# How does a CCD work?

I have read the textbook but it does not explain it in a very intuitive manner. This is the explanation (the bolded bits are the unintuitive bits):

Rectangular electrodes and an insulating layer are thin enough to allow light photons to pass through and liberate an individual electron in the light sensitive material underneath.

When collecting charge, the central electrode in each pixel is held at +10 V and the two outer electrodes at +2 V, which ensures that the liberated electrons accumulate under the central electrode.

After the pixels have collected charge for a certain time, the charge of each pixel is shifted towards the output electrode via the adjacent pixels. This is achieved by altering the voltage level of each electrode in a sequence of three step cycles.

Andy Resnick
After the pixels have collected charge for a certain time, the charge of each pixel is shifted towards the output electrode via the adjacent pixels. This is achieved by altering the voltage level of each electrode in a sequence of three step cycles.

Does this video help?

davenn
Does this video help?

Not really since youtube is blocked

Drakkith
Staff Emeritus
I don't get it after the electrons are liberated and are 'captured' by the potential wells.

Are you referring to when the charge in each pixel is transferred to the next pixel over?

Are you referring to when the charge in each pixel is transferred to the next pixel over?

Yes

davenn
Gold Member
2021 Award
Not really since youtube is blocked

why is it blocked ? I can view that video with no probs

Drakkith
Staff Emeritus
Yes

Alright, well, I'm not quite sure what to tell you. Let's start with the basic process of a voltage being applied to move the electrons from one pixel to another. Do you understand what that means?

phinds
Gold Member
2021 Award
why is it blocked ? I can view that video with no probs
Me too

D H
Staff Emeritus
why is it blocked ?
There are many possibilities:
• The OP is 17, with a device presumably supplied by his parents. His parents applied parental controls on the device to blacklist youtube.
• The OP uses his device at home, where his parents have blacklisted youtube at the router or modem level.
• The OP uses his device at school, which has blacklisted youtube at the internet provider level.
One final possibility (easily discountable in this case) is that the OP lives in a country that has blacklisted youtube at the gateway level.

davenn
Alright, well, I'm not quite sure what to tell you. Let's start with the basic process of a voltage being applied to move the electrons from one pixel to another. Do you understand what that means?

I get that bit, yes. But I don't get why.

There are many possibilities:

• The OP uses his device at school, which has blacklisted youtube at the internet provider level.

Correct

Last edited:
Drakkith
Staff Emeritus
I get that bit, yes. But I don't get why.

Electrons are charged particles. When you apply a voltage you exert a force on them that causes them to move. By applying this voltage in the right way, we can move the electrons from one pixel to another pixel to another until they wind up at the final pixel, which finally moves them into a charge amplifier to be "read out" by the electronics.

Imager
Electrons are charged particles. When you apply a voltage you exert a force on them that causes them to move. By applying this voltage in the right way, we can move the electrons from one pixel to another pixel to another until they wind up at the final pixel, which finally moves them into a charge amplifier to be "read out" by the electronics.

OK

Electrons are charged particles. When you apply a voltage you exert a force on them that causes them to move. By applying this voltage in the right way, we can move the electrons from one pixel to another pixel to another until they wind up at the final pixel, which finally moves them into a charge amplifier to be "read out" by the electronics.

Why don't they just put some multiplier tubes underneath the pixels, connected to a computer, which can then produce an image on the screen since they know which pixel each tube corresponds to? Why do they have to shift it to the next pixel?

Drakkith
Staff Emeritus
Why don't they just put some multiplier tubes underneath the pixels, connected to a computer, which can then produce an image on the screen since they know which pixel each tube corresponds to? Why do they have to shift it to the next pixel?

My best guess is that they're too large. CCD pixels are only about six to a dozen microns or so in size, way too small for multiplier tubes. The pixels have to be that small to pick up the detail from the image formed by the optical system. Also, photomultiplier tubes are entirely different beasts from CCD's. They use a whole other method for picking up the light, so you would never have a photomultiplier hooked up to a CCD. Put simply, CCD's and photomultipliers perform the same function (detecting light and converting it to a digital signal) using different methods, and each has their strengths and weaknesses.

My best guess is that they're too large. CCD pixels are only about six to a dozen microns or so in size, way too small for multiplier tubes. The pixels have to be that small to pick up the detail from the image formed by the optical system. Also, photomultiplier tubes are entirely different beasts from CCD's. They use a whole other method for picking up the light, so you would never have a photomultiplier hooked up to a CCD. Put simply, CCD's and photomultipliers perform the same function (detecting light and converting it to a digital signal) using different methods, and each has their strengths and weaknesses.

Thanks. That makes more sense.