Quick scan hints


  • dpi = "dots per inch" in the U.S. (expressed in dots per centimeter in many other countries); the meaning differs between scanners, image files, printed documents, and viewing devices
  • pixel = the smallest visual element in a monitor, device screen, or television
  • ppi = "pixels per inch"

Their meanings

  • dpi in image files = the number of samples per inch
  • dpi in printed documents = the horizontal resolution; vertical resolution may differ
  • dpi in printers and plotters = capability of printers and plotters; vertical resolution may differ; appearances of printed documents with identical resolutions will differ between different types of devices (inkjets vs laser printers, etc.)
  • dpi in scanners = the horizontal resolution being sampled; "dpi is a misnomer, because scanners capture digital "samples per inch", not dots; vertical resolution is usually assumed to be equal to horizontal resolution, but can be slightly different dependent on the quality of components
  • ppi in monitors and viewing devices = the horizontal resolution of a monitor, device screen, or television; vertical resolution may differ

Scanner resolution

  • The number of readings (samples) the scanner takes in every horizontal and vertical inch, expressed as "dpi."
  • Scanner software controls the number of samples (dpi) quantified; manufacturers give multiple resolution options to users in all but the most basic software.
  • Typical scanner software provided with homeowner-type scanners usually offers 150, 300, and 600 dpi options.

File types

  • There are approximately 49 different image file formats; some are no longer used except in special situations.
  • The most popular formats include JPG (JPEG), TIF (TIFF), GIF, PNG, BMP.
  • Most scanners I've seen allow users to save images as JPGs, TIFs, and PDFs.
  • Although homeowner-type scanners typically allow saving images in PDF format, it is not an image format per se. PDFs store images as raw binary data.
  • Image file formats can be grouped as lossless or "lossy." Lossless formats allow users to open, manipulate, and save images multiple times without noticeable degradation. Lossy formats compress images by different methods to decrease file sizes, but in doing so, discard information.
  • TIFFs are considered lossless and can be opened, manipulated, and saved multiple times with little loss of clarity, depending on the type of alteration performed. TIFFs can be compressed to a certain degree.
  • JPGs are considered lossy because information is compressed during saves. Most software scanning and manipulation software allows users to choose the amount of compression. Each time an image is decompressed and recompressed, degradation results; the severity of degradation depends on the software and amount of compression used.
  • PDFs are considered both lossless or lossy depending on the source file and the software used.
  • Image manipulation software allows conversion of TIFS and JPGS to many other formats (Photoshop offers 19 possibilities, most others offer much fewer conversions.)
Epson Expression 10000XL flatbed scanner

Epson 11x17 scanner with removable lid.

Data capture comparisons

  • When you double your scan resolution, you quadruple file sizes.
  • For instance, if you scan one square inch:
    • at 150 dpi, your scanner samples 22,500 points (150 x 150)
    • at 300 dpi, your scanner samples 90,000 points (300 x 300)
    • at 600 dpi, your scanner samples 360,000 points (600 x 600)
    • 1200 dpi, your scanner samples 1,440,000 points (1200 x 1200)

File size comparisons

  • Scanner and image manipulation programs all save information differently. Therefore you will probably never get the same faile sizes from different scanners or different programs.
  • Even TIF files can be compressed, some.
  • JPG files can be created and compressed at the point of origin (coming out of the scanner) or after scanning by software manipulation programs.
  • Here is a comparison of JPG images created by a scanner and after-the-fact by software programs. File sizes are expressed as percentages of an uncompressed TIF file.
to JPG using
to JPG using
TIF uncompressed !00%
JPG @ 100% quality (JPG compression only) 54% 35% 48%
JPG @ 75% quality (25% compression) 10% 13% 12%
JPG @ 50% quality (50% compression) 7% 11% 8%
JPG @ 25% quality (75% compression) 5% 6% 6%
JPG @ 0% quality (maximum compression) 1% 5% 2%

My experience

  • I do not claim to be an expert by any stretch of the imagination, but I have personal and professional experience scanning and manipulating well over a million images.
  • I personally save all "high-resolution" images for this project as 300 dpi, 75% quality, JPGs.
  • I convert all inbound TIF and PNG images to 300 dpi JPGs in order to save space both on local drives and on the Cloud. (TIFs and PNGs average 9 to 10 times larger than 75% quality JPGs.)
  • I use Photoshop for almost every image, although GIMP is superior for a few special tasks.
  • I use Microsoft image Composite Editor exclusively for stitching large certificates that have been scanned in pieces.

My recommendations for scanning CERTIFICATES

  • Unless someone is creating images for publication in major slick magazines or coffee table books, or capturing minute details, there is limited need for scanning certificates above 300 dpi. Ultra high resolutions are occasionally necessary to capture very small details.
  • Unless someone is planning on opening, altering, and re-saving JPGs repeatedly, there is very limited need for saving certificate images as TIFs of PNGs.
  • Use JPGs for posting images of certificates on the web INCLUDING EBAY; JPGs are much smaller than TIFs and PNGs and therefore offer much faster downloads and viewing.

Image manipulation

  • For certificates, image manipulation primarily means straightening, removing distortion, adjusting brightness and contrast, cropping, and changing resolutions.
  • I recommend Photoshop over every other program, although it is expensive for casual use. I have used several other programs, but the only other one I recommend for hobby use is GIMP. Proficiency takes a while, but GIMP does almost everything that Photoshop does (although not necessarily as easily.) And it's FREE. There are loads of hints and tricks on the web. Just word your searches like "straightening images with GIMP" or "rotating images with GIMP."

Third-party scanner software

  • The dedicated software that comes with scanners is usually very good for average scanning. Third-party software will take you further, because it will do more than the dedicated software was designed for.
  • I heartily recommend VueScan for any scanner made, new or ancient. That even includes slide scanners.
Epson Expression 10000XL flatbed scanner

Plustek Book Edge scanner with removable lid.

What kind of scanner?

  • I don't like to recommend scanner brands or models because I've probably used fewer than ten different ones in my lifetime.
  • I DO recommend a desktop scanner with a removable lid. That allows you to move certificates around with hanging them up on something and risking damage.
  • If you intend to also scan books, then consider a "book edge" scanner which has glass all the way up to the front edge. That way you can scan books and magazines without squashing books against the glass and breaking the spines. (Or the glass!) There are only a few out there and I have a small Plustek model that comes in handy. And it has a removable lid.
  • An 11 x 17 scanner (also known as "A3" size) will scan almost every large bond. They are a little pricey, so make sure you have the need. I advise against buying a large scanner unless it has a removable lid.
  • High-price scanners are often repairable; cheaper ones aren't. However, they tend to last a long time. If concerned, buy protection plans or locate repair options before buying.

A couple quick scanning tips

  • Scan in the middle of the scanner glass if you can. Certificates can get caught under the plastic frames on some scanners and lose a corner tip.
  • Never assume certificate borders are parallel to their edges.
  • If your scanner allows,
    • turn off automatic color adjustment
    • turn off automatic exposure
    • turn off automatic sharpening.
  • Scan against LIGHT gray or preferably VERY LIGHT blue posterboard. Most certificates have some yellow in the paper and white scanner lids enhance yellow. (Why blue? Blue is the opposite, aka "complimentary," color of yellow and helps remove yellow.)
  • Scan certificates full size; don't reduce.