dynamic range of digital image sensors

Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams

Mr.Adams wrote:
Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams

There are limits to the capacitor area directly related to the pixel size.
If you could capture more photo-electrons in the well, you would get an
improved signal-to-noise ratio, but needing more photon would result in a
decreased sensitivity. We already see sensitivities down to ISO 50 in
some 8MP cameras - do we want lower?

Cheers,
David

Mr.Adams wrote:
Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams

There are limits to the capacitor area directly related to the pixel size.
If you could capture more photo-electrons in the well, you would get an
improved signal-to-noise ratio, but needing more photon would result in a
decreased sensitivity. We already see sensitivities down to ISO 50 in
some 8MP cameras - do we want lower?

Cheers,
David

"Mr.Adams" wrote in message
m...
Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams

There is only so much room available at the photosites on the sensor. When
they pack, say, 4 mp on the same sized sensor as a 3 mp sized sensor,
everything must be made smaller, thus noise and dynamic range become a
problem. Many new cameras do okay in the noise department due to noise
removal algorithms, but that is not a cure all - especially at higher ISOs.

DSLRs use a large sensor and the sensor is designed with less electronics at
each photosite so you get amazingly low noise at very high ISOs. Due to this
design (not to mention the swinging mirror being in the way), the sensor
can't do live video. The images look smooth and unprocessed and have better
dynamic range.
bg

"Mr.Adams" wrote in message
m...
Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams

There is only so much room available at the photosites on the sensor. When
they pack, say, 4 mp on the same sized sensor as a 3 mp sized sensor,
everything must be made smaller, thus noise and dynamic range become a
problem. Many new cameras do okay in the noise department due to noise
removal algorithms, but that is not a cure all - especially at higher ISOs.

DSLRs use a large sensor and the sensor is designed with less electronics at
each photosite so you get amazingly low noise at very high ISOs. Due to this
design (not to mention the swinging mirror being in the way), the sensor
can't do live video. The images look smooth and unprocessed and have better
dynamic range.
bg

Mr.Adams wrote:

Hello Ng,

My questions regards the dynamic range of digital image sensors (CCDs
and CMOS). I understand that this is mostly limited by the size of the
capacitor / well that holds the electrons and the amount of electronic
noise produced by the sensor. But if the capacitor size is a limiting
factor, why don´t the manufacturers use larger ones ? Is there a
special relationship between the size of the capacitor and the size of
of each pixel that keeps them from doing so ?

Thanks for your input!
Mr.Adams


Because of a limitation on the number of layers in IC processing, the
area of the capacitor is directly proportional to the area of the pixel.
If we keep the overall size of sensor chips the same (silicon real
estate is expensive) and shrink pixel size to get more pixels on the
chip, then the size of the capacitor shrinks also, making a smaller
capacitance.

Note that the size of the capacitor is only one of many potential noise
sources. Whether the capacitance is the limiting noise factor on not is
a tough question for a particular sensor. However, if we reduce other
sources of noise, the capacitance issue will indeed eventually bite us.

The size of the capacitor is a volume problem. The area of the pixel
fixes two of the dimensions, the ability to create deep diffusions
fixes
the other dimension. If one tries to make a really deep capacitor, one
runs into the problem that one cell might short out to its neighbor.

So, as pixels shrink in area, the capacitance shrinks in volume.

Mitch

The size of the capacitor is a volume problem. The area of the pixel
fixes two of the dimensions, the ability to create deep diffusions
fixes
the other dimension. If one tries to make a really deep capacitor, one
runs into the problem that one cell might short out to its neighbor.

So, as pixels shrink in area, the capacitance shrinks in volume.

Mitch

In article , David J
Taylor says...

There are limits to the capacitor area directly related to the pixel size.
If you could capture more photo-electrons in the well, you would get an
improved signal-to-noise ratio, but needing more photon would result in a
decreased sensitivity. We already see sensitivities down to ISO 50 in
some 8MP cameras - do we want lower?

Well no, sensitivity is not decreased, because you don't need more
electrons. With a larger well you essentially have more dynamic range
(it takes longer before each pixel saturates).
--

Alfred Molon
------------------------------
Olympus 4040, 5050, 5060, 7070, 8080, E300 forum at
http://groups.yahoo.com/group/MyOlympus/
Olympus 8080 resource - http://myolympus.org/8080/

In article , David J
Taylor says...

There are limits to the capacitor area directly related to the pixel size.
If you could capture more photo-electrons in the well, you would get an
improved signal-to-noise ratio, but needing more photon would result in a
decreased sensitivity. We already see sensitivities down to ISO 50 in
some 8MP cameras - do we want lower?

Well no, sensitivity is not decreased, because you don't need more
electrons. With a larger well you essentially have more dynamic range
(it takes longer before each pixel saturates).
--

Alfred Molon
------------------------------
Olympus 4040, 5050, 5060, 7070, 8080, E300 forum at
http://groups.yahoo.com/group/MyOlympus/
Olympus 8080 resource - http://myolympus.org/8080/