jim wrote:> > Jeff Miller wrote: > > >>Hmmm, would it be fair to say there's a difference of opinion >>between Jerry and Fred in these latest responses? The sigma-delta >>converter I'm looking at has a nominal sample rate of 5Ms/S, but a >>nominal bandwidth of 2.45Mhz. You don't think that spatial data above >>2.45Mhz will simply get tossed, Jerry? Perhaps it's much the same thing >>as blurring the image. >> >>I'm a _little_ disturbed that I might be "wasting" half or 3/4 of my >>pixels. EM photos are breathtaking because they are so sharp and >>detailed. I wish I could turn the digital filter on and of. Don't think >>I can. But for final images, I don't think there's any real limitation >>on resolution. For display, I could sample at 2Kx2K and dither(?) down >>to 1Kx1K... though I may need IMAQ Vison (~$700) to do it in real time. >>For rendering to posters I should be able to capture 32kx32k. >> >>Now that Fred mentions it I am aware of "screening" issues when >>scanning. And sure enough as I understand it the solution is to scan at >>2X or more of the screening frequency. >> >>I'm leaning toward the sigma-delta converters with built-in filtering so >>I think I'll be fine in the vertical. >> >>But now Fred's got me thinking about the horizontal. How do I "filter" >>in that dimension? Or oversample for that matter? Perhaps if I can >>figure out the ultimate resolution (which has mostly to do with "probe >>size", which as I understand it is approximately equal to the area of >>the cathode that deos the actual emitting: about 25nm for Tungsten >>filaments, maybe 10nm for LaB6 cathodes, and 1-2nm for feild emission >>sources, motly I use Tungsten and LaB6) and then scale my scan for >>10nm/pass for Tungsten and 5nm/pass for LaB6... >> >>Hmm, well this will be interesting anyway. > > > Hi Jeff, > I haven't been following this before today, but from what I can tell > you have not really revealed the necessary information to address the > issue of aliasing. If I understand you have a signal that is both > digital and analog. In the vertical direction in order to correctly deal > with aliasing you need to know how many scan lines are being mapped to > how many rows in the array that will store the data. > In the horizontal direction that is where the sampling rate comes into > play. If you scan 500 lines at 10 times a second you have 5000 lines per > second. If your sampler is taking 5 million samples a second then you > will have 1000 samples per line. So that is the natural size of the > array that your data will fit into 500x1000. If you want to put it into > a 512X512 or 1024X1024 or whatever sized array you need to resample > using an appropriate resampling scheme. Spline based interpolation works > well for image processing resampling - especially if you use tension > splines with adjustable tension. That allows you to deal with some of > the tradeoffs between aliasing and image clarity on the fly. > > -jim > > ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =----OK, first off let me say I realized later I was confused (and likely confusing you guys a bit) by getting my horizontal confused with the vertical- not that I confused the two per se, but I confused them in the common contextual meaning. So now I see that Jerry and Fred do agree. Where Jerry says horizontal filtering is "easier than vertical. Just low-pass the data stream.", I think the sigma-delta converter/filter will do that for me whether I like it or not. I'm still not quite clear on how to over sample/filter the vertical, then. Jim, the SEM's I'll be working with were born strictly analog. Their operating principle is remarkably simple. They use a pair of scanning coils, one for x and one for y. They can be driven at virtually any speed, limited only by the frequency response of the coils and by secondary considerations such as response of the detector (generally 5-10Mhz nominal) and some sensitivity tradeoffs. Typically they are raster-scanned. In all cases the CRT deflection yoke is driven at the exact same speeds, in sync, by the same circuit that drives the SEM deflection coils. The signals are voltage divided down to drive the SEM coils depending on magnification selected: at low mags the scanning coils are driven hard, at high mags they're barely "tickled" so the beam is driven over a very small area of the sample. In most cases several nominal scanning speeds and formats are available to the operator, generally at least a "high speed" low-res scan with 500-2000 