Re: CD's and CD-ROM's

[Pete Lohstreter <petel@tenet.edu>]

Bob;

There is a fine article in The Physics Teacher (October '90 OR '91  I 
think) on just this topic.  From what I remember, the digital information 
is recorded in concentric rings that are spaced 1.6 microns apart.  The 
rings are composed of smaller depressions that range ins length from .9 
to 3.3 microns in .3 micron steps.  The depth of the depression is about 
..12 microns.  The article indicates that the ring spacing will act as a 
diffraction grating with a line separation of 1.6 micron. 

If you would like, I can look up the exact date and page of the article.

petel@tenet.edu

Pete Lohstreter                       "The first rule of intelligent
North Garland High School              tinkering is to save all the parts."
Physics Department                                           Aldo Leopold
2109 Buckingham
Garland, TX  75042


On Wed, 3 Apr 1996, Bob Muir wrote:

> This concerns the cause of the diffraction patterns seen in the reflected 
> light from CD's.  Looking at an audio CD with an incondescent bulb as a 
> light source, the continuous spectrum extends away from the light 
> beginning with blue and evolving into red.  This is consistent with 
> looking through a diffraction grating in that the blue lines are nearest 
> the central peak and the reds are farther from the central peak.  This 
> implies to me that there are very closely spaced, concentric rings of 
> varying reflectivity.  However, I looked at one with a fairly high 
> powered, stereo microscope and I don't see a ring pattern of anything.  I 
> think that I remember hearing that there are pits in the reflecting 
> surface which are a binary representation of the information, but I 
> didn't see what I expected to see.  I did see lots of tiny, whitish or 
> grayish dots, but they didn't seem to have much of a pattern.  There is 
> an outer region (about 3/8 " wide) with some appearance of concentric 
> rings and inside this region the dots don't seem to be in a pattern.  
> Since the interference pattern extends completely across the disk, I am 
> confused.
>
> Does anyone KNOW what the pattern actually is?  The dots I saw are so 
> tiny, I can't believe that a laser beam is focused to such a small area.  
> Is there a mask of some sort between the disk and the "read head"?  How 
> is the sequence of material (music) distributed on the disk?  How are the 
> "tracks" addressed?  Is there sectoring as on magnetic disks?  Does 
> anyone have a good reference for this information?
>
> Is there any fundamental difference between computer CD-ROMs and audio CDs?
>
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> * bob muir                 voice: (910) 334-5669        *
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