Back to Dye Printing Tricks & Tips
There is a difference, and it's good to know when to quote which.
PPI on your computer monitor or screen
These are imaging pixels per inch, little blocks of color. The higher number of pixels per inch, the finer detail you can see on screen.
PPI as image resolution
This is the number of pixels per inch as output by digital camera photos. The higher number of pixels per inch, the finer detail available for output. (Once captured, no further detail is actually available. Filters and fractals can fake some more guessed at color blocks, but no new information will be uncovered, only guessed at)
DPI printer output
The dots per inch produced on a deskjet, laserprinter or RIP station. In deskjets and laserprinters a single "Dot" is the smallest fleck of toner that can be sprayed or statically applied to the paper. Obviously one dot per inch would make any photo look like a series of colored building blocks. 100 dots per inch would reveal more detail. 300 would be fine. 600 or more makes it nearly invisible to the naked eye that there are even dots involved. On the film or platemaking RIP stations it takes 1200, 2400 or more dots per inch to create what appears to be curved lines from everything from the letterforms to graphics. In reality, under a magnifying glass, even the smoothest curve is but a series of square dots stairstepping their way to form a circle.
LPI printer output
The lines per inch generated by higher end printers to create the halftone patterns to create the "grays", the midtones, the gradients, etc. In the print dialog box of many applications you often see the two dpi/lpi numbers after the color separation area. This tells you the dots per inch. Often 150 lpi (for the halftones) and 2400 dpi (to create the images and text).
A Closer Look
Pretend this is a photograph (not a JPEG on your computer screen).
If we scan it into the computer, it can be viewed as Pixels Per Inch:
See the many blocks it takes to create the colors to create the image? Each is a pixel. A computer rendering of this rasterized image contains thousands of pixel color blocks. Everything is made up of some pixel or another. Unlike printing, there are white blocks as well.
In printing, white is the absence of ink, so the paper shows through. To create this same image on a printing press, the colors of the photograph are broken down into their "process color" components. Here, so much cyan, magenta, yellow and black process inks, mix visually to create the various shades of red.
Pretend you're looking at the photograph again. Now we capture it at a higher resolution and "separate it" for process color printing. We'll need four plates to create this full color image.
Notice the right half of the enlarged sample? That's just the black halftone dots it takes to create the image. This would represent the black plate used to put down black spots of ink. (We'll also need a cyan plate to put down the bluish areas, a magenta plate for the reds, and a yellow plate for the yellows. Where the color dots meet, the human eye mixes the various shades of all the colors.) Heavy dark areas below will go black, lighter smaller dots will allow more of the other inks to take over and mix the reds.
In this vector graphic sample we can see how the gray area is really solid black dots, but with some paper allowed to sneak through. That visual mix turns it to a "halftone" gray eventhough it's really teeny dots of 100% black ink. The gray areas of such a printed piece would not be a gray ink put on the press, but rather a tint of the black ink. The red ink put down for the other areas has a gradient to it. Overall, this multicolor job, is really a two-color press run.
Here's a deskjet close up of the vector graphic. See the multicolored sprayed dots? Even the "gray" area is a bit cyan, a bit magenta, and a bit yellow. Notice also how ragged the image really is up close?
