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Photo Forum / Film Photography / Darkroom / March 2005What are accepted figures for signal/noise (SNR) and dynamic range for CRT? LCD? Film? Human eye?
I'm converting my old analog scanning electron microscopes to digital/pc based. 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 images? 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? Thanks! -Jeff > I'm converting my old analog scanning electron microscopes to digital/pc > based. > > 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 --snip-- > 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? 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.  SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com 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. -- Tzortzakakis Dimitri?s major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr > I'm converting my old analog scanning electron microscopes to digital/pc > based. [quoted text clipped - 31 lines] > > -Jeff > I'm converting my old analog scanning electron microscopes to digital/pc > based. [quoted text clipped - 27 lines] > 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? 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 SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ... > 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 Correction: 30:1. > 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? Typical newsprint before the now-common biodegradable soy ink was 10:1. Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > > 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 > > Correction: 30:1. 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. SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ >>>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 [quoted text clipped - 5 lines] > I just took a measurement from a printed page on coated stock, it > yielded 1.27 => 1.3 => 10 ^ 1.3 => 20:1. 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 SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ >>>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 [quoted text clipped - 5 lines] > I just took a measurement from a printed page on coated stock, it > yielded 1.27 => 1.3 => 10 ^ 1.3 => 20:1. Unless print paper has changed in the twenty-five years since I made some of those measurements, paper doesn't quite get to 100:1. I have seen papers with blacks at 2% (50:1). Most papers have about 3% black reflectance, however (33:1) At one time I worked on a project finding black coatings for cameras and other EO sensors. It is amazing how hard it is to get a truly black black. Even the famed 3M Black Velvet was a 2% reflectance. The only thing we found was a coating by Martin Marietta that was close to 1%. The emulsion itself in photographic paper is what creates that reflection, regardless of how much black silver it has in the emulsion. I would assume in inkjet ink it would be the binder that holds the pigment or dye to the paper. With laser printers the toner has some wax in it. >>>> 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 [quoted text clipped - 22 lines] > pigment or dye to the paper. With laser printers the toner has some wax > in it. I once got down to about 0.2% edge-on to a stack of single-edge razor blades with the backs removed. (I was puzzled at first by poor performance, but it worked like a charm after being degreased.) That's about what one gets with a Tyndall tube*. I still have a can of 3M's Nextel Velvet Black, but as far as I know, they don't make it any more. (Nextel now means something else.) Kodak's Brushing Lacquer was pretty good, too, but I think that's also a thing of the past. I'm almost out of Edmund's flock paper, but that's still available. Jerry __________________________________ * I found no web reference to a Tyndall tube, so I figure a brief description is in order. Tyndall needed a good light absorber for his ultramiscrope, http://tinyurl.com/66e63 He drew a piece of glass tubing -- a side tube on his specimen chamber -- to a cone that curved like the toe of a jester's shoe. The outside of the tube was coated in soot from a candle flame. Light entering the tube is reflected deeper and deeper into the small end, suffering a small loss at each reflection. Eventually, a ray turns around and starts out, again reflecting many times. By the time it emerges, all those slight absorptions have pretty well attritted it to zilch. SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > Nicholas O. Lindan wrote: > > Photographic paper can get to 2.0 od reflected, a range of 10 ^ 2 = 100:1. > Unless print paper has changed in the twenty-five years since I made > some of those measurements, paper doesn't quite get to 100:1. I have > seen papers with blacks at 2% (50:1). Most papers have about 3% black > reflectance, however (33:1). It is a non-specula measurement. 2.0 isn't hard to get to. However, 2.0 is not a good value for making prints with any shadow detail as it is up on the shoulder. 