Do You Really Need All That Resolution?

It seems even the scanner makers are now caught up in the more-is-betterroutine. A few years ago 300dpi 16-shade grey was an incredible scanner, to-day it seems if you dont have a 600dpi 24-bit scanner youll 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 wont assume anything, so dont 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 engine manufacturers (many re-packgers) and they all buildtheir systems essentially the same way. The highest resolution flat bed scanning system currently is physically limited to 600 spots (pixels) per linear inch. You may say wait a minute Ive seen 1200, 2400, and even 9600 dpi ratings, but note I stated the physical limit is 600 spi. 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 600 right now.

3. This 600dpi 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) inthe 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.

   Another interesting development is the scanner manufacturers that are indicating their resolutions in non-uniform terms. For instance, Microtek currently indicates that their scanners are 600x1200 dpi. While this seems like a higher resolution scanner than one that is merely 600x600 dpi, think about it for a moment. This measurement reflects how much data that the scanner can acquire in a square inch, or X x Y. What would happen if we acquired 600 spots in the X and 1200 spots in the Y? Either we wouldnt have a square, we would have noticable gaps in one dimension, or the most likely scenario, there would be overlapping spots in one dimension. Scanners that have non-uniform resolutions dont actually give you the ability to acquire image data at this non-uniform resolution, they instead interpolate one dimension. At 600x600 they interpolate the 1200dpi dimension down to 600dpi (usually done by merely running the stepper motor that moves the light bar at twice its minimum rate), or at 1200x1200 they interpolate the X dimension.

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 greyscale scanner can determine if the pixel should be any number of shades ofgrey. 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 simulateshades 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, isnt using all of the pixels to create the dot patterned image, in most cases its 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 2400dpi imagesetter at 150 LPI then the normal maximum resolution you need to scan at is only 225dpi. 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 ofline art you can scan a large original and reduce it. Or better yet,autotrace it into your favorite illustration program and forget having towork with large, slow bitmaps.

9. High resolution color becomes a different story somewhat. It is possible tofind a continuous tone color output device where it would be nice to output atrue continous tone modification that would rival the original (i.e.-National Enquirer PhotoShop modification of a 35mm slide at high res and thenreoutput 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. I have worked on a photo retouch of a 35mm slide that we intend to re-output to 4x5 on a 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 I have used or currently use and found them to be very reliable:

    
    300dpi Grey Scale:
    
    	Hewlett-Packard ScanJet+                  $    800
    
    	Apple OneScanner                               700
    
    	Microtek ScanMaker IIg                         500
    
    
    
    400-600dpi Color Flat Bed:
    
    	Microtek ScanMaker IIxe (600dpi)             1,300
    
    	AGFA Arcus+(600dpi, 10-bit)                  3,750
    
    	XRS 3cx (AutoRads & X-Rays, 600dpi)          4,500
    
    	Sharp JS450 (400dpi)                         6,500
    
    
    
    600-2000dpi Color Flat Bed:
    
    	UMAX PowerLook (1200dpi, 10-bit)             6,000
    
    	Imapro XL (1200dpi, 12-bit)                 12,000
    
    	PixelCraft ProImager 8000 (1400dpi, 12-bit) 13,000
    
    	AGFA Horizon+ (1600dpi, 12-bit)             18,500
    
    	Artronic ViewScan (2000dpi, 12-bit)         35,000
    
    
    
    3000dpi Slide Scanners:
    
    	Nikon LS3500                                 7,500
    
    	Nikon LS3510 (12-bit)                       10,500
    
    	Leaf 35 (12-bit)                            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
    
    	

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    http://www.infomedia.net/scan/The-Scan-FAQ.html / 7.6.97 / jbone@jbone.com