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Date:         Fri, 22 Jan 1993 14:40:46 CDT
Reply-To:     Jeff Bone <JBONE@DOPIG.UAB.EDU>
From:         Jeff Bone <JBONE@DOPIG.UAB.EDU>
Subject:      Scanning FAQ Update Part 2 of 4

-------------         PAGEMAKER/QUARK LISTSERV               -----------------
-------------   S C A N N I N G    F A Q    UPDATE 1/22/93   -----------------
Part 2 of 4

               Do You Really Need All That Resolution?


It seems even the scanner makers are now caught up in the "more-is-better"
routine.  A few years ago 300dpi 16-shade grey was an incredible scanner, to-
day it seems if you don't have a 600dpi 24-bit scanner you'll likely to get
drummed out of the local DTP users club.

But do you really need 600dpi?  It is quite possible that, depending on your
application, you could produce some very pleasing and fully professional
results with less investment.

I suppose I should explain my first question now.  I won't assume anything,
so don't let me insult your intelligence, just overlook any riduculously
elementary parts.

1.
A flatbed scanner is merely a series of CCDs (charge coupled device = light
sensitive integrated circuit) mounted in a stationary row that light
reflected from a piece of flat art is allowed to pass over.  These CCDs
register presence or absence of light (ON/OFF) thus producing a pixel
electronically.  Since they are mounted in a single row that is the way the
electronic file is created, row by row.  Essentially the CCDs are reflected
one row of the flat art at a time until the image is completely built.

2.
That being the case, resolution or the number of pixels written based on what
is reflected is controlled two ways.  The number of pixels horizontally is
controlled by how closely the CCDs are placed next to each other along the
single row.  The number of pixels vertically is controlled by how slowly the
light bar and mirror inch along the length of the flat art thus reflecting
onto the CCDs.  Therefore, the more CCDs and the smaller the steps of the
advancing light bar the greater the resolution.  Currently there are five
major scanner motor manufacturers (many re-packgers) and they all build
their systems essentially the same way.  The highest resolution flat bed
scanning system currently is physically limited to 400 dots (pixels) per
linear inch.  You may say wait a minute I've seen 600, 800, and 1200 dpi
ratings, but note I stated the physical limit is 400 dpi.  This is true,
there currently is a real physical limit as to how many CCD ICs can be placed
side by side in one inch and that limitation is 400 right now.

3.
This 400dpi physical limitation has been breached by what is known as
interpolation.  Interpolation is a software/firmware process whereby the
scanner essentially samples two pixels and averages (often times using more
complex formulas) the two pixels together to form an extra pixel (or more) in
the middle.  Better scanners now do this in hardware, but some still rely on
their scanning software to do it (often uninvolving the user).  But
nevertheless, this higher resolution is only psuedo-data.  That is, it is
data being create by averaging and not by actually sampling it from the
original art.

4.
Grey-scale scanners are scanners with CCDs that can differentiate between
levels of light falling on them, rather than just being on or off, the grey
scale scanner can determine if the pixel should be any number of shades of
grey.  Most scanner manufacturers have stopped R&D once they acheived 256
shades of grey because the current version of PostScript can only recreate
256 levels of grey, however there are many high-end systems that can produce
much more levels (4096, 32768, etc.)

5.
Color scanners are nothing more than grey scale scanners that have filtration
(most commonly Red, Green, and Blue) and make multiple passes to generate 256
levels of each RGB component.  The software then recombines the three passes
to create full color.  Higher quality scanners perform all three scans in one
pass at the same time to perserve registration (although this is not usually a
problem in good three pass scanners).

6.
Imagesetters are unable to produce continuous tones, that is that
are unable to make a pixel different shades of grey like the scanner "sees"
them, instead they use a very complicated screening pattern to simulate
shades of grey to the naked eye.  At higher magnifications you can easily see
that a photo is not really a photo, but is a series of variable size dots.
This is called "half-toning."  Half-toning, therefore, isn't using all of the
pixels to create the dot patterned image, in most cases it's using only half
or less of a high resolution scans original data.

7.
So the question becomes why scan so high if the data won't be used.  There is
a formula for this of course.  It is, scan at 1.5 times the lines per inch
(LPI) of the final output device.  Therefore if you are outputting to a 2400
dpi imagesetter at 150 LPI then the normal maximum resolution you need to
scan at is only 200dpi.  So if the un-informed user scanned his photo at
600dpi thinking he needed that high res capability because he was going out
to a high res imagesetter, he would be sending over 9 times too much data to
the imagesetter.  This would result in a very long RIP time and possible
crash of the RIP.

8.
Line art scans (black and white) could, of course, be a good candidate for
the high resolution scanner, but more likely if you plan to use a piece of
line art you can scan a large original and reduce it.  Or better yet,
autotrace it into your favorite illustration program and forget having to
work with large, slow bitmaps.

9.
High resolution color becomes a different story somewhat.  It is possible to
find a continuous tone color output device where it would be nice to output a
"true" continous tone modification that would rival the original (i.e.-
National Enquirer PhotoShop modification of a 35mm slide at high res and then
reoutput to 4x5 negative on a film recorder to produce a retouched print).
However, continuous tone output requires very high resolution to produce
satisfactory results.  This requirement pushes the upper envelope of flat bed
scanning (800, 1200dpi) and becomes a job for the slide scanner and drum
scanner.  Unfortunately at these ultra high resolutions the personal computer
becomes a liability.  Today I have been working on a photo retouch of a 35mm
slide that we intend to re-output to 4x5 on our Solitaire film recorder.
Unfortunately the full resolution file (4800 dpi) was over 250 Megabyte!
This is definitely not a job for PhotoShop.

10.
Finally, I have listed a few scanners in different catagories to show
their relative merits and estimated pricing, they are all units that we
use or have used internally and found them to be very reliable:

300dpi Grey Scale:
    Hewlett-Packard ScanJet                     $  800
    Apple OneScanner                               700

400-600dpi Color Flat Bed:
    Microtek 300Z (300dpi)                       1,200
    Microtek 600Z (600dpi)                       1,500
    XRS 3cx (AutoRads & X-Rays, 600dpi)          4,500
    Sharp JS450 (400dpi)                         6,500

600-2000dpi Color Flat Bed:
    Sharp JS600 (600dpi)                         8,000
    Imapro XL (1200dpi)                         14,500
    AGFA Imagemaster (1600dpi)                  22,500
    Artronic ViewScan (2000dpi transmissive)    35,000

3000dpi Slide Scanners:
    Nikon LS3500                                 7,500
    Nikon LS3510 (12-bit)                       10,500
    Leaf 35                                     14,000
    Leaf 45 (35mm & 4x5 12-bit)                 19,000

4000-6000dpi Drum Scanners:
    Dangraf DeskDrum (4000dpi, 12-bit)          22,500
    Optronics ColorGetter (6000dpi, 16-bit)     85,000
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU