Hi guys,

I'm trying to find out (for certain, hopefully not just theories) of why a xerox copy of a CD comes out looking the way it does... I'm guessing it has something to do with optics and/or the different wavelengths of light (maybe the fact that the light on the xerox is green?)... any help in explaining this strange fact would be much appreciated. I don't have a picture of an actual xerox copy, but here's a link to a drawing I made of what it comes out looking like...

http://img651.imageshack.us/i/unle234d1copy.jpg/

 

What's wrong with you?

 

If anythink it probably has to do with the location of the detectors or how the grooves are oriented, but I've never seen a xerox copy of a CD...
You should post a picture...

 

These are interference patterns caused by double refraction.

http://www.britannica.com/EBchecked/topic/170003/double-refraction

 

Sorry for the lack of response on my part, everyone... I've been busy with some things. I'll try to get an actual photo soon... I'll also do some reading on that double refraction, studiot... thanks a lot

 

Hi guys, I was able to xerox a CD and scan how it comes out looking... its attached now (so you can see the actual thing rather than take my word for it)... any help in explaining this would be great

studiot: I read about double refraction, but can't seem to put it all together as to why it causes this phenomena... could you please explain more maybe?

thanks in advance everyone

 

Here is the image for all to see:

 

I found this with a moment's googling.

http://www.blumonster.com/050~Resources/053~Specifications/GlossTerms.pdf

Read the entry for birefringence

 

Thanks, studiot.... i also found this blurb about it:

http://books.google.com/books?id=Gk...cQ6AEwAg#v=onepage&q=cd birefringence&f=false

I can't seem to put 2 and 2 together though as to why it always causes that pattern when xeroxing....

 

OK here is another way to look at it.

The plastic base layer of the CD receives locked in stresses during manufacture. The base layer is reflective at the top surface.
The whole base layer including its top surface is distorted (bent) slightly by these locking in stresses (also called residual stresses). The distortion is very small - of a similar amount to the wavelength of the scanner light in the Xerox machine.
The transparent coating layer is similar to material used in photoelastic stress analysis. This is the layer that posesses the birefringence. It is put on top of the base layer after the base layer distortion so providing a flat top surface to the CD. But because the top of the base layer is not flat it varies slightly in thickness by just the amount of this distortion.

The light from the Xerox is polarised and reflects from both the bottom and top surfaces of the transparent layer.

These two reflections either constructively or destructively interfere depending upon the path length of the double journey through the transparent layer.

Where the transparent layer thickness causes destructive interference you see the black zones and where it allows constructive interference you see the bright zones.

This is the same effect as the colours caused by a thin film of oil on water.

 

Is the picture I attached what you mean studiot?

I'm more of a picture person, so I'm not entirely sure I understand your explanation just from text... thanks a lot for your help

 

You are finishing your report on time?

 

If you photocopy a mirror it will come out black. Just Google something like photocopy mirror for more info

 

Actually, this isn't for a report or even a homework assignment....

t06afre, is the mirror analogy what's really happening in the cd case as well?

Any help would be appreciated.... is my diagram wrong (am i understanding this wrong)?

 

Could someone please help me out with pictorially understanding what's going on? Is my drawing wrong? Thanks!

 

I could be wrong, but my impression is there are elements of polarization on a CD. Kinda goes with the territory. If the light source is also polarized, you would something similar. I know laser diodes are polarized big time, I think LEDs are too.

 

Is my drawing wrong

Well yes - and - no.

No it is the way the CD reading laser works. The size of the pits is set to accomodate the wavelength used by the reading laser.

But the Xerox scanner light has a totally different frequency.
It is the whole base layer of the disk as a unit that is affected in this case.

 

Hello,

Its all in the pits.
The CD has a long spiral with small pits.

When you scan that with a XEROX (wich has a straight scanline),
you will see dark parts where the pits are inline with the XEROX scanner.
When the pits are NOT inline with the XEROX scanner, there will be a light pattern shown.

The image comes from this PDF:
Optical storage: it’s all in the pits

Bertus

 

It is not often I disagree with Bertus but here is a scan of a blank recordable CD.

This has no pits whatsoever.

 

Hello,

A CD-R has a guidance track, that has the same shape as the groove with the pits on a CD-ROM.
So there is still h height difference visible with the XEROX.

This image is from this page:
http://encyclopedia2.thefreedictionary.com/Blank+recordable+CD

Bertus