Collectible Stocks and Bonds from North American Railroads     by Terry Cox

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Pixels, dots and samples

Pixels, dots and samples are not the same.

I suspect most people tend to express scanning resolutions, monitor resolutions and printing resolutions all in terms of 'dpi', meaning 'dots per inch'. I've been using the term interchangeably for twenty years and don't expect to change now. If you want to be more accurate, then understand that:

  • Scanning resolutions ought to be expressed in 'samples per inch' (spi).
  • Monitor resolutions ought to be expressed in terms of 'pixels per inch' (ppi).
  • Printer resolutions ought to be expressed in 'dots per inch' (dpi).

When you talk with me, feel free to use whatever terms you like. I know the differences, but almost always use 'dpi' for all three measurements. I will explain the differences and once we get through this little exercise, I hope you'll understand scanning resolution better.

Semantics anyone?

During scanning, scanners sample areas of documents and convert them to bits of information. Monitors interpret those bits of information and display them as pixels. Printers interpret those same bits of information and print them as dots.

(See bits to learn what the terms '8-bit grayscale' and '24-bit color' mean. )

The language of scanners.

When you place a certificate in your scanner and tell it to start scanning, it will sample every one-hundredth of an inch, every two-hundredths of an inch or whatever you tell it. In the U.S., scanners report scanning resolutions in terms of samples per inch. Elsewhere, resolution is commonly expressed in samples per millimeter or samples per centimeter.

When talking about the act of scanning, 'samples per inch' is the most accurate terminology. So why do so many people use 'dots per inch?'

Because so many scanner companies do!*

Scanner software manufacturer Resolution terminology
Epson 'dpi'
Nikon 'ppi'
Hewitt-Packard 'dpi' or none**
Ideal 'pixels per inch (dpi)' ***

* Terminology used in software versions I have. Other versions may not agree.
** One version avoids the issue by not using any terminology
*** Yes, the software actually uses both terms exactly like shown!

Monitors have pixels and they have non-uniform but measurable size.

Monitors create displays of images and words using pixels. The term pixel means picture element. A pixel is a single colored spot on your monitor. You now know that scanners create information based on samples per inch. What about monitors?

You are probaly reading this text on a monitor. Chances are very great that you have a different monitor than me. Most monitors manufactured in the last few years have a width to height ratio of about 16:9. Older monitors have shapes similar to older televisions with a width to height ratio of 4:3. These ratios indicate the shape of your monitor; it doesn't tell you anything about your resolution.

You probably don't know the real resolution of your monitor right at this moment. Let's find out.

If your operating system is a Microsoft Windows product, right-click in a blank area of your desktop. Choose 'Properties' from the menu that appears. Next, click 'Settings'. In the dialog box, you will see a box that reads 'Screen Resolution.' Chances are, it will say something like 1280 x 768 pixels, 1440 x 900 pixels or something similar. Write down those two numbers.

Those numbers tell you the entire width and height of the visible part of your monitor in pixels. Grab a ruler and measure the width and height of your monitor in inches. Divide your larger pixel number by your width measurement and your smaller pixel number by your height measurement. The resulting values will tell you your real viewing resolution.

You will probably discover your vertical resolution is not the same as your horizontal resolution. My monitor is set to 89 ppi wide by 85 ppi tall. Your resolution is almost certainly different than mine. This exercise is designed to let you know a few things.

  • Your screen resolution, as sharp as it may appear, is probably lower than you thought.
  • Whatever you see on your monitor will almost never accurately represent what you scan.
  • What you see on your monitor is likely not the same as everyone else's.
  • Your vertical and horizontal resolutions are different.

You now have a better feel for what a 'pixel' is and what 'pixels per inch' means. Now, let's look at how different scanning resolutions affect what you see on your monitor.

YOU get to set the size of pixels for YOUR monitor.