lines/image at 2-5 refreshes/sec and a low speed, hi res scan of 5-10K lines or so for photography: a single exposure can take 10 seconds or more. The phosphors used in the CRTs are of a special long decay type so the images persist for viewing by the operator in high speed mode, but of course that only works so well and is a major source of annoyance at higher resolutions. It's not uncommon to drive at NTSC rates as well, interlacing and all, so images can be recorded to tape, sent remotely, etc.: but at higher magnifications this is seldom done, for some reason S/N starts falling apart, if only because at higher mags the (DC driven) "condenser" lens is driven harder which narrows the beam and (I think) limits the number of electrons available to hit the target. I'll be retrofitting with a delta-sigma converter with a nominal sample rate of 5 Ms/sec and a built in digital filter with a nominal bandwidth of 2.45 Mhz. I say nominal because I'll be lowering the clock speed considerably for saving images to disk at high mags and high resolutions. The digital filter is driven by the same clock source as the sampling portion, therefore the bandwidth is always limited to half the sampling rate. It is important to understand that I'll be using down counters and DACS to drive the scanning coils in sync with the pixel clock: the whole awkward console is divorced. The PCI card doing the high level stuff has an on-board buffer of 32 Mbytes, and I'll be using 16 bit resolution. Therefore my maximum "native" sampled array will be 4Kx4K. I'll also have "fast scan" modes of 512x512, 1024x1024, and 2Kx2K. But ultimately there is no hard limit to the size of the arrays other than the resolution of my DACS, I don't think: the board can save to disk as fast as the disk subsystem and PCI bus allows. Driven at low speeds I suppose 65K*65K should be possible, even desirable for posters. Ultimate magnification and resolving power (i.e.: resolution) is dependent mostly upon the type of cathode. I believe the figures are approximately 25nM for Tungsten, 10 nM for Lanthanum Hexaboride, and 1-2nM for field emission sources. I'm using W and LaB6 now, but I'll be trying to upgrade to FE (aka FESEM) in the future. In fact I will need 1 nM resolution if I'm going to explore single electron transistor technology. So given that the horizontal is filtered by the A/D as a matter of course, I'm wondering first if keeping the scan line width at 1/2 or less of the spot sizes mentioned above qualifies as a type of oversampling and will eliminate vertical aliasing without post processing. If not, it might beg the question as to why I'm going with a delta-sigma with built in filter, given the fact that I'll need post processing anyway to clean up the vertical. I don't _think_ aliasing/Moire' is an issue in the traditional strictly analog arrangement, but I could be wrong. Although it's obvious why there would not be aliasing in the horizontal, I'd be stumped as to why it would not be an issue in the vertical, given that in a sense the line vs. line scanning is still discreet. Hmmm. There's nothing stopping me from grabbing a frame using a traditional top down scan pattern and then exchanging x for y drive and taking one side-to-side. By rotating, correlating, and differentiating the two images with the knowledge that one is anti-aliased horizontally and the other is anti-aliased vertically perhaps I can isolate aliasing effects. Complicated by the fact that the gain of the x and y drive channels would have to be closely matched. But I digress. Assuming my half spot size scan scheme doesn't work or is not generally applicable, what are the types of algorithms to be on the lookout for (or have you already answered that question, Jim) and are there any free utilities I should look for to do it? Will Photoshop do it, do you know? I'm sure NI's IMAQ Vision product would do it, and probably in RT too. Can't afford it. Looking forward to trying it but it may be a couple months before it's finished. The NI IMAQ PCI-1422 image capture board (designed to drive hi-res parallel digital output cameras, but very flexible and comes with friendly utilities for defining new camera types) set me back a pretty penny and it will be a while before I can afford the A/D, D/A, and LVDS conversion EVM's to go with it. Lots of wiring to do as well. Thanks for all the insight so far! I have to studd Fred's most recent post more carefully. -Jeff
Do Nyquist/filtering requirments hold for raster video digitizing?
