Editors Note: This article was originally written in 1994 when continuous tone printers were tens of thousands of dollars unlike a good ink jet printer today. Much of the information is still solid, but understandbly this are will be soon updated to reflect the current level of technology in digital cameras and ink-jet printers.

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 you'll likely to get drummed out of the local DTP users club. But do you really need 600, 800, 1200dpi?

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 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 engine manufacturers (many re-packgers) and they all build their systems essentially the same way. The highest resolution flat bed scanning system currently is practically (cost to value) limited to 600 to 1000 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 practical 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 around 1000-1200 right now.

3. This physical limitation has been breached in low-end scanners 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.

   Another interesting development is the scanner manufacturers that are indicating their resolutions in non-uniform terms. For instance, Microtek and Epson have in the past indicated 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 wouldn't have a square, we would have noticeable gaps in one dimension, or the most likely scenario, there would be overlapping spots in one dimension. Scanners that have non-uniform resolutions usually don't 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 most versions of PostScript or PCL found in laser printers can only recreate 256 levels of grey. Inkjet printers have custom drivers that can produce 256 levels of grey with custom gery tone ink cartridges. There are many high-end systems that can produce many more levels (4096, 32768, etc.) but they are usually not found in consumer printers.

5. Color scanners are nothing more than grey scale scanners that have filtration (most commonly Red, Green, and Blue) and make multiple passes of the light bar, or acquire three data streams to generate 256 levels of each RGB component. The software then recombines the three passes/streams 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/stream scanners).

6. When producing images for typeset output, Imagesetters (Linotype machines) 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 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 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 towork 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 thenreoutput to 4x5 negative on a film recorder to produce a retouched print). Of course even today, inexpensive inkjet printers can produce fairly good continuous tone images in "photo" mode. Continuous tone output requires can require significantly 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. Even in the late '90s I have worked on a photo retouch of a 35mm slide that we intended to re-output to 4x5 on a Solitaire film recorder. Unfortunately the full resolution file (4800 dpi) was over 500 Megabyte and we were using 20Mhz 486 computers with 64Mb of RAM (fast for the day). Fortunately machines have gotten faster and it is quite common to have a couple of gigabyte or more of RAM in your computer, heck I'm even adding this update on my Apple MacBookPro that has 2Gb of RAM--thank heaven for Moore's Law.

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 and found them to be very reliable:

    300-600dpi Color/Grey Scale Flatbed:
    	Hewlett-Packard ScanJet+                  $    200
    	Epson 4180 Scanner                             400
    	Microtek ScanMaker III                         500
    
    2000dpi Color Flat Bed:
    	Imapro XL (1200dpi, 12-bit)                    800
    	PixelCraft ProImager (1400dpi, 12-bit)       1,000
    	AGFA Horizon+ (1600dpi, 12-bit)              2,500
    	Artronic ViewScan (2000dpi, 12-bit)          5,000
    
    3000+dpi Slide Scanners:
    	Nikon LS Series (12-bit)                     1,500
    	Leaf 35 (12-bit)                             3,000
    
    4000-6000dpi Drum Scanners:
    	Dangraf DeskDrum (4000dpi, 12-bit)          10,000
    	Optronics ColorGetter (6000dpi,  16-bit)    20,000
    	

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