Pioneer has announced demonstrations of Blu-Ray discs with 20 layers, capable of storing 500GB of data on a single disc. Commercial players and discs are currently confined to two layers.

The multilayer rush began with video DVDs. Each DVD data layer holds only 4.7GB, making storing a complete movie at high quality impossible. Resolving this had drives reading two layers of data from a disc, the second layer underneath the first, with reading lasers passing through the translucent first layer to reach the second. The technology was expensive, and did not come to consumer drives for a number of years after its wide deployment in commercial movie pressing. Multilayer CDs were never commercially released.

Blu-Ray has had dual-layer availability since its inception, but plans for adding more layers are not clear. HD DVDs with three layers were demoed before that format's demise, but technical problems made manufacturing expensive. Fundamentally, each layer is an optically active interface, and accurately reading data through multiple layers is complicated by increasingly complex echo patterns or light bouncing through the layers. This interference gets worse as the number of layers increases, and provides the main technical barrier to heavier multilayer technology.

Pioneer claims to have made a breakthrough in mitigating these issues. They say that by using alternating layers of two different thicknesses, this "optical crosstalk" can be minimized. This and other, proprietary, technologies allow Pioneer to read accurately from no less than twenty layers of BD data. With BD at 25GB per layer, this means a single disc would hold 500GB of data. This improves on an announcement of 16 layers holding 400GB last month.

The usefulness of such a capacity is not immediately clear. The format's inherent limitations on bitrate and such prevent its use to drive ultra-high-resolution video, and portable computer storage at this level is a niche market. If the discs with very long shelf life can be developed, they might replace tape as backup for some applications. The biggest potential impact of this technology, though, is to allow the size of Blu-Ray discs to scale massively without fundamentally altering the technology involved. As needs expand, new BD formats can be launched with increased sizes, relying on fundamentally the same technology.

Since Pioneer claims the new demo involves proprietary technology, it is very unlikely current BD players can read 20-layer discs, even with new firmware. New players and drives will have to be distributed, but if the technology pans out, BD can scale to be more and more heavily multilayer for a goodly number of years. Longevity as a technology is desirable, because economies of scale and entrenchment will lower prices and increase quality over time. Giving BD room to expand is a very good thing, and Pioneer seems to be leading the way.