Talk:Charge-coupled device
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PizzaPuzzle, I am a little puzzled by the statement "CCDs commonly respond to 70% of the light to which they are exposed", because I'm not sure what the 70% is a measurement of. Is it the percentage of the CCD surface that is covered by sensing material, as opposed to inert structures such as metal conductors? Or is it the percentage of the incident energy in the visible spectrum that falls on the sensors that is converted to electric charge, instead of being reflected or absorbed as heat? Or is it the product of both of these? I would be grateful for clarification. -- Heron
The latter, 70% of the photons react with the CCD - the rest are "wasted". As opposed to only 2% usage by a photographic plate. It would probably be useful to divide this page into astronomical CCD use, and more general/technical aspects. Pizza Puzzle
Thanks. I added your 2% figure to the article, and moved the section to "Applications" to avoid repetition. Perhaps only the general/technical aspects should be described here, with the applications (astronomical and otherwise) moved to digital photography. What do you think? -- Heron
Why does light cause an accumulation of charge? Omegatron
You have to know a minimal bit of quantum physics for that. When a photon strikes an atom, it can elevate an electron to a higher energy level. CCDs are commonly made of negatively-doped silicon with many electrons - when light strikes them, it frees electrons to move around and they accumulate in the capacitors (I believe these are also called "wells.") Those electrons are shifted along the CCD and "counted" by a capacitor, giving an effective black & white image of how much light has fallen on each individual pixel. Several things can affect whether a photon releases an electron: polysilicon circuits on the front-illuminated CCD surface can block light from entering, certain depths of the CCD may be penetrated by longer wavelengths without interaction with the atoms, certain screens may reflect off or absorb some wavelengths of the light, etc. (Thus BOTH of Heron's interpretations above are accurate.) Some wavelengths of light can free more or less than one electron per photon. Knowing how many of the photons which fall on the CCD will release an electron is an accurate measurement of the CCD's sensitivity. This figure is called "quantum efficiency." Some CCDs can be made to reach as high as 95% QE. Several Japanese companies have developed many methods of increasing the sensitivity of the CCDs. For example, illuminating the CCD from the side without circuits, increasing the area of the CCD which is receptive to light. Socrates's Universal Disclaimer: I don't know anything about anything. - Nocturnal [edit:] Correction: the doped layers have spare electrons, but if they were free from the start that would of course defeat the purpose of the CCD! Sorry, I'm not a physicist, just an amateur photographer.