... the Bayer CFA deliberately blocks out a lot of light. Each of those red, green and blue filters is supposed to block out a large portion of the other 2 colors, is it not?
What proportion of light is blocked, do you know? Perhaps not as much as 2/3rds but maybe as much as a half. That doesn't sound particularly efficient to me.
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Indeed, that is the source of the hope that an X3 type sensor (one measuring three "colors" at each location) can have higher quantum efficiency and thus a sensitivity advantage at a given resolution level. The question is whether they can in fact achieve significantly higher QE than good CFA designs, given that there is some additional light loss due to the multiple absorption layers needed with X3. The inefficiencies of multiple layer sensors must be weighed against those of CFA's, and I trust experiment over the hand-waving theory of some X3 advocates to make such comparisons.
So here are some QE numbers, for Kodak sensors simply because Kodak lets it all hang out when it comes to sensor spec's, at [a href=\"http://www.kodak.com/US/en/dpq/site/SENSORS/name/ISSProductFamiliesRoot_product]http://www.kodak.com/US/en/dpq/site/SENSOR...iesRoot_product[/url]
The best I have seen so far is Kodak's new MF sensor with micro-lenses and 6.8 micron pixel pitch (most MF sensors lack micro-lenses, which about halves their sensitivity.) For overall luminosity sensitivity, the mid-spectrum green figure is probably the most indicative number.
KAF-31600, as in the H3-31 and the long expected Pentax DMF
QE 43% green, 37% red, 36% blue.
Another slightly older sensor with the same 6.8 micron pixel pitch:
KAF-10500, as in the Leica M8:
QE 40% green, 17% red, 32% blue.
That 17% red is anomolously low, and may be a typo.
An older sensor with smaller 5.4 micron pixel spacing:
KAF-8300, as in the (discontinued) Olympus E-300 and E-500:
Color version QE 40% green, 33% red, 33% blue.
Some new monochrome versions, to show how much light is lost to CFA's:
Monochrome, microlenses, no glass: 60%
Monochrome, microlenses, MAR glass (IR blocking?): 54%
Monochrome, microlenses, clear glass (IR blocking?): 52%
Monochrome, no microlenses, clear glass: 37%
Source:
http://www.kodak.com/ezpres/business/ccd/g...300LongSpec.pdfA new interline CCD with "tiny" 4.75 micron pixel pitch:
KAI-10100:
QE 42% green, 32% red, 40% blue. (Aside: the highest blue QE I have ever seen.)
Monochrome sensors have higher QE, so the QE figures here are as a percentage of all light of all colors, suggesting that they are close to optimal. Being over 1/3 already sounds strange, but there are overlaps in the sensitivity curves, and perhaps "green" pixels in particular are made sensitive to more than one third of visible spectrum to make them more useful as the primary luminance measure, while R and B pixels are mostly used for "chroma" information.
The fact that monochrome sensors only have about 1.5x the sensitivity of CFA ones (60% vs 40% for the KAF-8300) suggest that the X3 advantage might not be so great after all. One possibility I see is that "green" pixels could effectively be "white pixels" giving optimum QE for luminance measurement, with R and B pixels solely for color information. Given the eye's greater resolution of luminance than color (rod vs cone density and cones being single color sensitive) this might be a smarter allocation of resources than X3's "misguided egalitarianism" in treating R, G and B as equally important.
