Blu ray technology in terms of the blu ray disc structure varies in some key ways when compared to CDs and DVDs. The size or diameter of blu ray disc media is the same as compact disc or digital versatile disc media. The thickness, size of the centre hole, etc., are also the same. However, when looking at a cross-section or slice of the disc there is a significant difference between the blu ray disc structure and that of a CD or DVD.
To examine blu ray technology in terms of how a disc is put together, the focus will be on read-only discs such as blu ray movie discs. Other formats such as recordable and erasable have additional layers than discussed here to record the digital information but the general location of the layers is the same within the disc.
Below is a schematic of a cross-section of a blu ray read-only disc.
The most obvious difference between a blu ray disc and a CD or DVD is the position of the data layer, metal reflective layer, and protective layers. For a CD, these layers are near the top of the disc and for a laser to read the data layer, it has to travel through about 1.1 mm of plastic base. Therefore, the optical clarity of the base is extremely important. Also, because the laser is focused well into the disc structure, flaws on the base surface such as minor scratches and dirt will not cause significant problems when the CD is being played or read. For a DVD, these layers are in the middle of the disc structure. The laser in this case has to travel through 0.6 mm of plastic base in order to reach the data layer. Once again optical clarity of the plastic base is very important and because the laser is focused deep (although not as deep as the CD) in the disc structure, there is some tolerance for minor scratches and dust.
Now, in blu ray technology, the thick plastic portion is actually the top side of the blu ray disc and not the base. The laser does not travel through the plastic base to read the disc. Instead, the laser will only travel through two very thin layers, approximately 0.1 mm in total thickness. The first layer that the laser encounters in blu ray technology is the hard coat layer and this layer is 0.002 mm in thickness. The next layer after the hard coat layer is the cover layer and it is 0.098 mm thick. The cover layer will protect the data layer underneath and provide separation so that the laser can properly focus on the data layer. The hard coat layer is a specially formulated material to prevent scratches, dirt and dust build-up, and fingerprints. Having a layer that will resist these elements is a critical component in blu ray technology because the focus point of the laser is very close to the base of the disc and any irregularity in the base will interfere with the laser light beam. Without a resistant hard coat layer, blu ray discs would experience plenty of read or play problems. Both of these thin layers obviously also require excellent optical clarity.
These are the main structural differences in the disc layers between a blu ray disc and a CD or DVD. Of course, a blu ray disc also has smaller size “pits” (which contain the digital code) than a CD or DVD and also the track of pits are closer together in a blu ray disc. To read these smaller “pits” a blue laser which has a shorter wavelength is required.