If you look at the technical specs of
Blu-ray, compared to HD-DVD,
you'll see that the main difference that allows 67% more storage is the Numerical Aperture (NA). But what is NA and how does that
relate to Blu-ray?
If you are familiar with high power lab quality microscopes then you probably have some idea of what NA is and can probably guess
how it might affect Blu-ray. I won't go into the technical definition but I will breifly explain how it relates to microscopes and
show how that helps to understand how it affects Blu-ray.
When you look through a microscope the magnification (power) is the power of the objective lens, the one closest to the object,
times the power of the eye piece, the one you look through. So if you use a 40 power (40X) objective with a 16X eye piece you'll get a power of 640X.
A typical 40X objective has an NA of 0.65 (the NA of air is 1), if we multiply the the NA by 1000 we get the maximum power the lens
will support and still be able to give a clear image. So 640X is close to the maximum power this lens will support.
If we used a 20X eye piece we would get a power of 800X but there would be very little additional detail, so the image would
not be clear. This is is like taking a small graphic and simply making it bigger. Although it is larger, there is no more detail and
it is not very clear. To get a really clear image at 800X, you would need an objective with an NA of at least 0.8
The NA of HD-DVD is .65 whereas the NA of Blu-ray is .85 this means that the laser can see a much smaller mark when reading and the
writer can make a much smaller mark when writing. This allows for more marks in the same space and thus more storage on the same
size disk.
This brings up an intersting qustion. If we could increase the NA even more, perhaps making it greater than 1, let's say 2.5,
could we get even more storeage in the same place? Well, the difference between .85 and 2.5 would be squared so an NA of 2.5 would
in theory give you 8.65 times as much storage on the same size disk. Could this actually be done?
NO!!!
To see why not, let's return for a minute to our microscope.
A 100X objective lens on a microscope typically has an NA of 1.25, which would allow a usable magnification of 1250X. But wait,
didn't we say the NA of air was 1? If the objective lens is looking through air, how can it see more detail than air itself will
allow?
IT CAN'T!!!!
This lens is known as an oil immersion lens because the lens is immersed in special oil with an NA of 1.5, thus allowing no air
between the object and the objective lens.
The glass used in the coverslip and the slide also has an NA of about 1.5 and thus we see the practical limit of light microsopy.
This oil must be cleaned off the lens with a (usually toxic) solvent such as xylene and if you want to view the slide in the future
it will have to be cleaned as well.
Thus an NA greater than 1 can only be achieved by using oil between the laser's lens and the disk.
This procedure is obviously very messy and is far beyond the capabilities of the average DVD viewer. It is obvious, then, that it
is not practical to achieve an NA greater than 1. We haven't even looked at the NA of the layers in the DVD itelf which may actually
be less than one!
It is clear, then, that the most you could sqeeze of a 12 cm optical disk per layer is maybe 38.4% more than Blu-ray offers. How,
then, could a future optical storage tehcnology go beyond the 50GB offered by DL Blu-ray?
It would have to either use more than two layers, a larger disk, or a completely different technology!
