Monday, August 2, 2010

Blu-ray Discs

Blu-ray discs are optical storage media and look similar to regular CD/DVDs (they are all of the same size). But they can store 10 times the data in a single DVD.

NA = nsinθ ; n = refractive index. Since θ1>θ2, NA(BD) > NA(DVD)

The name "Blu-ray" comes from the blue laser being used to read and write. There are GaN (Gallium Nitride) diodes that produce photons of 405nm directly. 400nm is the wavelength of the violet component of the visible spectrum so the "blue laser" actually appears violet to the human eye. By decreasing the wavelength from 650 and 780nm used for DVDs and CDs respectively, and increasing the aperture from 0.60 to 0.85, the laser can be focused to a smaller area. Hence, more data can be stored in the same amount of space.

In BDs, the distance between the surface of the disk and the surface upon which the data is actually recorded is 0.1mm. This is 1/6 times the distance for a regular DVD. Since the discs are made of plastic, the incident angles can change a little due to disc warping. To check this problem, the NA is increased (to an extent) and the cover layer's thickness is decreased.

Cover layer thickness is reduced to check distortion of the laser spot.

Typical BDs can store 25GB if they are single layered, and 50GB if they are double layered.
"Double-layered" refers to the presence of two recording layers inside the protective cover layer. Each recording layer can store 25GB of data. Pioneer Corp. recently developed a BD with 20 recording layers which allows it to store 500GB of information inside a single BD!

Structure of single-side, double-layer BD

And finally, regarding the popularity of BDs, they are mostly used to store HD videos since the regular DVDs can only store Standard Definition (SD) videos. When HDTVs become widespread, BDs are expected to replace DVDs. For the time being, companies such as Sony, Panasonic, Phillips, Samsung, Pioneer and LG have developed BD drives that are compatible with BDs and DVDs both. This is known as Backward Compatibility and is a measure to ensure that once newer media like BDs become popular, the older storage media do not become obsolete.

Photographs courtesy :


  1. I know this deviates from the topic, but this was the first article on the smblog that can be read in under 5 minutes. Thanks for the detailed diagrams. I hope others can also follow this example.

  2. good post :)
    quick question. You mentioned that "the incident angles can change a little due to disc warping."

    What is disc warping and how does it happen?

  3. Amol, Karan - That was the aim ;) Thanks :D

    Shayan - Discs that are made of plastic can bend a little due to heating. So in that case, the laser will fall obliquely and travel through a thicker cover layer than it was intended to.

  4. Hey nice post you've written. A good read.

    @ Amol : Is 5 minutes all you can give to learn something new from the internet?? Are you so impatient that you cannot read longer posts and feel good about learning new stuff??