Single Unit of CCD/CMOS?

[doofus]

I was thinking about how CCD's and CMOS's almost exclusively come in multiples. To be clear, what I am referring too is how each chip is fabricated to feature many pixels. Does the fundamental unit of a CCD/CMOS have a name "commercially"? Something like a "single pixel camera"? Looking at wikipedia I see the fundamental unit is called a "MOS capacitor". Which has me thinking about phototransistors, but I was under the impression these did not have a concept of storing charge/exposure. Photodiodes being in a similar position. I think it would be very fun to have a single pixel that had a concept of an exposure time, but I'm not sure if this is a thing other people use, or if my understanding of phototransistors needs some brushing up.

Would a single pixel basically be a photodiode hooked up to several transistors?

 

[phyzguy]

Why would anyone want a "single pixel camera"? It would not be useful for building up an image. So the manufacturers build them with many pixels so they can detect an entire image. Also CCDs and CMOS imagers have a lot of overhead circuitry that surrounds the imaging region to be able to read out the image. To build all of the support circuitry for a single pixel would be very inefficient. You can always take a multi-pixel CCD or CMOS imager and just look at one of the pixels if you want.

For a CCD imager, the basic unit is indeed a specially designed MOS capacitor, where the charge is stored in a potential well in the silicon.

 

[berkeman]

CMOS's almost exclusively come in multiples.

Can you give a couple examples of what you are calling "CMOS's"? The term CMOS (complementary metal-oxide semiconductor) usually refers to a type of transistor construction. Those transistors can be used in the circuitry of an imaging device, but they are not generally used as imaging sensors on their own:

https://en.wikipedia.org/wiki/CMOS

 

[rbelli1]

The light sensing part of the CMOS image sensor is a photo-diode. There are also integrated amplifiers. You could build a singe one of them with a discrete photo-diode and a high impedance operational amplifier (op-amp).

BoB

 

[doofus]

Why would anyone want a "single pixel camera"? It would not be useful for building up an image. So the manufacturers build them with many pixels so they can detect an entire image. Also CCDs and CMOS imagers have a lot of overhead circuitry that surrounds the imaging region to be able to read out the image. To build all of the support circuitry for a single pixel would be very inefficient. You can always take a multi-pixel CCD or CMOS imager and just look at one of the pixels if you want.

For a CCD imager, the basic unit is indeed a specially designed MOS capacitor, where the charge is stored in a potential well in the silicon.

The idea on its surface is goofy. The use case isn't for taking images, more-so for a typical photodiode like application. But, the nice thing about being able to store charge from light exposure is, well, assuming thermal noise and other-wise are OK, you could measure low light levels without wiring up an amplification stage (or two).

It's true that someone could ignore the other pixels, but it has me wondering about the minimal elements and how available they are.

 

[doofus]

Can you give a couple examples of what you are calling "CMOS's"? The term CMOS (complementary metal-oxide semiconductor) usually refers to a type of transistor construction. Those transistors can be used in the circuitry of an imaging device, but they are not generally used as imaging sensors on their own:

https://en.wikipedia.org/wiki/CMOS

Sorry this is my ignorance. I am referring to CMOS imaging sensors because I am uncultured and unaware of their other use cases. I'm specifically interested in what the smallest unit is, and if its possible to build/purchase for low light level measuring. Where I can learn more about this, or circuits I could explore to understand this area more or if it's all semiconductor fab level stuff.

 

[doofus]

The light sensing part of the CMOS image sensor is a photo-diode. There are also integrated amplifiers. You could build a singe one of them with a discrete photo-diode and a high impedance operational amplifier (op-amp).

BoB

Thanks Bob, that's pretty enlightening. How do CMOS image sensor pixels "store charge" to be read out later?I'm somewhat familiar with op-amps and amplification of photodiode current in general, but I am curious how CCD and CMOS pixels have a concept of "exposure".

 

[rbelli1]

The CMOS sensor's diodes don't have charge storage. If charge storage is used then it will be in a capacitor after an amplifier. I don't know if that is used or some other readout mechanism is.

The exposure will start by discharging the storage capacitor or zeroing out an accumulator register or some other initialization. The input from the photo-diode will be connected to the measurement system (or some combination of diodes to measurement/accumulation systems) for an amount of time then disconnected. That is the exposure time.

For a CCD the system simply sets the CCD to a known initial condition then measures at a later time the change to charge in the cells. This is often done by reading the first line then clearing that line and then shifting the charge from all of the other lines over one place and repeating.

BoB

 

[Tom.G]

Try this site for an explanation:
https://global.canon/en/technology/s_labo/light/003/05.html

Above, and many more, found with:
http://www.google.com/search?hl=en&q=how+cmos+image+sensor+works

Have Fun!

Cheers,
Tom

 

[Borek]

For light intensity measurements you can always use the smallest resolution CMOS imaging sensor you can find (alternatively: the cheapest, no matter the resolution), and just sum up (or average) signals from all pixels.

 

[Rive]

I was thinking about how CCD's ... almost exclusively come in multiples.

The very point behind the CCDs was about reading them out through their Coupling. A single unit cannot be coupled

If you are flexible enough you can take any PV item as single pixel. Even up to a complete rooftop solar panel.

 

[doofus]

Try this site for an explanation:
https://global.canon/en/technology/s_labo/light/003/05.html

Above, and many more, found with:
http://www.google.com/search?hl=en&q=how+cmos+image+sensor+works

Have Fun!

Cheers,
Tom

Thanks Tom.G I am a fan of LMGFY as well :D. The link is cool. I love the diagrams. Here's what I still don't "know" yet. How is charge stored with an open circuit? Most photodiodes have relatively speaking very low capacitance. Often by design, because high capacitance loads are difficult to stabilize in a transimpedance sense compared to their low capacitance counterparts. Charge isn't getting stored there I don't think. Maybe it's charged on the series capacitor over the gain resistor in a typical TIA circuit.

If we put a capacitor in series with the output of an op-amp would that be enough? They demonstrate the gating with a switch, but I assume they are actually using a transistor.

I wish I could find a simple analog circuit for a single "pixel" CMOS. I think it would be really fun to breadboard and play with to understand what goes into the design for this sort of thing.

 

[doofus]

For light intensity measurements you can always use the smallest resolution CMOS imaging sensor you can find (alternatively: the cheapest, no matter the resolution), and just sum up (or average) signals from all pixels.

True! Its a little limiting in some characteristics. For example, if someone wants a high ADC count, you may be buying a lot of unused functionality that goes unused. Its still a good point and worth making.

 

[doofus]

The very point behind the CCDs was about reading them out through their Coupling. A single unit cannot be coupled

If you are flexible enough you can take any PV item as single pixel. Even up to a complete rooftop solar panel.

That would be part of the problem with my searches, to be fair though looking for "charge device" in google probably isn't going to be very helpful either :P. For some reason PhotoVoltaic's didn't cross my mind as a search term. Thanks for this