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Barcode scanning on LCD display

Discussion in 'Electronic Basics' started by MS Ian, Jul 6, 2004.

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  1. MS Ian

    MS Ian Guest

    Hi all,

    Stupid question...can a normal barcode scanner scan from an LCD
    display displaying a barcode (instead of from scanning from paper and

  2. John Fields

    John Fields Guest

  3. Kevin R

    Kevin R Guest

    You might do it with the CCD type if you are very lucky, but definitely not with the
    laser type.
  4. Bob Myers

    Bob Myers Guest

    I'm guessing the multiplexing or lack thereof is only one
    concern. Are we talking about a simple reflective LCD here,
    or an active-matrix backlit type (like most monitors or notebook
    displays). If the latter - then given the way most barcode scanners
    work (i.e., scan a light beam, often a low-power laser, across the
    barcode and watch for variations in the reflected light), I think this
    may be a pretty hit-or-miss proposition. There's no reason to
    think that the LCD is going to reflect significantly more of the
    light in those areas that appear lit up to the eye than it will in
    the "dark" areas.

    Even in the case of the simple reflective LCD, the basic effect
    utilized by the display is often strongly wavelength-dependent -
    it's still not a sure thing. Can't say more since I've never tried

    Bob M.
  5. John Fields

    John Fields Guest

    Since the active-matrix backlit types are, I believe, universally
    muxed, they'll be automatically ruled out. What'll be left then, will
    be transmissive, transflective, and reflective dot matrix and
    character based displays. All dot matrix will then go away, since
    they're muxed, and so will all of the muxed character based displays,
    so we'll be left with statically driven transmissive, transflective,
    and reflective character based displays. Of the three, the
    transmissive types contain no reflector, so we'll be left with
    transflective and reflective displays. transflective can be either
    backlit or depend on external light, or both, for the source of
    illumination, and reflective depends on external light. Of the two,
    the reflective will give a better contrast ratio and will therefore be
    more likely to work properly as a target for a scanner.

    As far as wavelength dependency goes, I just illuminated a reflective
    display with red light in an otherwise totally dark room and the
    contrast between light and dark segments was excellent, so I don't
    expect a scanner with a red laser diode would have much trouble
    differentiating between the light and dark ares of the bar code

    I've fooled around with using LCD's for optical attenuators and there
    are no color problems until you get down to IR, and then the rules
  6. Bob Masta

    Bob Masta Guest

    Is muxing really likely to be a problem? My understanding
    is that although the drive is muxed, the slow response
    of the liquid crystals themselves means that they are
    essentially static. (Unlike, for example muxed LEDs that
    really do flash at the mux rate.)

    I'd be more concerned about the reflectivity issues, but
    this topic begs for a hands-on test.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  7. Bob Myers

    Bob Myers Guest

    Actually, they're not - that's the whole point of the active-
    matrix drive scheme (yes, they're multiplexed in terms of
    the row drive, but the point of the TFT is to hold the pixel in the
    desired state during the entire frame time). I included them
    since it seemed to be reasonable from the standpoint of the
    original question - that someone might be attempting to scan
    a barcode as displayed on, say, an LCD monitor.
    So how would you plan to display a barcode on a
    character-based display in the first place?

    Bob M.
  8. John Fields

    John Fields Guest

    I don't recall that that was ever mentioned as a design criterion. You
    might want to check the original post or get ahold of the OP to see
    what he had in mind...
    That's simple. I know how to do it, and I can prove it, but I won't
    divulge unless I got paid to do it.

    But, the original question wasn't about what _I'd_ do to set up a
    universal barcode scanning system, it was about how to detect bar
    codes displayed on LCDs, and I think my response was reasonable, in
    terms of actively scanning a statically displayed bar code.

    Dynamically displayed? Either dictate or figure out the timing and
    you've got it...
  9. Bob Myers

    Bob Myers Guest

    Oh, I can think of several rather simple ways to do it,
    but I guess I DO need to go back to the OP - I really
    did have the impression that he or she was just thinking
    of putting up a barcode on a standard LCD display, and
    trying to scan it.

    Bob M.
  10. MS Ian

    MS Ian Guest

    Thanks guys for the feedback. Wow you guys really know your stuff!

    And sorry for the confusion caused. I was just trying to determine if
    I could scan a barcode displayed on my PalmPilot. Guess it is not
    really feasible with the laser-type scanners.

    Thanks again!
  11. the Wiz

    the Wiz Guest

    From my experience with Palm PDAs, the answer is no.

    Generating a valid 3 of 9 code on the Palm screen (non-backlit screens on Palm
    III & m125), then trying to scan it with a Symbol 1500 series laser scanner does
    not work.

    Capturing the Palm screen and printing that image on paper scans perfectly.

    I think the key here is the reflectivity of the Palm screen - shiny glass is
    still shiny glass whether what's behind it is light gray or dark gray.

    If you have a Palm device, the program is Code3of9 and can be downloaded
    (including source) from this page:


    More about me:
    VB3/VB6/C/PowerBasic source code:
    Freeware for the Palm with NS Basic source code:
    Drivers for Pablo graphics tablet and JamCam cameras:
    Email here:
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