CD/DVD laser wavelength and track widths

[AndyW]

I have read that that 'spot size' for reading a track on a CD is twice the wavelength, e.g. on a CD the track seperation is 15 microns which can be read by a 700nm laser. But where does this physical rule come from?

Also, the DVD standard uses a 640nm laser. That would imply a track seperation of about 13 microns, but in actual fact it is 7.4 microns. Why is this?

Thanks

[Nice coder]

I would also like to know this.

[wimms]

Check your math. 2x 640nm is 1.28um not 13um.
Track separation is different thing from spot size. Both are on safe side to avoid reading neighborig tracks.

Twice the wavelength size rule comes from fact that with smaller spot light diffraction goes nuts, and you can't get exact reading, reflection gets blurred.

[Loren Booda]

Maybe it insures substantially more than one wavelength for reading, thus less destructive interference.