Started by ●February 10, 2005
Reply by ●February 14, 20052005-02-14
Reply by ●February 14, 20052005-02-14
Jeff Miller wrote:> OK, first off let me say I realized later I was confused (and likely > confusing you guys a bit) by getting my horizontal confused with the > vertical- not that I confused the two per se, but I confused them in the > common contextual meaning. > > So now I see that Jerry and Fred do agree. Where Jerry says horizontal > filtering is "easier than vertical. Just low-pass the data stream.", I > think the sigma-delta converter/filter will do that for me whether I > like it or not. > > I'm still not quite clear on how to over sample/filter the vertical, then. > > Jim, the SEM's I'll be working with were born strictly analog.Well, yes and no. If you are raster scanning then each scan line is effectively a digital sampling. If I'm understanding correctly, in the vertical direction the data you will be collecting is essentially just like a digital camera. In the horizontal direction it is of course analog. So you have several potential problems in regards to aliasing. As you say your D/S sampler should take care of aliasing in the horizontal direction. In the vertical direction you may not be able to do much about aliasing other than decrease the distance between scans (i.e. increase the spatial frequency resolution). Or the system may have ways to filter the data you haven't revealed. Can you control the width and shape of the energy that is being mapped to data? Your comments elsewhere suggest the answer is yes. You talk about spot size and probe size. If you have an ideal sampler (doesn't exist but for the moment pretend it does) then you would be gathering data at a singular point. When you filter the incoming energy you are just spreading out the area from which the data is gathered. The size and shape of the function that represents where the energy comes from is the impulse response of the sampling system. In other words the probe size and shape is important in controlling aliasing as well as any other type of filtering. Also, I'm not so sure that a delta/sigma converter is the best choice for this application. The frequency response will be good but I'm pretty sure it doesn't have a linear phase response (maybe some one else can comment on this). Linear phase is pretty important for image processing. The best scheme would be to collect sample points at distance in the horizontal that is many times smaller than the vertical scan line spacing. Then on the fly you can apply a FIR linear phase filter to the stream of data and decimate down to something close to the vertical spacing. Or if you can afford it save the data without decimating. I have some experience with a similar sampling scheme for reverse engineering surfaces using data gathered with a Renishaw Cyclone probe scanner. Here's how it works. The surfaces are raster scanned with a probe. The probe size can be from 1/16" to 1/2" diameter. Large probes produce data with lower frequency content than small probes. Typically the data is gathered in rows that are .02" apart. Every .0002" movement along the scan line the scanner measures the location of the probe. It stores the data in a buffer. At the end of each line it resolves the points in the scan line buffer into linear segments to within .0001". That is if the surface is dead flat the line resolves into 2 points at the beginning and end of the line and if the surface is very rough there might be points just about every .0002 inches. The data is then stored in a file. The surface is scanned in this manner with scan lines going both in x and y direction. From both sets of scanned data a polygon mesh representation of the surface is created (at .02" X .0002" spatial resolution). Both surfaces are then resampled at some fairly high resolution (typically around .005" in X and Y) and the results are compared and the highest point between the two is used and the lowest discarded. Then that data is filtered and downsampled to a lower resolution (.06" to .01" mesh depending on surface detail) and that represents the final surface. On a good smooth hard surface the accuracy is good enough to reproduce the surface to better than +/- .001" (provided the curvature is never smaller than the probe size). The scanner it self is rated for +/-002" absolute position accuracy, but the relative accuracy is much better than that. If you tear off a piece of masking tape (.005" thick) and stick it to the surface that is to be scanned. when rendering the final surface it is possible to not only clearly see the masking tape but the jagged torn edge is also quite clearly visible. The smaller the probe the clearer the detail but the greater the chance of aliasing (high frequency surface rughness showing up as low frequncy artifacts). -jim>Their > operating principle is remarkably simple. They use a pair of scanning > coils, one for x and one for y. They can be driven at virtually any > speed, limited only by the frequency response of the coils and by > secondary considerations such as response of the detector (generally > 5-10Mhz nominal) and some sensitivity tradeoffs. Typically they are > raster-scanned. In all cases the CRT deflection yoke is driven at the > exact same speeds, in sync, by the same circuit that drives the SEM > deflection coils. The signals are voltage divided down to drive the SEM > coils depending on magnification selected: at low mags the scanning > coils are driven hard, at high mags they're barely "tickled" so the beam > is driven over