If you go back to the dialog box where you found your screen settings, you will see that you can easily change your screen resolution. By doing that, YOU control the sizes of the pixels on your screen. These days, average users set their monitors to display around 96 pixels per inch. I am now 62 years old and although I hate it, I need larger pixels than I used to. Adjust your screen for your comfort level, but be aware that what you will see will definitely not be the same for everyone else.

I need to underscore the fact that screens merely display pixels; they are not meant to accurately display measured dimensions. To make matters worse, you are now aware that monitors frequently display horizontal and vertical features at different scales! That freaks some people out.

Want to see engineering professionals get the heebie-jeebies? Ask them to draw a perfect circle on their laptop computer using a high-tech program like AutoCAD. The circle will appear on their non-calibrated monitors as an ellipse. They are generally amazed and want to know what they can do.

"Don't worry. Trust me. Everything will print perfectly."

They, of course wonder why.

Monitors and printers display different things. Monitors display pixels. Nothing more.

Below are four images. I scanned a section of a certificate at four different resolutions and then cropped each image to exactly 125 pixels wide.

Since your monitor only displays pixels, it has no idea what my original sizes were. Nor does your monitor know anything about my original scan resolutions. All it cares about is displaying pixels.

100 dpi
original = 1.25 in
200 dpi
original = 0.63 in
300 dpi
original = 0.42 in
400 dpi
original = 0.31 in

Since all four images are 125 pixels wide, they all display the same size.

Printers print dots and dot sizes depend on specific printers.

Image setters, used for printing books, often have the capability of printing at 2400 dots per inch. Average laser printers and most ink jet printers can print 600 dots per inch. At 600 dpi, printed dots are very small, approximately 0.0025 to 0.0033 inches in diameter.

In order to make letters more readable, most printers overlap adjacent dots very slightly. This gives an illusion of solid letters with sharp edges. However, if you magnify printed letters 20 to 25 times as in this example, you will see that they are really quite 'jaggy.'

300 dpi
ink jet printer

The charaters come from the same text file printed on three different printers. The character at the left was printed on a typical Hewlett-Packard inkjet printer of recent vintage. The character in the center was printed on a Hewlett-Packard inkjet plotter used for making maps and the one on the right came from a Konica-Minolta Bizhub laser copier.

The greater your printer resolution, the less jagginess you will see. Among these examples, the laser printer delivers superior quality for printed letters. However, if we printed photos with these same three printers, we would see the 600 dpi ink jet printer would be the best choice. It has the capability of rendering objects in shades of gray and colors whereas the laser printer does not. The 300 dpi plotter, made for plotting large maps, delivers superior quality when printing line drawings and large maps.

In the 300 dpi plotter example above, you can see several individual colored dots around the edges of the letter. This is because the 'black' letter was actually constructed from black, cyan, magenta, and yellow ink droplets. Fortunately, the dots overlap sufficiently that the eye is fooled into seeing the letter as a single black object.

This illustration also shows the importance of the type of paper used for different printers and applications. The 300 dpi ink jet printer (center) used a high-quality coated paper. Various magenta and cyan dots are visible in the high-resolution image because the ink set on top of the paper as opposed to soaking into it. Similarly, the 600 dpi laser printer (right) used toner that adhered to the surface of the paper before being fused by the printer's heating element. Note however that the 600 dpi ink jet printer (left) sprayed ink on a cheap, somewhat fibrous, copy paper. From arm's length, the letter was perfect. Under high magnification, absorption and spread into the paper is evident.

While it is easy to throw around resolution abilities of printers, it is important to understand that the diameters of dots matter too, both in laser and ink jet printers. In the examples above, dots are very tiny, but overlap sufficiently to make letters appear solid. Coverage and appearances are also affected by the quality of the inks and printer heads you use. (We no longer use third-party inks because was have found that quality matters!)

Here is an example of how the letter 'e' might be constructed from dots of different sizes. By comparison with the highly magnified images above, these letters would probably be the equivalent of printing with an old dot matrix printer at about 125 dpi. Dot size greatly affects clarity.


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(Last updated November 20, 2015)


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