More on DPI, PPI and LPI for genealogists

I wasn't quite satisfied with all I said previously about digitizing, so here is some more. My discomfort was caused by a question from one of my friends, who asked, whether the DPI could be changed when I scanned a photograph? DPI? This is the old dots per inch. It is really a measure for printing and not for digitizing, but by overuse the term "DPI" has slopped over into a lot of areas where it doesn't belong since the DPI values have only an indirect correlation with image resolution.

Digitizing documents and photographs does several things for genealogists. Digitizing a document allows a copy of the document to reside in many locations at once. For old documents that cannot be replaced, it is a way of preserving the information in the original. For those of us who cite sources, it is a way of attaching an actual copy of the source to our file so we don't have to around the house hunting in boxes and drawers all the time. If those aren't enough reasons to digitize documents and photos, then think what would happen if your house burned down, how many valuable documents would you lose? If you digitized everything and stored a copy somewhere else, they wouldn't be lost. OK, now back to the technical stuff:

DPI or Dots per Inch, is properly used to indicate the number of discrete dot images are lineally produced in one inch by a printer and sometimes, inaccurately, a video display such as an LCD (liquid crystal display). The term first had its ascendancy with dot matrix printers which applied ink to the paper by having tiny rods strike an inked ribbon. I recently had my car serviced at one of these chain service locations and they still used a dot matrix printer to print out my receipt.  Notwithstanding this retro use of the machine, nearly all printers today are either ink jet or laser toner based. Even with an ink jet printer, although the little blobs sprayed by the ink head are relatively small, usually the maximum effective resolution is about 600 DPI. On the other hand laser printers can effectively go to about 1200 DPI (or higher up to 9600 x 600 DPI in advertising claims).

Laser printers use toner. Toner, if you didn't already know is powdered plastic. Very simplistically, the laser printer melts the little tiny plastic powder onto the paper to make the image. 

With all print systems, there is an upper practical and physical limit to the resolution that can be obtained by putting an image onto paper. In optical systems, the Dawes Limit is the smallest resolvable angle of an optical system. In technical terms, to calculate the Dawes Limit of a lens system, divide 4.56 (seconds of arc) by the Aperture in inches, or 115.8 (seconds of arc) by the Aperture in mm. To give you some idea of this theoretical limit, it has been calculated that the Hubble Space Telescope (the largest visible-band optical system in an affordable, sustainable reasonably low orbit) can capture just under 10cm (just under 4 inches, perhaps as little as 3 inches) when at perigee, and that’s presuming a perfect atmosphere and a minimum distance to a Mean Sea Level object.

OK, you are saying, what does this mean to a genealogist? Simply put, there are physical limitations to any imaging system whether digital or analog.  In addition, there is the practical cost consideration that says I can't afford the Hubble Space Telescope. Let's get back to DPI, after printing about a million pages, I find 300 DPI is perfectly adequate for almost all applications except photo printing. Given today's market, however, you can buy an inexpensive 1200 DPI laser printer for under $100. Laser printers are now cheaper than the replacement toner cartridges.

But we are talking about digitizing not printing so now let's switch to Pixels per Inch or PPI. PPI is a measurement of the resolution of devices such as monitors, image scanners and digital cameras image sensors, not usually the output of a printer. PPI is the number of discrete imaging units in the device's display or sensor. The higher the number, the greater the resolution. It is claimed that the unaided human eye cannot differentiate detail greater than 300 PPI. This statement is commonly made, but not actually true. If you would like to know exactly how much detail the human eye can see, look at Notes on the Resolution and Other Details of the Human Eye. Most of the time, when resolution is an issue, there is a comparison to film reproduction. The light sensitive elements in film are microscopic and so film is usually referred to as having a continuous tone (i.e. no discrete dots). This isn't exactly true because all film has a "grain." But as nearly all of the world's photographers have figured out, the higher end digital cameras now produce an image that, when printed, cannot really be distinguished from film reproduction. I took a few thousand print and slide pictures in my film days and I can tell you, my digital photos are mostly far superior to anything I did with film.

So, if I want to digitize my documents and existing film photographs, what can I use? We really have two choices: scanners of some sort, usually flat bed, and digital cameras.

Most people will not have a digital camera that will produce the same quality of image as a flat bed scanner. Flat bed scanners available today for less than $300 exceed the resolution needs of some of the most strict professional archivists. The problem with scanners is the physical size of the machine. The bed on most scanners is limited to 9 x11 or maybe 9 x 14 inches. What about larger documents?

You can use a digital camera to copy documents but photographs don't work so well. Large documents and photos can be scanned in pieces and stitched together by software.

Now to Lines per Inch or LPI. I already discussed this in detail in a previous post, but it bears repeating. This is not something you will see in any advertisement. LPI is a standard for determining if an image meets the qualifications of an archivist.

Oops, I am running out of time on this post. I will continue it tomorrow.