Ordinary CDs and DVDs use a laser that produces red light (whose wavelength is about 650nm, a little more than half a micron). Blu-ray and HD-DVD units use a blue-light laser, with wavelength of 405nm (a little less than half a micron). That tiny difference in wavelength makes a large difference to many people.

Standard DVDs can hold about 4.7GB of data. That data can be computer documents or thanks to clever software movies or audio. That permits storing a two-hour film and a few extras to display on standard TV sets. It isn't nearly enough to store a high-definition film to show on 720p or 1080i or 1080p HDTV sets.

The number refers to the number of lines of pixels on the screen, the 'i' or 'p' indicates whether those lines are painted in two passes - every other line interlaced - or progressively in one pass. 1080p has the best picture, all other things being equal - which, of course, they rarely are.

Now for HD (high definition) discs and players.

Blu-ray and HD-DVD can both store enough data for a HD film and extras - HD-DVD up to 30GB using dual-layer discs and Blu-ray up to 50GB or more. Both achieve this by using smaller pits in the aluminum disc, spaced closer together in spirals layed out from the center to the edge.

A smaller wavelength laser is neeeded, mainly because of some interesting physics of waves, to 'read' the more closely spaced pits. Some 100GB discs and players are under development using four layers inside a single disc. That could store 10 years of standard TV episodes on a single disc.

Though important for computer applications, the different storage capacities mean little to consumers interested primarily in watching HD films and other on-disc content. Either will display the same quality image on an HDTV. Both will offer far superior audio compared to today's DVDs.

But there is behind-the-scenes activity that does make a difference to consumers.

Most of the major studios are lined up to deliver Blu-ray format content, though some are supporting both. HD-DVD players and some content are now available and the machines are currently priced about half (+/-$500) the anticipated prices of Blu-ray machines. Blu-ray machines are expected to launch in the summer of 2006. Either format will play a standard DVD.

This time, Sony (makers of both Beta and Blu-ray) seem determined not to repeat the experience of the 1980s. They've lined up a great many large company supporters, including Panasonic, Samsung and others.

Toshiba (originators of HD-DVD), Hewlett-Packard, Microsoft and others are pushing HD-DVD. The advantages are much less need (and therefore lower cost) to modify existing disc duplication equipment and other technical factors.

It's too early to tell which format will win out, or whether they'll both carve a large enough niche withconsumers to remain viable. It's likely at this stage that Blu-ray will win out for films and HD-DVD for PCs, but it's too soon to be sure. Much will depend on costs and available content over the coming years. See what fits in with your family, budget and other home improvement plans.