[moses lawn (OP)]

Why is it that recordable DVDs that you buy in the shops have dye or coloring in the recordable side, whereas prerecorded DVDs do not, being usually just silver in colour.

Is it to do with preventing piracy, or is there a more technical reason?

Life can be lousy, but the alternative's even worse!

[neilep]

This should be a question that I can answer, but, I'm going to have to ask around some colleagues. Admittedly, it's not required learning as we sell the equipment but I'm going to speculate that it may be due to the fact that a pre-recorded dvd is just that...pre-recorded,  and that a recordable one still has all it's layers intact(it's a virgin disc).....once you've recorded onto it, you'll notice a change of hue where the laser has burned the data onto the disc,(same with cd's), but I'll endeavour to find out......I'm probably still not right as they are obviously still a different colour BUT (thinking), it must have something to do with the fact that some discs are re-recordable (you can record up to a thousand times on them) and others you can only record onto once and once only. I don't think ' preventing piracy' has anything to do with it in this case.

Men are the same as women.... just inside out !!

[daveshorts]

DVDs and CDs work by shining a laser at a disk and looking at what gets reflectd back, a 1 is represented by something that reflects the light and a 0 by something that doesn't.

The way they make the shiny and not shiny bits on a bought CD is to etch a glass plate so it has lots of bumps on it. they then push this into a thin piece of aluminium in the disk which makes lots of little pits (these scatter rather than reflect the light) they then seal the aluminium up in polycarbonate as it would be a bit delicate otherwise. This is a cheap way to make loads of disks as you can essentially stamp them out.

A recordable CD works differently as having a photolithography suite (to make the glass masters) in your computer would be unwieldy. They make the disks with a piece of aluminium covered with a photosensitive dye. when your CD writer hits this dye with a big fat laser it changes how transparent it is, so recording the information.

The rewritable CDs have a dye that is thermosensitive and will go cloudy or transparent depending how hot it is heated to, so you can erase the disks...

[neilep]

Well there you go, I was about to say the same thing (yea right !!)[]

Men are the same as women.... just inside out !!

[moses lawn (OP)]

Well, I didn't know that!

Thanks

"You play ball with me, and I'll scratch yours"

[moses lawn (OP)]

OK, so why are recordable CDs silver and not dyed?

"You play ball with me, and I'll scratch yours"

[moth]

Re-recordable cds use a alloy as the recording layer. The alloy is chosen so that it has two different phases (patterns of arranging its atoms) that are stable at different temperatures and adsorb light differently. The alloy starts out in a crystaline phase that is stable at low temperatures. To record a pit the laser heats it rapidly and it changes into an amorphous (non-crystaline, disorderd) phase that is stable at high temperatures, it then cools rapidy and so doesn't have time to change back to the crystaline form before it is too cold to rearrange its atoms, so stays trapped in the amorphous phase. To erase the disc the laser heats the alloy to a lower temperature, but lets it cool much slower, so all the alloy returns to the crystaline phase. (A process know as annealing).

[moth]

Singly recordable cds can be made silver too by using the right dye. CD reading lasers use infared light, so you can use a dye that stops infared but is transparent to visible light.

[moses lawn (OP)]

Thanks again for very enlightening replies.

"You play ball with me, and I'll scratch yours"