dynamic range of digital image sensors

Discussion in 'Digital Photography' started by Mr.Adams, Apr 5, 2005.

  1. Mr.Adams

    Mr.Adams Guest

    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, Apr 5, 2005
    #1
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  2. 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
     
    David J Taylor, Apr 5, 2005
    #2
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  3. Mr.Adams

    BG250 Guest

    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
     
    BG250, Apr 5, 2005
    #3
  4. Mr.Adams

    Don Stauffer Guest


    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.
     
    Don Stauffer, Apr 5, 2005
    #4
  5. Mr.Adams

    MitchAlsup Guest

    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
     
    MitchAlsup, Apr 5, 2005
    #5
  6. Mr.Adams

    Alfred Molon Guest

    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, Apr 5, 2005
    #6
  7. Mr.Adams

    Alfred Molon Guest

    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, Apr 5, 2005
    #7
  8. With a constant pixel area and lens f/number, the photon flux is constant.
    To get the improved SNR, you need to collect more photons, therefore you
    need to expose for a longer time. I.e. to take advantage of a bigger
    well, and get a better SNR, you need to reduce sensitivity. Simply
    getting a larger dynamic range may only help you capture specular
    highlights better.

    Cheers,
    David
     
    David J Taylor, Apr 5, 2005
    #8
  9. Mr.Adams

    Justin Thyme Guest

    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.
     
    Justin Thyme, Apr 5, 2005
    #9
  10. Mr.Adams

    MitchAlsup Guest

    "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 ? "

    Verticle electron storage blocks the incomming light! This would not be
    an
    issue with back illuminated sensors.
     
    MitchAlsup, Apr 5, 2005
    #10
  11. Mr.Adams

    Alfred Molon Guest

    Then let's make a vertical storage back-illuminated CCD - more
    sensitivity and dynamic range!
     
    Alfred Molon, Apr 5, 2005
    #11
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