a very small area of the sample. In most cases several > nominal scanning speeds and formats are available to the operator, > generally at least a "high speed" low-res scan with 500-2000 lines/image > at 2-5 refreshes/sec and a low speed, hi res scan of 5-10K lines or so > for photography: a single exposure can take 10 seconds or more. The > phosphors used in the CRTs are of a special long decay type so the > images persist for viewing by the operator in high speed mode, but of > course that only works so well and is a major source of annoyance at > higher resolutions. It's not uncommon to drive at NTSC rates as well, > interlacing and all, so images can be recorded to tape, sent remotely, > etc.: but at higher magnifications this is seldom done, for some reason > S/N starts falling apart, if only because at higher mags the (DC driven) > "condenser" lens is driven harder which narrows the beam and (I think) > limits the number of electrons available to hit the target. > > I'll be retrofitting with a delta-sigma converter with a nominal sample > rate of 5 Ms/sec and a built in digital filter with a nominal bandwidth > of 2.45 Mhz. I say nominal because I'll be lowering the clock speed > considerably for saving images to disk at high mags and high > resolutions. The digital filter is driven by the same clock source as > the sampling portion, therefore the bandwidth is always limited to half > the sampling rate. It is important to understand that I'll be using > down counters and DACS to drive the scanning coils in sync with the > pixel clock: the whole awkward console is divorced. > > The PCI card doing the high level stuff has an on-board buffer of 32 > Mbytes, and I'll be using 16 bit resolution. Therefore my maximum > "native" sampled array will be 4Kx4K. I'll also have "fast scan" modes > of 512x512, 1024x1024, and 2Kx2K. But ultimately there is no hard limit > to the size of the arrays other than the resolution of my DACS, I don't > think: the board can save to disk as fast as the disk subsystem and PCI > bus allows. Driven at low speeds I suppose 65K*65K should be possible, > even desirable for posters. > > Ultimate magnification and resolving power (i.e.: resolution) is > dependent mostly upon the type of cathode. I believe the figures are > approximately 25nM for Tungsten, 10 nM for Lanthanum Hexaboride, and > 1-2nM for field emission sources. I'm using W and LaB6 now, but I'll be > trying to upgrade to FE (aka FESEM) in the future. In fact I will need 1 > nM resolution if I'm going to explore single electron transistor > technology. > > So given that the horizontal is filtered by the A/D as a matter of > course, I'm wondering first if keeping the scan line width at 1/2 or > less of the spot sizes mentioned above qualifies as a type of > oversampling and will eliminate vertical aliasing without post > processing. If not, it might beg the question as to why I'm going with a > delta-sigma with built in filter, given the fact that I'll need post > processing anyway to clean up the vertical. > > I don't _think_ aliasing/Moire' is an issue in the traditional strictly > analog arrangement, but I could be wrong. Although it's obvious why > there would not be aliasing in the horizontal, I'd be stumped as to why > it would not be an issue in the vertical, given that in a sense the line > vs. line scanning is still discreet. > > Hmmm. There's nothing stopping me from grabbing a frame using a > traditional top down scan pattern and then exchanging x for y drive and > taking one side-to-side. By rotating, correlating, and differentiating > the two images with the knowledge that one is anti-aliased horizontally > and the other is anti-aliased vertically perhaps I can isolate aliasing > effects. Complicated by the fact that the gain of the x and y drive > channels would have to be closely matched. > > But I digress. Assuming my half spot size scan scheme doesn't work or is > not generally applicable, what are the types of algorithms to be on the > lookout for (or have you already answered that question, Jim) and are > there any free utilities I should look for to do it? Will Photoshop do > it, do you know? > > I'm sure NI's IMAQ Vision product would do it, and probably in RT too. > Can't afford it. > > Looking forward to trying it but it may be a couple months before it's > finished. The NI IMAQ PCI-1422 image capture board (designed to drive > hi-res parallel digital output cameras, but very flexible and comes with > friendly utilities for defining new camera types) set me back a pretty > penny and it will be a while before I can afford the A/D, D/A, and LVDS > conversion EVM's to go with it. Lots of wiring to do as well. > > Thanks for all the insight so far! I have to studd Fred's most recent > post more carefully. > > -Jeff----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Reply by ●February 14, 20052005-02-14
