Halftone Color Dot Pattern -- LPI vs PPI

Discussion in 'Digital Photography' started by Ken Hall, Dec 8, 2003.

  1. Ken Hall

    Ken Hall Guest

    First my, perhaps flawed, understanding:

    When a printing process (inkjet printer of offset) that is not
    continuous tone creates an image from a digital file it represents a
    pixel by applying tiny dots in clusters that the eye sees as the color
    of the pixel.

    When I picture this in my mind I imagine a square that represents the
    pixel, and (for a 6 color printer) I imagine a the square filled, as
    necessary, with six rows and six columns of the smallest color dots
    the printer can make. Thus, for example, a 2400 dpi printer would
    generate 400 of these squares per inch. Hence, it's a 400 lpi color

    Given my mental picture of how this works it seems like a ppi that
    equals the lpi the printer makes for an ideal match.

    But, there's something (maybe many things) wrong with my understanding
    because conventional wisdom says to create an output image with a ppi
    1.5-2.0 times the lpi of the printing device.

    I have searched the web unsuccessfully for an illustration of how
    these dots are arranged to print a pixel.

    Can someone explain why it takes 1.5 - 2 times the printer's lpi in
    pixels to match the printer ideally?

    Ken Hall, Dec 8, 2003
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  2. Ken Hall

    Ray Murphy Guest

    RM: The professional printing process has gone through several major
    transitions since printing was invented about 450 years ago. First we
    had the type which was assembled as single letters 1 inch high. We
    also had carved wooden blocks which were for large type and drawings.

    The drawings were considered "line drawings" because they were made up
    of either solid lines or (later) batches of lines which were thick and
    thin, which created the illusion of tone. The more sturdy versions of
    these blocks were engraved in solid copper which was mounted on a
    wooden block, also 1 inch high (to match the height of the type).
    You may have seen references in old books to "Plate 6 or PLate 7"
    These are referring to the "copper plates" which were used to produce
    the pictures.

    At one point printing diverged to what later became modern offset
    printing. This occurred after someone drew on a very porous Barvarian
    stone which had a very fine grain. They used a greasy pencil and then
    inked the stone after wetting it. They then were able to print single
    copies from the stone. This was later automated, whereby printers
    loaded the large blocks of stone into machines. This was called
    lithography ("stone-graphy").
    Some of the work produced by this original method appears in old books
    and also in prints of old paintings. They are all quite valuable now
    because of their rarity.
    This type of printing is very much like "continuous tone" but in
    reality it is not. The prints only appear to have a tonal range
    because the smooth stone was of such a fine texture that we cannot see
    any distinct "dots" in the image (for the high and low areas of the
    stone). No modern technology has ever replicated this quality. It's
    basically the equivalent of random dots but they are tooo fne to see.

    After photography was invented, someone decided to place the
    photographic emulsion on the copper plates and then etch out the
    non-image areas. This required the "breaking-up" of the photographic
    image into "dots" of varying sizes because the printing machine could
    only print in single solid colours. This breaking-up of the continuous
    tone image was achieved by photographing the artwork with a gallery
    camera which is a very large bellows camera which might be 8 - 12 feet
    in length.

    In front of the film was a glass screen about 7mm thick which had a
    series of millions of dots laid out in rows. They were dense black at
    the centre and got lighter as they radiated out. These were in effect
    - millions of "lenses" and as the exposure took place, the light
    formed small and larger dots (according to how much light managed to
    penetrate the fuzzy dots).
    These dots were arranged at an angle of 45 degrees and they were
    separated in distance from each other according to the "screen ruling"
    that the scrren was made to do.
    In the case of newspaper photos, the glass screens were 65 lines per
    inch (65 LPI) and a reasonable quality screen for high quality
    reproduction might have been 133 LPI. (I used this system myself in
    the mid 70's until "contact screens" replaced them). These are .007"
    acetate film "halftone screens" which are kept in hard contact under
    vacuum, and do the same job.

    Later when full colour printing was introduced, the glass screens were
    made to be rotated to specific angles so that the colours CMY (Cyan
    Magenta and Yellow) were all photographed with a slight "shift" so
    they would not overlap exactly. This also prevented an undesirable
    pattern from emerging.

