Skin colour in 24MP DX cameras

Started Jun 14, 2013 | Discussions thread
goosel
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Re: Good Question! Among many.
In reply to MarkJH, Jun 20, 2013

MarkJH wrote:

As I understand it, "CCD" and "CMOS" actually refer not to the photo-sensing capacitor or photodiode of each pixel that actually translates received light into a charge state, but rather, to the underlying technology by which that charge state is converted to a voltage quantity that can be remembered or processed.

With a CCD sensor, after you've snapped your photograph and the sensor's exposed, collected charges for each pixel "shift" down rows and columns, where they're read by a single amplification circuit for the row (or column) that converts them, one by one, into a voltages that can be stored or processed. Each pixel gets "dumped" this way. ("CCD" = "Charge Coupled Device," which describes the "shift" of charges down each pixel "bucket".)

By contrast, in a CMOS or "active pixel" sensor, a distinct amplification circuit is a part of each pixel, coupled directly to the sensing photodiode or capacitor. So the voltage conversion process happens on exposure, per pixel, and image data can be stored or processed right away, even on the same integrated circuit as the sensor itself. ("CMOS" = "Complementary Metal Oxide Semiconductor," which refers to the design style of the circuitry, in which pairs of transistors are balanced to perform logic and gating functions.)

Just looking at the raw architecture of it, it's not entirely clear (to me?) why images from a CCD sensor would look different than those from a CMOS sensor. The CMOS advantage of being able to integrate photosensing, reading, storing, and processing functions on a single silicon chip makes plenty of sense from a production and design standpoint. Smaller, lighter, faster, more efficient etcetera.

It is (or was) a matter of the geometry of the silicon. A CMOS sensor has the transistor pair or pairs right on the chip directly next to the pixel vs. a CCD which doesn't. This reduces the amount of area/volume available to the light gathering photo-diode. So other mechanism were employed to compensate. For instance  CFA's which may not be as discerning. Another important aspect is the AD converter/sampler which is probably a lot different and tweaked differently as well.

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