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Why doesn't DVD blank media have a silver bottom?

Discussion in 'DVD±R media' started by fettfan, May 4, 2007.

  1. fettfan

    fettfan Member

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    I'm wondering why there isn't a silver bottom blank dvd? Blank
    CD-R's are usually silver bottomed, so why do blank dvds have a purple/blueish bottom? Shouldn't CD-R's have to have this too in order to be burned to? The recording dye I mean. Why is this?

    -Keith
     
  2. 300bowler

    300bowler Regular member

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    a short version why,
    Commercial dvds are pressed not burned

    wait for Joe or other members to answer with better details
     
  3. BIGTOXY69

    BIGTOXY69 Regular member

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    Also The Silver Bottom on CDR's is Not really The Dye itself Because if you Scratch it It will most likely be blue or Bluish green The true color of the Dye ! The SIlver , Black or whatever Color the Underside of your CDR is is More about Reflectivity Than anything else ! Burned media is less reflective than Commercially Made CD's Or Dvd's Which are stamped or Pressed out Much like the Vinyl Lp Records were /Are ! With Dvd Media keep in mind that You are also Cramming Roughly 7 times the data of a CDR onto a disc the same physical size and that's Just on single layer DVD media ! DVD+R DL is even more ! There are similarities in CDR's & DVD Media as well as differences ! But for me I don't care what color the readable side of my media is as long as it works ? so basically it's a moot point! Ciao!
     
    Last edited: May 4, 2007
  4. JoeRyan

    JoeRyan Active member

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    Light passes through the dye layer to the reflective layer and back out again for recordable optical discs. The simulated "pits" are spots that are where the dye is burned to prevent light from passing back out. The important part of the mark is how well defined the edge of the burn mark is, just as it is the edge of the pit. (The reading laser gets data from the changes in reflected angles when it hits the edges of pits/marks, not the pits or marks themselves.

    CDs and DVDs have no dye. The light passes through the clear polycarbonate until it hits an aluminum mirror layer, then bounces back. That's true for both lasers and the human eye, but the human eye reads all wavelengths of light passing through the plastic. The light from a laser is a single wavelength. CD-R dyes can be deep blue (azo-cyanine), blue-green (cyanine), or faint yellow-green (phthalocyanine). All these dyes are tuned to react to a laser burning infra-red wavelengths, and the reflected light bounces off a layer of silver alloy or gold (aluminum corrodes in the presence of these dyes). DVD dyes are cyanine, azo-cyanine, or oxonol; and they all look deep purple to our eyes when visible light bounces back from their silver alloy layer (gold is less reflective and much more expensive). They are tuned to darken when burned by a laser at "ruby-red" light wavelengths (generally 635 to 650 nanometers.)

    "Colored" discs use a tinted polycarbonate to look different. The "black" discs use a very dark red/purple color that lets so little natural light back that the discs look black to our eyes. To the laser with its single wavelength, the light passes right through the "black" and all other colors except green--green aborbs red light.

    In short, it is the dye that gives the discs their color unless the polycarbonate is also tinted for a color effect. CDs and DVDs have no dye. You are seeing their aluminum mirror layer, and that is why they look silvery.
     
  5. alamo55

    alamo55 Member

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    @ JoeRyan,

    WOW, what a great explanation! It's people like you that keeps me reading the post here at AD.

    Thanks
     

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