BlastProcessing
Member
But doesn't dvd playback make that moot?
DVD and CD also use different lenses.
Because consumers have already invested agood amount of money in their CD audio and CD-ROM collections, it was considered a top design priority thet DVD palyrs read axisting CDs as well as new disc.To make DVD payers with this ability required specific optical design features. The simplest design is to mount two lenses in a single optical head-one optimized for a 1.2 millimeter-thick substrate and another for a0.6 millimetre-thick one -and then to switch mechanincally from one to the other as needed.
A more elegant solution that emerged uses a single molded optic with a holographic element at its center.Theelement at its center.The light passing throght the outer annulus of the lens is unaffected by the hologram and focuses to a spot size in the plane small enough to be suitable for DVDreadout.About one thid of the readout beam incident onthe centralpart is focused by both the the and the hohgram to aspot suitable for reading pits on the thicker CD.
http://library.thinkquest.org/26171/diffrent.html