Originally posted by Francis This link is about very small sensor, intended for cell phones or compact cameras (about 1/10th the area of the APS-C sensor in the K-5). It happens to be of similar resolution, but that is just a coincidence. It is not the K-5 sensor.
Anyways, I don't think that the K-5 sensor is back-illuminated. I would guess that it uses micro-lenses to steer light away from support circuitry and into the light sensitive wells (that is what back-illumination would achieve also - that all the available light hits photo sites (but then of course some gets filtered away by the bayer filter, and after that there is maybe 50% light collecting efficiency of the silicon, but this is the same whether it is back-illuminated or not)).
Lastly, I am no expert, and if anyone would be willing to describe micro-lenses and back-illumination in more detail, I would be happy to read it.