DVD questions

[mantis69666]

How exactly does the laser eye on a DVD-ROM or player able to read the second layer on a dual layered disk? Does it read thru the first layer and how does it know which layer is which?

 

[Elledan]

The lens through which the laser beam is focused is refocused, so that it 'reads' the second layer, which is further away than the first layer.

I'm not certain how the laser beam passes through the first layer, though. It has to be partially transparent, but further I wouldn't know how it works.

 

[Superdoopercooper]

I think the layers are SHINY, yet transluscent.... sort of like those silver anti-static bags that you get with video cards/other electrical junk sometimes. So, when the focal length is set to hi the first layer, it will reflect... but when focused at a different lenght (second layer), some of the light is reflected/diffused on the first layer... but the focus still hits the second layer adequately enough to be read back by the optical pickup.

This is just a guess, so don't go telling all your friends what I said.

 

[miguel]

I think that's correct.

But don't worry - it's not like my friends would believe someone named "sooperdoopercooper" anyway...

 

[kendogg]

<< I think that's correct.

But don't worry - it's not like my friends would believe someone named "sooperdoopercooper" anyway... >>



lol thats pretty funny.. sorry i cant answer the question though..

 

[CTho9305]

<< The lens through which the laser beam is focused is refocused, so that it 'reads' the second layer, which is further away than the first layer.

I'm not certain how the laser beam passes through the first layer, though. It has to be partially transparent, but further I wouldn't know how it works. >>


I think that if you separate the two layers enough, and the beam is wide enough, as long as the first layer is somewhat transparent, its not all that hard. think of it like this: take double-pane glass. scratch the inside and outside different. focus on the near side, and you can't really see the scratches on the other side. focus on the far side and you can't see the scratches on the near side.

For this to work, I would think the laser has to be greater than a certain width. if it is too small, it would be more affected by the near layer.

Another example is a microwave that has a grille in front. you can see either the grill or the inside, but not both at the same time (at least, not easily )

 

[Shalmanese]

I am not 100% certain on this but I think that it uses the principle of thin film reflection.

That basically means that the light reflecting off the first layer destructively inteferes with the light coming off the second layer so only the second layer is read (bad explaination I know, go read a physics textbook)

 

[Bozz]

What is actually happening is the two beams are always focused on the same spot, one higher and one lower. There is a film on the lower layer that reflects light that is 90 degrees out of phase yet passes 0 degree phase light. Pretty simple really, eh

 

[Elledan]

<< What is actually happening is the two beams are always focused on the same spot, one higher and one lower. There is a film on the lower layer that reflects light that is 90 degrees out of phase yet passes 0 degree phase light. Pretty simple really, eh >>


Makes sense Thanks, Bozz!

 

[CTho9305]

<< What is actually happening is the two beams are always focused on the same spot, one higher and one lower. There is a film on the lower layer that reflects light that is 90 degrees out of phase yet passes 0 degree phase light. Pretty simple really, eh >>


very cool