How do digital cameras capture images using the photoelectric effect?

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jabers
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So they use the photoelectric effect right? Does that mean when the camera takes a picture it allows photons of light to interact with a sensor for a certain period of time? How does that create the picture? If i understand it correctly the different photons have different frequencies and their wavelengths correspond with colors. Any more insight or corrections would be very much appreciated.
 
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The simplest possible photo detector works thusly. You basically have a diode with reverse bias applied to it. The current can't flow, because there are no carriers (electrons or "holes") in the depletion region. (Look up diode for details.) However, if a photon strikes one of the electrons and excites it into the conduction band, you end up with a free electron and a free hole, which will immediately flow in opposite directions. If you have enough photons striking the depletion region, you'll have a current flowing. The current will depend primarily on the intensity of light, allowing you to measure it. I'm sure the rest is self-evident.

Oh, the idea here is that so long as the energy is sufficient to knock out an electron, a higher energy doesn't make much difference. The low threshold for cameras is usually in near-IR region (about 900nm), so all visible light produces roughly equal effect. To get color, there is a filter mosaic overlaid on the sensor.
 
jabers said:
So they use the photoelectric effect right? Does that mean when the camera takes a picture it allows photons of light to interact with a sensor for a certain period of time? How does that create the picture? If i understand it correctly the different photons have different frequencies and their wavelengths correspond with colors. Any more insight or corrections would be very much appreciated.

Yes - it's 'a' photoelectric effect in which electrons are 'moved about' by photons but many digital (compact and movie) cameras expose the sensors to the incoming light continuously (they don't use a shutter as with a film camera). Each sensor delivers a signal which is proportional to the intensity of light (rather than the energy admitted in 1/125 of a second). The signal is 'read' by the camera circuitry at the time you make the exposure and over a short period (the 'exposure' time). This arrangement is simple and cheap but suffers from the fact that the sensor is exposed to light all the time and low light performance suffers. Posh cameras (SLR for example) use a shutter, as with film cameras, and the sensors only get a short burst of light energy. Along with the fact that the sensors are larger area than in compact cameras, the different system gives much better low light performance.
Here's a pretty description of digital camera workings: "[URL htm"]http://www.cambridgeincolour.com/tutorials/camera-sensors. htm[/URL]
 
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