The Difference Between DPI and PPI and How it Affects Inkjet (Giclee) Printing

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People and even software manufacturers freely exchange the terms PPI and DPI.
These are two very different measurements. We often receive calls and messages regarding what resolution is needed to print images.
While high fidelity professional inkjet printers can render a DPI (Dots per Inch) of 1440 to 2880, this is not how a file should be prepared. Let's take an example.
You have a file that measures 20X30" at 150 PPI. If you want to print on a fine art paper, this is all is needed.
The confuision comes as of why it is sufficient to have a file at 150 PPI when the printer is imaging at 1440 DPI. The answer is that PPI and DPI are two very different measurements.
PPI refers to the number of pixels per inch in a file. It is an electronic figure, purely digital.
PPI is important because it affects print quality and even the way an image displays on a screen. While about 72 PPI is all is needed for web images, it is generally too low for printing.
The result would be jagged edges and pixels showing in the print. Giclee printing in particular is demanding because professional grade nozzles are not very forgiving of low resolution files.
Generally speaking, textured materials such as canvas or watercolor paper will print well at a lower resolution. Smooth, photo type papers will need a higher PPI.
DPI on the other hand refers to the printer resolution. One pixel does not equal to one dot.
One dot represents one color while one pixel will render 4 or more. DPI is an important measure of quality for printers but not the only factor.
There are expensive machines that only have a 300 DPI capability but produce extremely high end prints. This has to do with the screening technology employed and it is beyond the scope of this article.
Interpolation of a file to boost PPI before printing sometimes helps but it has to be done with care. A 20 PPI will not print well, no matter what software and hardware is used.
Generally an interpolation factor of about 200% will create a good print. Be aware not all files size up with the same quality.
Digital photographs are easier to enlarge because there are no dots to start with. A film scan from ie a 35mm negative will not be as easy to interpolate because of the inherent film grain.
A well exposed image with good values is also a better candidate than a poorly prepared file. The old say in digital imaging, "garbage in, garbage out" is true.
Start with a good resolution file, proper highlites and shadows, and you will be able to produce outstanding prints. A poorly prepared image cannot not be salvaged to create an equally beautiful reproduction.
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