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Tuesday, October 5, 2010

Megapixels And ISO: Have We Reached The Limit?

Although there’s no question that the pace of increase has slowed, noise-reduction advancements are poised to fuel continued resolution boosts


This Article Features Photo Zoom

In a CMOS sensor, by contrast, there’s circuitry at each individual photodiode to convert the charge into a voltage value. This requires considerably less power since the charge itself doesn’t need to be moved throughout the circuitry. This circuitry also enables the voltage values to be read in parallel rather than in serial form, leading to faster transfer of the capture data.

So far CMOS is sounding rather superior by comparison, but there’s another issue that creates some challenges for CMOS image sensors. With a CCD sensor, the circuitry that supports the work of the photodiode isn’t co-located with the photodiode the way it is with CMOS sensors. As a result, the entire area occupied by the photodiode can be used for the exclusive purpose of recording light, which is the whole point of the image sensor in the first place. Despite the inherent advantages of the CMOS sensor architecture, having the extra circuitry at the photodiode site means the light-gathering area is compromised, thus reducing signal quality and dynamic range.
 
At a fundamental level, the laws of physics limit the potential for increased resolution. The individual components of an image sensor are made from molecules, so those components can’t possibly be smaller than a molecule, at least not until we figure out how to make an image sensor out of subatomic particles. But clearly we haven’t even gotten close to this limit.
 
Resolution Potential
At a fundamental level, the laws of physics limit the potential for increased resolution. The individual components of an image sensor are made from molecules, so those components can’t possibly be smaller than a molecule, at least not until we figure out how to make an image sensor out of subatomic particles. But clearly we haven’t even gotten close to this limit.

Today’s digital cameras that employ the APS-H image sensor size (commonly referred to as a sensor having a 1.3x crop factor) offer resolution of about 16 megapixels at the top end. Demonstrating that 16 megapixels is nowhere near the limit of this particular sensor design, Canon recently announced it successfully has developed an image sensor that’s approximately the size of today’s APS-H sensors, featuring a resolution of approximately 120 megapixels. What’s more, this prototype image sensor is capable of about 9.5 fps at that incredible resolution.

How A CCD Works
One pixel’s electrical charge is transferred to the next row in a bucket relay, and it’s also moved horizontally. The converter at the end converts it into a voltage signal.



Advantages: Proven record of technologies and commercialization, low noise, high S/N ratio
Disadvantages: High power consumption, higher speed difficult, on-chip peripheral circuits difficult


 

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