FOUR
I read that Bayer sensors yield something like 70% resolution for the pixel count.
That's per dimension as well, do to interpolation and dithering.(and the AA filters that need to be applied to counter it of course). The technology is adequate but really needs to be replaced with something that works more like film did/does. 8MP is fine to replace 35mm for instance, if all of the pixels are full color locations and have no screens, filtering, AA, or so on applied. Too bad nobody actually does this yet*. I've pretty much given up on Foveon doing anything with their technology at this point - maybe when they go under and the tech gets bought by Sony or somebody.
*There are a couple of 6-12MP black and white digital backs that of course have none of these issues. Gorgeous results for the money. Now if they could only get the same thing for color...
Oh - quick calculation - 2400dpi scan for 6x6 film(56x56mm/2.205 Inches) is roughly 28MP. ~5290x5290 dpi.
The same with a Bayer sensor: 2400DPI*0.7=1680 effective pixels per inch. You have to multiply the number of pixels by ~1.425 to re-equalize for the losses. So 2400DPI is ~3200DPI on Bayer X 2.205 = ~7500*7500(7540 but let's keep the math simple - heh). 56MP.(ouch) Thankfully there are 50-60MP DBs out now.
Even if you take into account the maximum theoretical possible results for a digital back at 0.75 net pixels per dimension, you'd end up with a factor of 1.33X more pixels per dimension. So 2400x1.33x2.205=7038 Or 49MP.(slightly more reasonable I guess).
It's not that the technology it's great - it's fantastic. It's that the sensor pattern and construction itself is a decades old kludge and needs to be replaced. Getting back to your original point(s), yes, there is a huge need for a good film-like sensor in the 16-25mp range.
Oh - an interesting note - the Fuji sensor is closer to .8x due to the hexagonal layout and he fact that it's blending between the two images to get a single cleaner result. That may not sound huge, but it results in: 2400X1.25=3000X2.205=6600 Just under 44MP for a DB using this technology. IF SUCH A THING EXISTED. Sigh.
For 35mm film? ~15.6MP with a CCD/Bayer(0.7x), ~13.6MP with a DB type design, for the Fuji, 12MP(!yay!), and Foveon(why not...), 7.8MP(oh, so close at 4.7MP...NOT). Obviously 20-25MP in a 35mm format/camera is silly overkill.... I'd rather have better quality and less pixels.
Now, if Fuji or Foveon could get of their butts and do something about larger formats...