dynamic range of digital image sensors

Archived from groups: rec.photo.digital (More info?)

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

Archived from groups: rec.photo.digital (More info?)

"Mr.Adams" <mr.adams@gmx.net> wrote in message
news:ffeb7fc.0504050223.444ef308@posting.google.com...
> 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

Archived from groups: rec.photo.digital (More info?)

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

Archived from groups: rec.photo.digital (More info?)

In article <1112718936.188191.47470@l41g2000cwc.googlegroups.com>,
MitchAlsup@aol.com says...
> 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.

Not sure if I understand this. With DRAM we are at the point where they
make vertical capacitors, because if they made a simple horizontal
capacitor, this would not be able to store enough charge. Why can't they
make vertical capacitors also for CCDs ?
--

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

Archived from groups: rec.photo.digital (More info?)

"Mr.Adams" <mr.adams@gmx.net> wrote in message
news:ffeb7fc.0504050223.444ef308@posting.google.com...
> 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 ?
They do in fact make sensors with larger sensels (each individual sensor
element), and they do deliver better signal/noise ratios. But there are some
drawbacks:
The overall sensor size equals sensel size*megapixels, so in the push to get
more megapixels, either the overall sensor size must be larger, or the
individual sensel size must be smaller.
Larger sensor size = larger lens=bigger heavier camera=(generally) less
market appeal.
Just like all integrated circuits, the larger the sensor, the lower the
yield of acceptibly good chips, hence manufacturing costs climb
significantly.
Larger sensels require more light to saturate them, hence they have a lower
native ISO equivalence. So while a sensor made with larger sensels may
deliver exceptional SN ratio, it might be at it's best at ISO 25
equivalence. Once amplification has been applied to deliver ISO 100 or
higher equivalence, the SN ratio will have dropped, and may not be
significantly better than a smaller sensel that has a native ISO 100
equivalence.
Thus, the actual sensor chips used are at a size where the manufacturer
feels they have the best trade-off between SN ratio on the one hand vs cost
and overall size on the other hand.

>
> Thanks for your input!
> Mr.Adams

Archived from groups: rec.photo.digital (More info?)

In article <1112736057.898236.305320@z14g2000cwz.googlegroups.com>,
MitchAlsup@aol.com says...

> Verticle electron storage blocks the incomming light! This would not be
> an
> issue with back illuminated sensors.

Then let's make a vertical storage back-illuminated CCD - more
sensitivity and dynamic range!
--

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