What are accepted figures for signal/noise (SNR) and dynamic rangefor CRT? LCD? Film? Human eye?

Discussion in 'Digital Photography' started by jeff miller, Feb 22, 2005.

  1. jeff miller

    jeff miller Guest

    I'm converting my old analog scanning electron microscopes to digital/pc

    One of the important questions is how many bits of resolution ie: levels
    of greyscale are required. I've heard the analog front ends of the
    scopes have a maximum SNR of about 72 dB, or 12 effective bits, and to
    give me some flexibility for digital image processing that's about what
    I'll shoot for: a bit better if I can get it.

    But on the other end of the scale, I'm wondering what are accepted
    figures for signal to noise or dynamic range of various display
    technologies, and of the human eye.

    It seems to me the published "contrast" figures for LCD displays of
    about 600:1 peg them at about 49dB dynamic range.

    What is generally accepted for B&W film? Is it highly dependent on film
    type and processing? Anyone know some typical figures for various
    processes? Is "silver print," which in my mind is marked by high
    contrast, a special technique or just a fancy word for black and white?

    How about for CRT's? Deos it depend in part on the design and
    construction of the CRT, and do dedicated monochrome CRTs have a greater
    dynamic range for greyscale than color CRT's used to display greyscale

    And what about the human eye, for that matter? It's been suggested the
    human eye has only about 6 bits or 36 dB dynamic range/SNR for greyscale
    images. Is that about right? Bonus question: I know the sensitivity of
    the human eye varies with color, being most sensitive at about 555 nM
    green. How about dynamic range and SNR? Deos it vary with color, too?


    jeff miller, Feb 22, 2005
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  2. jeff miller

    Tim Wescott Guest

    Most of what you ask I don't know, but what I _do_ know is this:

    In infrared imaging (which is what I'm familiar with) you take as many
    bits as you can get at the sampling rates you need -- this is currently
    14 bit ADC's at 10MHz on high-end systems. Even so you need some
    up-front gain selection (in the form of allowing the user to select a
    few different integration times) to get the best picture for the
    conditions. This data gets processed, gained up linearly or
    non-linearly, possibly spatially and/or temporally filtered, then
    presented to the user on video with an 8-bit DAC. Needless to say you
    throw away a _lot_ of information when you go through the
    user-presentation layer.
    Tim Wescott, Feb 22, 2005
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  3. I am not certain about the technical data, you should ask some electronic
    engineer, but it's very difficult to construct a lcd or tft scrren that can
    compete a good crt, because the picture in the crt is made by tiny bits of
    phosphorus struck by the cathod rays, also emitting their own light.Besides
    that, crts have a developing history of more than 50 years.
    Dimitrios Tzortzakakis, Feb 22, 2005
  4. jeff miller

    Jerry Avins Guest

    Man! you ask a lot of good questions! I suspect that many of the answers
    are to be found in papers of the SMPTE.

    I recall that the contrast between printer's ink and glossy paper is
    about 10:1 and B&W prints on glossy paper are a bit better (but on
    matte, a bit worse). The contrast ratio of newsprint can be as low as
    3:1 before the ink starts to look gray. Discouraging, no?

    Look at your TV screen when the set is off. No part of it gets darker
    when the set is on, but it sure looks like it does.

    Jerry Avins, Feb 22, 2005
  5. jeff miller

    Jerry Avins Guest

    Jerry Avins wrote:

    Correction: 30:1.
    Typical newsprint before the now-common biodegradable soy ink was 10:1.

    Jerry Avins, Feb 22, 2005
  6. Photographic paper can get to 2.0 od reflected, a range of 10 ^ 2 = 100:1.

    I just took a measurement from a printed page on coated stock, it
    yielded 1.27 => 1.3 => 10 ^ 1.3 => 20:1.
    Nicholas O. Lindan, Feb 22, 2005
  7. jeff miller

    Jerry Avins Guest

    Thanks for the reality check. The 30:1 figure was the best available,
    typically found in (new) eye charts and photographic resolution targets
    when I played with that stuff 40 years ago.

    Jerry Avins, Feb 22, 2005
  8. The eye is comfortable with a brightness range of ~50-100x in one scene
    with no pupilary/retinal adaptation, 1000:1 is viewable without noticing
    much adaptation - after images or time for pupil to adjust. The eye can
    accommodate a brightness range from reflected starlight on a field
    (remember clean air and no streetlights?) to sun on fresh snow (except
    in extreme cases, mountains, clean air ...). This about a 25 bit/zone
    range or ~32,000,000:1 and has no relevance to viewing a CRT.

    Within a 2.0 OD range it can easily discriminate to .005 OD in the middle
    of the range. However, OD is logarithmic and photocells and CRTs aren't.
    Figure you will need about 12 bits for very good fidelity, but 8 bits
    looks just fine on a CRT.

    Numbers above are from memory, you should check with a good text on
    human visual response if the values are important.

    Negatives have close to no relation to photoptic response. Exposure
    and development of film are such as to yield the thinnest (least dense)
    negative that will yield a good print. This figure depends on scene
    contrast, film contrast, paper contrast and the method used to image the
    negative on the film.