    So photography allowed printers to produce these "dot images" or more
    correctly "half-tone images" on both copper plates (and later)
    aluminium offset plates.
    The copper plates (or rather - blocks) were used in letterpress
    printing, and the aluminium plates were (and still are) used in offset
    lithography - or offset printing as it is now known.
    The copper blocks were later replaced by zinc and magnesium to save
    money and may still be in use today.

    If you cannot find an enlarged picture on the net of the
    "rose-pattern" of a full colour image, you can always see it with a
    very strong magnifying glass. It is much more obvious in the light
    areas of any 4 colour print.
    Incidentally, the common 4 colour process of CMYK, was originally only
    CMY, with the K (or black) being introduced later in order to give a
    slight increase in contrast). You can see how little Black is actually
    in the average full colour image if you isolate the Black in

    Until you get to see a real example of what a 4 colour printed image
    looks like when greatly enlarged, you could either go out and find a
    bus and look at the large advertising signs - or read on :))
    Basically what the image looks like is a series of circular dots of
    varying sizes in CMYK. They are all slightly out of alignment.
    The white areas will have minute circular dots and the mid tomes will
    have almost square dots like a checker board pattern, and the very
    dark areas will have minute white dots.

    When Photoshop and the Mac came along, non-specialists were able to do
    desktop versions of what the printing industry was doing with million
    dollar equipment. It became possible to hook up a Mac to industrial
    strength "imagesetters" which produce roll film where raster imaging
    constructed each halftone dot with incredible precision - 2540 and
    3356 dpi.
    This means that if we are printing a 100 line screen photo , there are
    335.6 raster dots (in each direction) constructing ONE halftone dot.
    [There may be some error in this simplified explanation].

    The film which is produced by the imagesetters (or process cameras) is
    then used to make the offset plates or letterpress blocks. As
    mentioned earlier, the film has only "image or non-image" areas which
    are all halftone dots of varying sizes.
    These dots are referred to in terms of percentage, and a good
    halftone negative for reasonable quality work will have a 5% dot in
    the white areas and a 95% in black areas (highlight or shadow to be
    more precise).
    In simple black and white printing the dot sizes should be larger, and
    their size is largely dictated by screen ruling and paper surface.
    This means for example if you are printing a halftone on a laser
    printer *without* adjusting the dot size for the highlight and shadow
    areas it will usually look far too contrasty.
    If you use "auto" settings in Photoshop it will *guarantee* a bad

    Now back to the difference between professional printing and inkjet
    printing. In the former we always use a halftone screen which might be
    say 133 LPI for reasonable colour work. This requires using 266 pixels
    per inch when working at finished size.
    Photoshop recommends 2 to 2.5 ppi, but you can go slightly outside
    these limits with only a small degradation of image.

    Inkjet printers don't use a halftone screen, but instead print
    microscopic dots which are so small, most people never see them
    because there may be 720 per inch (720 dpi). They appear to be "all
    over the place" in a random order, but of course they are not. They
    are not "pixel for pixel" as you see on your monitor, because you are
    using a Tiff which might be 200 ppi or 300 ppi, so your pixel image is
    interpolated to print at 720 dpi.
    I don't know what would happen if we made a Tiff which matched an
    inkjet's dpi resolution, but presumably we would get a pixel-for-pixel

    You said earlier:
    RM: It's 2 to 2.5 for Photoshop, but no, I couldn't tell you, but at a
    guess I would say that the imagesetter needs to know (a) What colour
    the halftone dot is, and (b) How big it is supposed to be.

    Before I go, let's get the terms straight:
    LPI = Lines per inch for printing screens - traditionally
    --- 65, 75, 85, 100, 120, 133, 150, 175, 200, 300.
    DPI = Dots per inch of the output device - such as
    --- a laser printer, imagesetter or inkjet.
    PPI = Pixels per inch - in computer,
    --- monitor, scanner and digital camera.

    * 90% of the world, including inkjet manufacturers and authors are
    still referring to ppi as dpi incorrectly. They got off on the wrong
    foot and very few people have been game to challenge it or correct it.
    It's true of course that inkjets produce 720 dpi, but it is incorrect
    to refer say that photo's in computers have dpi because they don't
    have ANY dots -- they only have pixels.

    Ray Murphy, Dec 8, 2003
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