How does the mechanism of a dual layer DVD read the data from both layers?

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I read things like "When a dual layer DVD burner reads a dual layer disc, the read-laser readjusts its focus past the semi-transparent first layer of physical data, on to an additional layer of data." and "Reflectivity of both recording layers of a dual layer recordable disc is the similar: greater than 18 percent. The reflectivity between the L0 and L1 layers, however, is greater than 50 percent because the upper (second) recording layer absorbs and reflects some of energy that is directed at the lower (first) recording layer L0 in order for organic dye to be recorded.". Ok, so the first layer is semi-permeable, but how does the signal differentiates what is reflected by one layer from what is reflected by the other layer? I imagine, and this applies to a single layer DVD or CD, in the differentiation between lands and bumps that the refractivity in the polycarbonate layer allows the reading or not reading (0 or 1) of the reflected signal - "The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes."; and therefore I assume that it may also play a part in detecting from witch layer it is reading.
Can someone elucidate me on this? And please correct any wrong assumption that I may have made!
Thank you
Regards
 
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I have seen that site in my search, but it does not answer the question. Thank you anyway.
 
Charles123 said:
I have seen that site in my search, but it does not answer the question. Thank you anyway.

You answered you own question in your original post and the link the other poster gave backed that info up. The laser is focussing shallower or deeper into the disc to access the two different layers.
It will ONLY read (write) the data for the layer that is in focus

Cheers
Dave
 
Davenn,
Thank you for your answer! Can you develop on the actual mechanism? Is it based on diffraction, and the reflected laser not reaching (0) or reaching (1) the receiving sensor? Is that done by approaching the laser gun? A lens?
regards
 
Charles123 said:
Davenn,
Thank you for your answer! Can you develop on the actual mechanism? Is it based on diffraction, and the reflected laser not reaching (0) or reaching (1) the receiving sensor? Is that done by approaching the laser gun? A lens?
regards

Hi Charles

I'm no expert in the field, there's probably others here that know more than me. But I have "played" with a lot of laser mechanisms over the years. What I have found is that either the laser itself is physically moved or the lens is physically moved. They take advantage of standard optics just as with a camera lens where they use a lens with a very shallow Depth Of Field. DOF. Google the term and learn a bit more about optics. Because the DOF is very shallow it allows the lens and or the laser to be slightly moved to have it focus at a different point within the DVD layers. This is even done on a single layer DVD or CD To ensure good data read and write.
Pull an old DVD. Etc apart and look closely into the actual laser mechanism, you will find the laser and or the lens is mounted on fine springs with tiny electromagnets to control its positioning

Cheers
Dave