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Monday, January 9, 2012

300dpi? 600dpi? Higher dpi?

You know what they say about old genealogists? They never die, they just scan away. Which is my way of bringing up the subject of scanning. There is one thing that is both frustrating and misleading about scanners and scanning documents, that is the claims concerning the scanning resolution, usually referred to in dots per inch or dpi. If you look at the "specifications" for a new model Epson, Canon or HP or whatever flat bed scanner you will see something like this:
  • Canon CanoScan LiDE110 Color Image Scanner 2400x4800 dpi 
  • Epson Perfection V30 Color Scanner 4800x9600 dpi optical resolution
  • Canon CanoScan 9000F Color Image Scanner 9600x9600 dpi
  • HP ScanJet 5590 Digital FlatBed Scanner up to 2,400 dpi optical resolution
and so forth and so on.  Do you realize that these claims are so much bunk? Dots per inch? What dots?

What set me off on this was receiving a "newsletter" from a large genealogy company (that will remain nameless) advising me to scan my photos at 600 dpi or the highest resolution available to my scanner. This did not give me very much confidence in the technical expertise of the company sending out this notice. Unfortunately, the explanation of why this is so is very complicated. A fairly high percentage of the "explanations" about scanner resolution online are inaccurate. So why would you believe me? Good question. But here goes the explanation.

Optical resolution is measured in ability to perceive fine detail. There is nothing new or innovative about the issue of resolution. Microscope and telescope manufacturers have been concerned with the issue of optical resolution for many, many years. In addition, if a object or document has very fine detail and is viewed with a constant illumination, there are at least two physical limitations in perceiving the fine detail; the quality of the eye's optics and the ability of the retina to transmit the image to the brain and resolve the details. See Optical and Retinal Factors Affecting Visual Resolution by F. W. Campbell and D. G. Green, from the Physiological Laboratory, University of Cambridge as reported in J. Physiol. (1965) 181, pp. 576-593.

There are also physical limitations to resolution imposed by the wave length of the light used to illuminate the subject. What does this mean? It means that regardless of the claims of scanner manufacturers and the recommendations of technology publications, there are physical limits to the degree to which any physical system can resolve fine details in any object or document.

Let me give you an example of a rather simple physical limiting factor. Let's suppose you have a document printed on a laser printer that you want to digitize into your computer. Should you follow the recommendations of the tech guys and scan it at 600 dpi? (Assuming dpi has any relevance). The answer is easy to understand, you can't get water out of a dry well. You can't scan the document at any higher resolution than it was printed. If the laser printer printed the document at 300 dpi, scanning the document at a higher resolution is just a waste of time and memory space on your hard drive.

So the first rule is match your scanning resolution to the detail in the item to be scanned. But what about photographs? If you didn't know, photographs are also made up of grains. If you magnify a photo you can see the grains making up the image. So there is always an upper limitation to any digitization product.

By the way the reason the term "dpi" doesn't apply to scanners is that the imaging mechanism in a scanner is an array of CCD (charged coupled device) light sensors. The actual optical resolution of any scanner is limited by the number of sensors in the CCD imaging array. A claim by a scanner manufacturer that the scanner can scan at 9600 dpi is simply false. In order to create a scan of 9600 dpi, the scanner would need 81,600 sensors. For example, a row of 7,650 sensors will produce an scan of 300 dpi. Here is a brief explanation of the process from
Let's take a simple example: If a scanner's resolution is 300x300 dpi, and that scanner is capable of scanning a letter-sized (8.5x11-inch) document, then the CCD has 2,550 sensors arranged in each horizontal row -- 8.5 (inches across) x 300 (x-direction sampling rate) = 2,550. A single-pass scanner would have three of these rows for a total of 7,650 sensors. The stepper motor in our example is able to move in increments equal to 1/300ths of an inch. See My scanner has a resolution of 9600x1200 dpi -- what do those number mean?
 Claims of higher resolution are based on software interpolation, that is, adding extra pixels by calculating the steps between the physical image's pixels.

Would it help you at this point to know that the Library of Congress standard for scanning is 300 dpi?

Now, let me go back to the dpi issue. Optical resolution is determined by a the ability of a lens or whatever optical device, including scanners to resolve a grid of lines on the SiverFast Resolution Target or the EIA 1956 video resolution target. Here is an example of the Resolution Target:

Here is the scanner target:

By making a scan with this target, you can measure the actual resolution of the scan or other optical device. There is also a IEEE Std 208-1995 target. These targets take into account the actual physical resolution of the device, not the claims of the manufacturers or your personal perceptive ability. The true measurement of resolution should be made in terms of lines per inch (lpi) or millimeters or whatever.

From there resolution gets very complicated. Back to the Library of Congress. I you want to know what the archivists recommend you can look at the Library's Digital Preservation pages.


  1. Thanks for this. As a rule of thumb, I scan documents at 300dpi (unless there's really small print or writing), and photographs at 600dpi to 1200dpi. The photograph resolution depends on the quality of the photo, and the eventual optimum print size you want to get out of it. On my scanner, I find that 600dpi scans tend to print smaller, but are crisper on screen than the 1200dpi.

  2. Because 35 mm slides are very high resolution I scan them at 3200 ppi which produces about a 12 megapixel image. I seldom scan photos higher than 600 ppi since it is rare to have a photo that has high enough resolution to justify it. As far as the third dimension (pixel color) I have found 24 bit (sometimes called "true color") to be more than adequate.