1.8 OD is a better max value, closer to 2%, as you indicated. > At one time I worked on a project finding black coatings for cameras and > other EO sensors. It is amazing how hard it is to get a truly black > black. Even the famed 3M Black Velvet was a 2% reflectance. The only > thing we found was a coating by Martin Marietta that was close to 1%. Not only isn't it black, it's yellow, or blue or red ... Black, like white seems to be an imaginary concept. SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ >>>Photographic paper can get to 2.0 od reflected, a range of 10 ^ 2 = 100:1. >> [quoted text clipped - 15 lines] > Not only isn't it black, it's yellow, or blue or red ... Black, like > white seems to be an imaginary concept. Carbon lamp black is probably the darkest diffuse reflector. It's reflectance in the visual range is about 1.6%: http://pubs.usgs.gov/of/2003/ofr-03-395/ofr-03-395.html specifically: http://pubs.usgs.gov/of/2003/ofr-03-395/PLOTS/M/carbon_black_gds68.1533.gif Spectralon is the brightest: "Spectralon reflectance material gives the highest diffuse reflectance of any known material or coating over the UV-VIS-NIR region of the spectrum." It is >99% in the visual range. http://www.labsphere.com/products.asp?parent_id=187&catId=188 Roger > >>At one time I worked on a project finding black coatings for cameras and > >>other EO sensors. It is amazing how hard it is to get a truly black [quoted text clipped - 8 lines] > reflector. It's reflectance in the visual range is > about 1.6%: The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level corresponding to this part of the chart is deemed not to have come from the chart at all but from light scattered in the lens, and the circuitry is adjusted accordingly. Rod. > The BBC's Test Chart No 57 included (includes?), for the purpose of > setting [quoted text clipped - 11 lines] > > Any value of the output signal above dark current level Why not just test with the lens cap on? > > The BBC's Test Chart No 57 included (includes?), for the purpose of > > setting [quoted text clipped - 13 lines] > > Why not just test with the lens cap on? Test what? The superblack box is for setting flare correctors, which of course won't do anything if there isn't any flare. All you can test with the lens cap on is black balance, clamp balance ("pulse cancellation") and black shading. Superblack has to be visible simultaneously with a large area of high brightness in order to set and balance the flare correctors. Rod. That should be slightly different, as the black rectangle will pick up light not meant to be focused in that area (the stuff that lowers contrast). >Why not just test with the lens cap on? Signature<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<> John P Sheehy <JPS@no.komm> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>>< > In article <422E63F7.9070008@qwest.net>, Roger N. Clark > Carbon lamp black is probably the darkest diffuse > reflector. It's reflectance in the visual range is > about 1.6%: The color of the lamp black varies with the source of the paraffin. North Sea Brent produces a different color than West Texas Light, produces a different color than .... This just for plain-ole black paint. 1.6% is quite a bit of reflectance - 1.8 oD, 6 stops down from white. SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ > In article <422E63F7.9070008@qwest.net>, Roger N. Clark (change username > to [quoted text clipped - 31 lines] > from > light scattered in the lens, and the circuitry is adjusted accordingly. Ah - so that's where my licence fee goes...... [...] > > Any value of the output signal above dark current level corresponding to > > this [quoted text clipped - 3 lines] > > Ah - so that's where my licence fee goes...... Yes, it goes into maintaining broadcast picture and sound quality in various ways that most people probably don't even know about. It also goes into training people to be able to do this, and these people gradually migrate into the ITV and freelance market, where there are hardly any training courses except manufacturers' proprietary ones on specific items of equipment unless individuals are prepared to pay for them at costs in the region of 1000ukp per week. Thus, a great deal of the expertise in daily use in broadcasting *everywhere* in the UK is provided through the BBC. Rod. > [...] > [quoted text clipped - 16 lines] > > Rod. But on other side of "Pond" we find FREE enterprise works well. I never did understand licensing receivers. Transmitters ( and operators ) yes. Receivers *NO* ! > > > Ah - so that's where my [BBC]licence fee goes...... > But on other side of "Pond" we find FREE enterprise works well. Nothing comes for 'FREE'. Enterprise on both sides of the puddle is pretty much the same. It's that the British find commercial enterprise so very tedious. It's just not on, don't you know. > I never did understand licensing receivers. It's to collect fees. It's about the money, it's always about the money. No commercials on BBC ... No PBS begging marathons. I think I would rather pay a fee. Got cable? -- ~$300/year -- So it's not like it's free over here either. SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ >>>>Ah - so that's where my [BBC]licence fee goes...... > >>But on other side of "Pond" we find FREE enterprise works well. > > Nothing comes for 'FREE'. I *AGREE* ;) > Enterprise on both sides of the puddle > is pretty much the same. It's that the British find commercial > enterprise so very tedious. It's just not on, don't you know. I was thinking more along line that if a commercial station could not send a quality signal they would extremely quickly lose audience and thus advertisers. I see no reason to insert any arm of government into process. >>I never did understand licensing receivers. > > It's to collect fees. It's about the money, it's always about the > money. Take it from advertisers not from me. OK, so in end consumer ends up paying. I just suspect free market is better filter IMHO ;} > No commercials on BBC ... No PBS begging marathons. > I think I would rather pay a fee. Don't get me started on PBS. They are just as blatantly commercial as commercial stations. > Got cable? -- ~$300/year -- So it's not like it's free over here > either. Who is crazy enough to have cable &/or sattelite. There is not that much worth paying for in anycase. Jeff miller wrote: > I'm converting my old analog scanning electron microscopes to digital/PC based. > One of the important questions is how many bits of resolution i.e.: levels > of grayscale are required ... the eye ... film ... 