Transparent CDs?

Recently when inserting CDRs into my car CD player, I noticed something peculiar: I could see the audio face through the CD, well enough to see the readout clearly. Since the CD player works by reflecting a laser off of the CD back to the lens, how can this work with a translucent CD? Wouldn’t the laser pass through the CD or is there just enough reflective covering on the CD to send the light back to the lens?

(This doesn’t happen with all of the CDRs but Sanyos and CompUSAs are most pronounced)

The CD has little areas that reflect the laser light to a sensor. The areas don’t have to be opaque to change the reflective properties of the CD. IIRC in a CD-R the laser is used to create the areas of altered reflectivity to store the information. The process could be as simple as changing the shiny surface of the CD to a matte finish that no longer reflects. Still transparent, just not as shiny.

To avoid being flamed, I’m aware that it is not, in fact, the surface of the CD that is altered, but an interior layer.

“If ignorance were corn flakes, you’d be General Mills.”
Cecil Adams
The Straight Dope

How CDs work.

The CDs are still somewhat reflective; When I hold them up to the light, I can see through them and pretty well at that (I’m doing it right now). In fact, I don’t even need to hold them up to a light source. I can see my hand through the CD when I hold it. Why doesn’t the laser pass completely through the CD instead of bouncing back to the lens?

I checked the provided link and it reaffirmed what I already knew- the laser has to bounce off a reflective surface to work properly.

Try thinking of a steel panel, dotted with tiny, tiny holes all over it. Enough holes to see through somewhat easily, but enough solid to reflect back, also.

–Tim


We are the children of the Eighties. We are not the first “lost generation” nor today’s lost generation; in fact, we think we know just where we stand - or are discovering it as we speak.

I’ve noticed that all CD’s I’ve looked at are transparent. You just don’t notice it on most of them because they have labels or because they are mostly opaque.

I don’t know how small the pits in the CD are, but they’re nearly indistinguishable by the human eye. I’m looking at a CD I just burned and comparing it to a blank. If I hadn’t marked it, I wouldn’t be able to tell the difference.

Forgive my ignorance, but wouldn’t metal CDs be better than plastic ones? They’d be harder to burn, but they would be less liable to be scratched and ruined, wouldn’t they? I’d love to see someone invent a CD that can’t have its integrity compromised by tiny scratches.

i don’t know if metal cds would be feasible. the reason scratches krunk up cds is because they scatter the laser’s light. if the metal was shiny, it’d do the same thing. metal cds would probably be too heavy to spin at the rpms needed.

besides, cds are metal anyway. ok, some of it is.

you can get scratch resistant films to put over the cd. libraries use them. there’s been an ad in billboard magazine for a company that sells films to retailers to use on a counter display or whatnot. don’t know the specifics. i’m so anal about taking care of my massive cd collection that i almost cry when i get a scratch on one. i haven’t had to replace one yet.

As I understand it, the bit that reflects light is a 1 and the bit that doesn’t reflect light is a 0. That’s how the information is coded.
(maybe it’s the other way around though) The ‘0’ bits can either absorb or transmit light. I’m guessing mostly they transmit some or all the light that falls on the 0 bits.

I should point out that it is NOT safe to look at the sun through a CD! Something with numerous tiny holes does NOT block enough light (esp. infrared) to be safe. I mention it because I’ve seen people doing it during an eclipse.

Most CDs are transparent; try holding one up to a light. The thing that makes most of them opaque is the printing on the other side.

Speaking of CDs; Sony Playstation games are on black CDs. How do those work?


“Drink your coffee! Remember, there are people sleeping in China.”

Dennis Matheson — dennis@mountaindiver.com
Hike, Dive, Ski, Climb — www.mountaindiver.com

I don’t know exactly what the optical path in a CD is, but the part in bold below is the gist of what I’m saying:

The reflective properties of CDs don’t depend on the light to bounce straight back, like a mirror. The light reflects obliquely, like the surface of a lake. You can see through water, but it also reflects images if you look at it at the proper angle. It’s (relatively) hard to change the reflective properties of a material for light coming in at 90 degress. It’s simpler to change the angle at which it starts to reflect oblique light.

“If ignorance were corn flakes, you’d be General Mills.”
Cecil Adams
The Straight Dope

Cd’s you can see thru are cheap cheap cheaply made. Don’t use them especially on your cd/r

Mojo, Mojo, Mojo. You aren’t paying attention to your CDs. You can easily tell a burned from an unburned, for the grain of a burned tends to be circular, whereas the grain of an unburned is, well, there isn’t really a grain. Also, on a burned CD, there will be a VERY visible line at the edge of the session, a slick, shiny looking area about a mm thick, then another burned area about a mm thick. Compare them again, there’s TONS of differences. Also, about 1/2 inch from the inside, on a burned one, it will change grain, no matter how much stuff is on it. Really, there are TONS of differences.

–Tim


We are the children of the Eighties. We are not the first “lost generation” nor today’s lost generation; in fact, we think we know just where we stand - or are discovering it as we speak.

I like to write on cds with a Sharpie. Didyou know the folks there at sharpie said to never do that as the acid in the pen leaks thru the cd.

tanstaafl:

I’ve wondered about the black CDs in the playstation too. They read just fine in a regular CD reader in a PC though. My theory is that the laser in a CD drive is infrared and the “black” surface passes the IR light just fine.

Having burned my fair share of CDs (and made my fair share of coasters), I’m very familiar with the data “line” you see on burned CDs. I was referring specifically to the transparency of the CD.

CD lasers are, in fact, near-infrared – you can still see a reddish glow, but most of the light is somewhere around 680nm, IIRC. The “black” CDs are probably very dark maroon, just like the windows on IR remotes.

Transparency per se is not the problem, since those long IR wavelengths can’t pass through like theother visible light. Consider the metal mesh in your microwave oven door: it stops the ~1cm microwaves just fine.

Something that IS a problem, though, is the use of greenish dyes in burnable CDs, and especially those Kodak picture CDs. They aren’t as efficient as aluminum, so old CD players (and my old 6x CD-ROM!) with weak lasers won’t read them.

Playstation cd’s have a barcode on them that is read by the laser of the machine but cannot be duplicated in a copy cd.

Actually, handy, what the problem is with duplicating the playstation cd’s is that the playstation looks for a number of inserted ‘errors’ and bad sectors on a cd, which is not transferred by copying a cd. In order to use the copied cds, you need to get either a special program (difficult to find), a solderable eprom (easy to find) or a box that snaps into the back of the playstation unit (kind of easy to find)

–Tim


We are the children of the Eighties. We are not the first “lost generation” nor today’s lost generation; in fact, we think we know just where we stand - or are discovering it as we speak.