    I have to confess I don't see much relevance in all this to an SEM. All
    you want is the range of signal that presently goes to the current
    analog display and be able to accommodate that. 8 bits resolution should
    be adequate.
    Nicholas O. Lindan, Feb 22, 2005
  9. jeff miller

    Tim Giles Guest

    In normal lighting, the cones on the retina can detect contrasts of aprox.
    2% over a range of 2 orders of magnitude. Their dynamic range is closer
    to 4 orders of magnitude, but their ability to differentiate shades drops
    rapidly at the lower (darker) end of the scale.

    A 13 or 14 bit (linear) number would cover the same range, with the same
    (or better) ability to separate shades. (Aside: Computer displays manage
    with 8 bits, partly because they don't have a contrast ratio of 1:10^4 and
    partly because they are not linear.)

    There's a lot of good stuff at
    http://faculty-web.at.northwestern.edu/med/fukui/Human eye.pdf
    The response of B&W film covers about 3 orders of magnitude. For B&W
    paper, the reflectivity ratio between white and the deepest black is about
    100:1. Ilford publish a lot of useful data on photographic materials at

    Tim Giles, Feb 22, 2005
  10. jeff miller

    Jerry Avins Guest

    Nicholas O. Lindan wrote:

    Yes. There's more light reflected from a lump of coal in sunlight than
    from a bowl of snow indoors. The eye/brain isn't fooled.
    For high-resolution work, we used to dye the emulsion black to avoid
    exposing interior grains. After all, the image can't be in focus
    throughout the emulsion depth. To make the film a little faster, we
    would etch away the top few microns of the emulsion, leaving an exposed
    layer of grains, like in sandpaper.
    The question of which 8 bits can be important. I can adjust images from
    my 12-bit flat-bed scanner in ways that just don't apply to 8 bits.

    Jerry Avins, Feb 22, 2005

  11. But that is one of the attractive features of this group.
    ( OK, so I'm reading comp.dsp ;)

    By reading one learns so much more than one narrow area of knowledge!
    Richard Owlett, Feb 22, 2005
  12. I think that touches on a question I raised in another subthread.

    Why a *FAST* *16 bit a/d* ?

    Would it not be more suitable to have 2 A/Ds in "parallel"?

    A very fast 6-8 bit A/D for use while positioning sample.
    A 16+ bit A/D for analytical output.

    Caveat Lector ;}
    Richard Owlett, Feb 22, 2005
  13. Bart van der Wolf, Feb 22, 2005
  14. As others have said, you should capture and manipulate your original images
    at the highest bit depth you can handle, but what is usually transmitted by
    TV companies, recorded on DVDs, or saved in JPG files for display on CRTs
    is based on 8 bit linear coding after gamma correction, and it generally
    looks just fine. i.e. 8 bits seems to be quite sufficient for display.

    From personal observation, dynamic range of monochrome and colour CRTs are
    about the same, though the detail resolution can be slightly higher on a
    monochrome CRT displaying a monochrome signal.

    Roderick Stewart, Feb 22, 2005
  15. jeff miller

    jeff miller Guest

    Hmmm.... I'd thought about that but not to those extremes: in fact it
    makes much more sense at the extremes you suggest and I may well
    incorporate the idea. It will also give me a chance to jump-start the
    project before I have the funds for the higher performance A/D.

    Interestingly enough the 16 bit 5MSPS sigma-delta A/D I was thinking
    about has a 10MSPS "turbo" mode with 14 bit resolution.

    This thread is providing some very useful information about SNR of media
    and human eyes... keep it coming!
    jeff miller, Feb 22, 2005
  16. jeff miller

    jeff miller Guest

    Yes good point as there is no "dot pitch" issue.

    jeff miller, Feb 22, 2005
  17. Roger N. Clark (change username to rnclark), Feb 23, 2005
  18. jeff miller

    jeff miller Guest

    Great resource. Based on that data, I'd conclude that images captured
    from my microscope might benefit for 65K x 65K resolution, so I'll use
    16 bit D/A's to drive my scanning coils. Of course that kind or
    resolution can only be rendered in print. And represents an 8GB file
    size. And will take 2 hours to collect.... but might as well build it in.

    The figure of 10,000:1 dynamic range in any "one view" corresponds to
    about 12 bits. That's pretty much exactly the nominal dynamic range I've
    heard quoted for the front end of an SEM.

    But it might seems no current reproduction technology acheives quite that.

    jeff miller, Feb 23, 2005
  19. jeff miller

    jim Guest

    Hi Jeff
    If you have a set of 40 cards with gradations of gray going from black
    to white about 90% of the population will be able to lay them on a table
    sorted into the correct order. If you increase the number of cards to 64
    about 2/3 will be able to sort them correctly. If you up the number to
    80 the number of people that can sort them drops to something like 20%
    and by the time you get to 128 cards almost nobody can put them in the
    right order. I'm recalling this from memory but the numbers are
    something like that.
    The sensitivity to brightness for film and the human eye is not
    strictly linear. That is even when you give people 128 cards with linear
    gradations of grayscale cards to sort there are certain ranges of
    brightness wherein they will mostly get it right. So 8 bits (256
    brightness levels) will pretty much handle the non-linearty of both the
    display and the viewer. That is, most people will not be able to see the
    difference between any two adjacent gray levels.

    If your next question is - Is it spelled "grey" or "gray"? I don't know
    jim, Feb 23, 2005
  20. jeff miller

    Jerry Avins Guest

    jim wrote:


    Both are used, but "grey" is used less. The disappearance of "gaol" in
    favor of "jail" is now nearly complete :)

    Jerry Avins, Feb 23, 2005
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