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| QE | QE = quantum efficiency | | Quadlet | Four bytes of data | Quantum Efficiency
| Quantum efficiency (abbr. QE) is the measure of the effectiveness of an
imager to produce electronic charge from incident photons. Especially
important to perform low-light-level imaging.
The quantum
efficiency (Q.E.) of a sensor describes its response to different
wavelengths of light. Standard front-illuminated sensors, for example,
are more sensitive to green, red, and infrared wavelengths (in the 500
to 800 nm range) than they are to blue wavelengths (400 - 500 nm). This
is most noticeable in tri-color imaging with color filters, where
exposures taken with the blue filter need to be much longer than the
exposures taken with the green and red filters, in order to achieve
proper color balance.
Back-illuminated CCDs have exceptional
quantum efficiency compared to front-illuminated CCDs. This is due to
the way they are constructed. How do you make a back-illuminated CCD?
Simple. Just take a front-illuminated CCD, thin it to only 15µ thick and
mount it upside down on a rigid substrate. The incoming light now has a
clear shot at the pixel wells without those pesky gate structures
blocking the view (CCD102).
Note that CCDs with anti-blooming
have about 1/2 the Q.E. of those without anti-blooming. CCDs with
backside illumination (such as the E2V 47-10 or 42-40 can boost quantum
efficiency to over 85%. The Blue Plus CCDs from Kodak reach a peak of
about 65% and are about 30% at 400nm. |
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