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. SignatureNicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ ... > 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 [quoted text clipped - 3 lines] > 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. 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. > 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. [quoted text clipped - 9 lines] > contrast, film contrast, paper contrast and the method used to image the > negative on the film. 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. > 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. 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 SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > [snip] >> I have to confess I don't see much relevance in all this to an SEM. [quoted text clipped - 6 lines] > > Jerry 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 ;} >> [snip] >> [quoted text clipped - 19 lines] > > Caveat Lector ;} 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! > This thread is providing some very useful information about SNR of media > and human eyes... keep it coming! See: Notes on the Resolution of the Human Eye How many megapixels equivalent does the eye have? The Sensitivity of the Human Eye (ISO Equivalent) The Dynamic Range of the Eye The Focal Length of the Eye: http://www.clarkvision.com/imagedetail/eye-resolution.html Contrast thresholds data: http://www.clarkvision.com/visastro/omva1/index.html Roger >> This thread is providing some very useful information about SNR of >> media and human eyes... keep it coming! [quoted text clipped - 11 lines] > > Roger 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 >>> This thread is providing some very useful information about SNR of >>> media and human eyes... keep it coming! [quoted text clipped - 25 lines] > > -Jeff Ooops, 10K:1 is more like 13.5 bits. But I'm finding a wide range of figures online for dynamic range of the human eye. I visited Barco's website in hopes of finding contrast figures for CRTs and in a white paper comparing CRT and LCD displays, I think they tossed out a figure of 100:1 for the human eye. 1,000 to one has been mentioned here as well. The site referenced above so far holds the title for claiming the widest range. The white paper also tossed out a contrast figure for CRT's of 3,000 to one, about 11.5 bits. The human eye figure of 100:1 is the most surprising. The low contrast of film is also surprising. I guess it (re film) shouldn't be, I know that MgO is the most reflective substance as far as diffuse reflectors go. I forget the figure. -Jeff ... > The human eye figure of 100:1 is the most surprising. The low contrast > of film is also surprising. I guess it (re film) shouldn't be, I know > that MgO is the most reflective substance as far as diffuse reflectors > go. I forget the figure. Photographic paper is often coated with barium sulfate -- baryta -- under the emulsion. Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ >>>> This thread is providing some very useful information about SNR of >>>> media and human eyes... keep it coming! [quoted text clipped - 48 lines] > > -Jeff Do you mean the contrast ratio of film or of photographic paper? Photographic paper, especially colour paper, is quite limited. The contrast ratio of film can be phenomenal, depending on its type. Slow black and white film reversal processed into slides can have extreme contrast, and usually has to be bleached down to the point where it looks reasonable to our eyes. When considering the contrast ratio of the eye, remember this is an instantaneous figure. Our pupils are constantly adapting as we scan around a scene, and the overall constrast we perceive after flitting our eyes around a scene can be far higher. Regards, Steve ... > Do you mean the contrast ratio of film or of photographic paper? > Photographic paper, especially colour paper, is quite limited. The [quoted text clipped - 7 lines] > around a scene, and the overall constrast we perceive after flitting our > eyes around a scene can be far higher. I understood that the accommodation to light levels provided by pupil size is a rapid but small part of the overall range, and that most of it is provided by light's bleaching of the photoreceptors. Was I wrong? Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > > > > > This thread is providing some very useful information about SNR of > > > > > media and human eyes... keep it coming! As long as you realize you are talking to amateurs in this field, I don't think there are any Opthamologists here. All advice worth price charged. > > > 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 pixels you need are determined by the spot size of the e-beam and the size of the object to be scanned. From published SEM photos I would say 2-3 megapixels may be more that enough. > > > 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. Quite possible. Realize that photography compresses or expands the range of subject luminance, be it 1000:1 or 10:1 and fits it to the 100:1 standard of the printed image. The 100:1 range of the printed image is there because that is what the eye can look at without adaptation: concentrating on the shadows doesn't result in blinding glare from the white part of the photograph. Printing evolved to accommodate the human eye. If the range of the eye were to be 10,000:1 then photographs and printing would be of such low contrast as to be worthless - we would see it as pale grey on pale grey. -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ >>>>>>This thread is providing some very useful information about SNR of >>>>>>media and human eyes... keep it coming! > > As long as you realize you are talking to amateurs in this field, I don't > think there are any Opthamologists here. All advice worth price charged. Speak for yourself ;-). I have written books and articles on how the human eye operates and detects contrast. But one need not be a professional to do basic observations to prove something. >>>>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. There are several ways everyone can test this. Use stars. Many people can see the Galilean satellites with their unaided eyes. Jupiter's moon Callisto averages stellar magnitude 5.5, and Jupiter -2.5, for a range of 8 stellar magnitudes. One magnitude = fifth root of 100, or 2.5119, so 8 magnitudes is 1585 (2.5119^8, where ^ means power). Next time venus is up high in a dark sky, it will be about magnitude -4.4. What are the faintest stars you can see? In a dark sky away from the city, magnitude 6 is possible, and in very good skies, 7. That is a range of 10.4 to 11.4 magnitudes, or a range of 14,000 to 36,000. If you have a telescope, you can use star clusters to test brightness ranges. For example, in the Pleiades star cluster, I have seen magnitude 14 stars in the same field of view as magnitude 2.9 Alcyone, for a magnitude difference of 11.1 or a range of 27,500. The full moon is magnitude -12.5 and in the city people can see stars of magnitude 3, and in the country magnitude 5 or perhaps fainter. That is a range of 15.5 to 17.5 magnitudes, or 1.6 million to about 10 million! (But the moon is an extended source, so this comparison might be biased somewhat.) A million to one is not unreasonable. > Printing evolved to accommodate the human eye. If the range of the eye > were to be 10,000:1 then photographs and printing would be of such low > contrast as to be worthless - we would see it as pale grey on pale grey. Why? I've demonstrated above that greater than 10,000 to one is easy, yet we still perceive photographs that appear nice and not of "low contrast." The eye+brain is a contrast detector, not an intensity detector. And it is non-linear. Roger SNIP > 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. See e.g. <http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html> Bart > > 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. 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! [...] >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. 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://webvision.med.utah.edu/ and http://faculty-web.at.northwestern.edu/med/fukui/Human%20eye.pdf >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? 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 www.ilford.com -Tim > 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 > images? 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. Rod. >>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 [quoted text clipped - 12 lines] > > Rod. Yes good point as there is no "dot pitch" issue. -Jeff 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 > I'm converting my old analog scanning electron microscopes to digital/pc > based. [quoted text clipped - 31 lines] > > -Jeff ... > If your next question is - Is it spelled "grey" or "gray"? I don't know > :} ... Both are used, but "grey" is used less. The disappearance of "gaol" in favor of "jail" is now nearly complete :-) Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > ... > [quoted text clipped - 8 lines] > > Jerry The contrast sensitivity of the eye has been stated to be about 1% , which makes it a logarithmic sensor. When a JND is nearly a constant ratio, the sensor is nearly logarithmic. Let us not forget what we learned in the early days of HI-FI. It is not sufficient to match the bandwidth of the human ear. The amplifier-peaker system must be flat over the whole range of human hearing in order to be sensed as real. The Nyquist rule says that the sampling rate of a system must be twice the bandwidth of its input. This applies to spatial frequencies as well as sonic or electronic. > .... The amplifier-peaker > system must be flat over the whole range of human hearing in order to be > sensed as real. ... That's not entirely true. Some speakers in the 50s, notably Electro- Voice, had an intentional broad response peak at around 5 kHz to produce "presence". It brought an immediacy to in-store A-B comparisons that sold their speakers, but made listening at home more than an hour or so unpleasant. Or is that what you meant by "peaker system"? Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ >> .... The amplifier-peaker system must be flat over the whole range >> of human hearing in order to be sensed as real. ... [quoted text clipped - 7 lines] > > Jerry No, I was having rouble with the s key. I hink I go i fixed now. ... > No, I was having rouble with the s key. I hink I go i fixed now. <vgb> Jerry SignatureEngineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ |
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