Jeff Miller wrote:> jim wrote:...> OK, first off let me say I realized later I was confused (and likely > confusing you guys a bit) by getting my horizontal confused with the > vertical- not that I confused the two per se, but I confused them in the > common contextual meaning. > > So now I see that Jerry and Fred do agree. Where Jerry says horizontal > filtering is "easier than vertical. Just low-pass the data stream.", I > think the sigma-delta converter/filter will do that for me whether I > like it or not. > > I'm still not quite clear on how to over sample/filter the vertical, then.... I'm not clear yet either. Consider this, though. In an analog raster scan, the vertical is inherently sampled. Whether the signal is digitized or not, a 1000-line image of a 501-line grating will be queer indeed if the grating lines and scan lines are parallel. Also, unless I'm mistaken, the internal oversampling and filtering in a delta-sigma converter are a matter of hardware design. The final samples that converter provides don't differ from those of flash or successive- approximation converters. Don't count on the converter's internals to suppress moires. Moires are also a problem when printing color halftones. The problem is easily dealt with by rotating the screens so they are not nearly parallel, which raises the beat frequency to obscurity. You can do that in the SEM as well. In the event that a periodic structure in the subject creates an objectionable moire, rotate the subject or the scan direction, or slightly change the magnification. After all, microscopy is an art as well as a science. Going digital won't change that. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 14, 20052005-02-14
jim wrote: (snip)> Well, yes and no. If you are raster scanning then each scan line is > effectively a digital sampling. If I'm understanding correctly, in the > vertical direction the data you will be collecting is essentially just > like a digital camera. In the horizontal direction it is of course > analog. So you have several potential problems in regards to aliasing. > As you say your D/S sampler should take care of aliasing in the > horizontal direction. In the vertical direction you may not be able to > do much about aliasing other than decrease the distance between scans > (i.e. increase the spatial frequency resolution). Or the system may have > ways to filter the data you haven't revealed. Can you control the width > and shape of the energy that is being mapped to data? Your comments > elsewhere suggest the answer is yes. You talk about spot size and probe > size.> If you have an ideal sampler (doesn't exist but for the moment pretend > it does) then you would be gathering data at a singular point. When you > filter the incoming energy you are just spreading out the area from > which the data is gathered. The size and shape of the function that > represents where the energy comes from is the impulse response of the > sampling system. In other words the probe size and shape is important in > controlling aliasing as well as any other type of filtering.As far as I know, in the days of scanning electron beam camera tubes, which as far as I know are still used in broadcast cameras, the beam size is adjusted as a form of analog low-pass filter to reduce aliasing due to scanning. For CCD detectors, that won't work and I am not sure what they do. Maybe the optical system reduces the resolution enough. -- glen
Reply by ●February 15, 20052005-02-15
glen herrmannsfeldt wrote:> > jim wrote: > > (snip) > > > Well, yes and no. If you are raster scanning then each scan line is > > effectively a digital sampling. If I'm understanding correctly, in the > > vertical direction the data you will be collecting is essentially just > > like a digital camera. In the horizontal direction it is of course > > analog. So you have several potential problems in regards to aliasing. > > As you say your D/S sampler should take care of aliasing in the > > horizontal direction. In the vertical direction you may not be able to > > do much about aliasing other than decrease the distance between scans > > (i.e. increase the spatial frequency resolution). Or the system may have > > ways to filter the data you haven't revealed. Can you control the width > > and shape of the energy that is being mapped to data? Your comments > > elsewhere suggest the answer is yes. You talk about spot size and probe > > size. > > > If you have an ideal sampler (doesn't exist but for the moment pretend > > it does) then you would be gathering data at a singular point. When you > > filter the incoming energy you are just spreading out the area from > > which the data is gathered. The size and shape of the function that > > represents where the energy comes from is the impulse response of the > > sampling system. In other words the probe size and shape is important in > > controlling aliasing as well as any other type of filtering. > > As far as I know, in the days of scanning electron beam camera > tubes, which as far as I know are still used in broadcast > cameras, the beam size is adjusted as a form of analog low-pass > filter to reduce aliasing due to scanning. > > For CCD detectors, that won't work and I am not sure what they > do. Maybe the optical system reduces the resolution enough.Well its the same for a camera only not adjustable. That is, there is an area and a distribution of light energy that is gathered for each pixel. Ideally, the distribution of energy that is being recieved and measured should have a profile shaped like a sinc to completely effective as an anti-aliasing filter. In practice with most devices about the best you can do is to approximate the main lobe of a sinc. -jim ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Reply by ●February 16, 20052005-02-16
