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Quantum Efficiency

The quantum efficiency (Q.E.) of a sensor describes its response to different wavelengths of light (see chart). 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. The different response to wavelengths also accounts for the fact that unfiltered CCD exposures of spiral galaxies will typically suppress star-formation regions (blue light) in the arms, and accentuate emission nebulae and dust lanes (red and infrared light).

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).

Quantum Efficiency
Typical Q.E. curves for front- and back-illuminated CCDs

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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