Determining Scanner Resolution:
1) Multiply the specified finished graphic dimension by the desired print resolution to determine the TOTAL pixels required for correct sizing.
2) Divide the total pixels by the input dimensions to determine the scan PPI (pixels per inch) setting to use on the scanner.
3) Set the scanner ppi (dpi) resolution. Use the next higher resolution if the scanner cannot be set to the exact resolution needed.

All our images are saved and manipulated, scanned and scaled to size using millimetres. Dots per inch is a common term but not used in Australia.

DPI Dots per inch
Printer ink dots and image pixels are very different concepts, but both use the term dpi in their own way.

People sometimes initially assume that because inkjet printer ratings use dpi, then therefore image pixel spacing MUST be called ppi, to distinguish them differently. These are indeed different concepts, but while ppi is fine as a personal preference for resolution if you wish, to imagine it is a rule ignores the vast bulk of existing real world practice.

Some literature does attempt to reserve use of the term dpi to be used exclusively for the ink dots made by inkjet printers, because that's so different from everything else. Then we hear scanner resolution called spi (Samples Per Inch), and that is indeed what it is (we call these color samples "pixels"). We hear image resolution called ppi (Pixels Per Inch), and that is indeed what it is. The spi and ppi usage is precisely correct.

But pixels are a kind of colored dot too (more a concept than a physical colored dot), and the term dpi is absolutely correct for "pixels per inch". This is true because dpi is in fact the printing industry's long established name for the spacing of image pixels on paper. Its origin may be in jargon, but dpi has always been the proper name for the term "pixels per inch".

When referring to images, or to printing images (as opposed to inkjet printer ratings), we can treat these three terms, spi, ppi, and dpi, as the same equivalent concept - all three simply refer to image pixels per inch - the spacing of image pixels on paper. You may use your own preference, dpi or ppi, but you must understand it either way, because you are always going to see it both ways.

The term pixel is a computer abbreviation for picture element, the smallest element of a digital image. Pixels are a very different concept than the ink dots that inkjet printers can make. The idea of printing these pixels in digital images is always about pixels per inch on paper. All three terms (dpi, ppi, spi) are the same image pixels per inch - the spacing of image pixels on paper. Spi and ppi are excellent tries in concept, and are good names too, there is nothing wrong with them, but the fact remains that common and historical usage has always called it dpi.

So yes, inkjet printer rating dpi is something entirely different, referring to inkjet printer ink dots instead of image pixels. The inkjet printer tries to simulate the color of one 250 dpi pixel by making several ink dots of four CMYK ink colors, which are located perhaps on 1200 or 1440 dpi spacing (See the Printer Basics section). The printer is trying the best it possibly can to reproduce the pixels (pixels is all there is), but inkjets cannot reproduce colored pixels directly. Image pixels and inkjet printer ink dots are NOT the same thing at all.

However, continuous tone printers (dye-subs, and Frontier-Noritsu-Lambda-Lightjet types) don't print discrete ink dots of three colors like inkjet printers must - instead they mix the color of the pixel directly. There are no dithered ink dots then. But these printer ratings still refer to the spacing of those image pixels with the term dpi, simply because dpi has always been the name for "pixels per inch". Scanner ratings also always call it dpi, also referring to pixels of course (scanners don't use ink dots). I always say dpi too, simply because that has always been the name for pixel resolution.

Ppi is a relatively recent new term, and we are seeing ppi used more now, and it is a perfectly fine name too. Recent photo editor software often says ppi, while scanner software generally says dpi (but there are exceptions to both, preferences allow this). But either term is correct, and in fact, the long established name for image resolution has always been dpi, for many years before inkjet printers could print photo images. So you will often see either dpi or ppi used, and you must understand both as the same term. It may be a bit confusing at first, but that's simply how things are. Think of this as training to understand what you will see elsewhere. <grin>

There is really no problem understanding the two uses of the word dpi if you know the basics, and realize the context. It always means the only thing it can mean in context. This should be no big deal, the English language thrives on multiple context definitions.

If the usage context pertains to images or printing pixels (and it almost always does), then dpi always means "pixels per inch". So does ppi, same thing exactly. It cannot mean anything else, printing is about spacing pixels on paper. The two terms are fully interchangeable, use either according to your whim. If we have a 300 dpi image, both terms mean it will print at 300 pixels per inch (pixel spacing on paper), so 300 pixels will cover one inch.

If the usage context pertains to inkjet printer ink dot ratings, dpi means "ink dots per inch" (but since the ink dots are actually larger than their spacing, the rating is more specifically about carriage and paper motor stepping intervals). If the printer rating is 1440 dpi, it means its motors can space 1440 ink dots per inch while trying to simulate the color of the pixels in that 300 dpi image
 

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