Jerry Avins wrote:> Jeff Miller wrote: > > >>jim wrote: > > > ... > > >>OK, first off let me say I realized later I was confused (and likely >>confusing you guys a bit) by getting my horizontal confused with the >>vertical- not that I confused the two per se, but I confused them in the >>common contextual meaning. >> >>So now I see that Jerry and Fred do agree. Where Jerry says horizontal >>filtering is "easier than vertical. Just low-pass the data stream.", I >>think the sigma-delta converter/filter will do that for me whether I >>like it or not. >> >>I'm still not quite clear on how to over sample/filter the vertical, then. > > > ... > > I'm not clear yet either. Consider this, though. In an analog raster > scan, the vertical is inherently sampled. Whether the signal is > digitized or not, a 1000-line image of a 501-line grating will be queer > indeed if the grating lines and scan lines are parallel. Also, unless > I'm mistaken, the internal oversampling and filtering in a delta-sigma > converter are a matter of hardware design. The final samples that > converter provides don't differ from those of flash or successive- > approximation converters. Don't count on the converter's internals to > suppress moires. > > Moires are also a problem when printing color halftones. The problem is > easily dealt with by rotating the screens so they are not nearly > parallel, which raises the beat frequency to obscurity. You can do that > in the SEM as well. In the event that a periodic structure in the > subject creates an objectionable moire, rotate the subject or the scan > direction, or slightly change the magnification. After all, microscopy > is an art as well as a science. Going digital won't change that. > > JerryTrue. I suppose moire must be an issue in the vertical with traditional analog arrangements, I've just never observed it. I still think the digital filter in the D/S converter will eliminate it in the horizontal, though. Glen's comment about adjusting the beam size in camera tubes is interesting, and tends to support my theory that I might get a type of anti-aliasing if my spot size is 2X or more of my line width. Spot size is adjustable using the SEM "condenser" control, but I'm not sure what the limits are, hence not sure if it's applicable at low mags. But I think it was Jim's comment that has me concerned about phase linearity now. I can't find any information on that directly, I would think if that were true the mfr. would supply a phase vs. frequency graph in the data for the chip. But in searching for that info in the docs, I took a much closer look at the figures for settling time for the D/S converters. At a sample rate of 5 Mhz (40Mhz oversample/clock) the settling time for a full step is 47 samples out, or 9.4uS! What the heck! That's to .001%, but the graph is steep: it's %1 at 6.4uS, about %6 at 5.4uS. The impulse response graph isn't quite so bad but it looks bad enough, at least %10 garbage 1uS wide. Perhaps this absurd settling time can be understood as phase non-linearity. So I think I'll be going with a 10MSPS 14 bit SAR. The THD and SNR figures aren't much worse than the 16 bit parts. Deosn't seem like anyone makes a 16 bit SAR faster than 3Mhz, and those parts are only spec'd out to 100Khz for some reason. Actually ADI makes an 80MSPS part but it's $500: $900 for the EVM. Anyone happen to know the generally accepted figures for SNR of a grayscale image on a CRT? LCD? Film? As I mentioned earlier the SNR of the electron collection/amp system is generally understood to be 72 DB, and bandwidth generally 5Mhz. Decay time of the P47 phosphors I usually use is 50-80nS, YAG is about 50nS, YAP:Ce 40nS (or maybe as fast as 20-nS). I was hoping to have quite a bit of headroom for reasearch. I wanted to experiment with direct electron multiplcation, for example. It's generally not done because the vacuums and contamination levels aren't quite low enough to operate an electron multiplier. But my main focus for the last few years has been hivac and I know I can get there with a small handfull of special techniques. Thanks for the insight guys! I'll have to study Jim's most recent post more carefully. -Jeff
Reply by ●February 16, 20052005-02-16
jeff miller wrote:> Jerry Avins wrote:...>> Moires are also a problem when printing color halftones. The problem is >> easily dealt with by rotating the screens so they are not nearly >> parallel, which raises the beat frequency to obscurity. You can do that >> in the SEM as well. In the event that a periodic structure in the >> subject creates an objectionable moire, rotate the subject or the scan >> direction, or slightly change the magnification. After all, microscopy >> is an art as well as a science. Going digital won't change that. >> >> Jerry > > > True. I suppose moire must be an issue in the vertical with traditional > analog arrangements, I've just never observed it. I still think the > digital filter in the D/S converter will eliminate it in the horizontal, > though. Glen's comment about adjusting the beam size in camera tubes is > interesting, and tends to support my theory that I might get a type of > anti-aliasing if my spot size is 2X or more of my line width. Spot size > is adjustable using the SEM "condenser" control, but I'm not sure what > the limits are, hence not sure if it's applicable at low mags.Someone will shoot me down if I'm wrong (it happens all too often!) but a converter is a converter. If it accurately reports the intensity at a point, how it achieves that can't suppress frequencies that another accurate converter would show. As for spot size, be aware that a raised cosine the width of two scan lines in the scanning direction completely suppresses the line structure. (Spot wobble is a way to simulate that.) A similar effective along the line of the scan should provide excellent filtering, but I haven't done the math. ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
