disney Company Vivendi Universal Games Warner Bros.	Buena Vista Home Entertainment New Line Cinema Paramount Pictures The Walt Disney Company Universal Studios Warner Bros. The Weinstein Company/Genius