Resolution of 600 DPI and Illustrator

Question

Someone would like a small 1 inch by 1 inch logo for invoices at 600 DPI. However I understand that illustrator doesnt work in DPI or PPI since it's vector. Would I have to transfer the small image over to photoshop. I did try and save a 1 inch by 1 inch logo at 600 PPI, however when I saved the image it was no longer 1 inch by 1 inch. It had increased significantly in size regardless of what file format I saved the image as.
I would really appreciate help in this matter as Im unsure how to proceed.
Thanks

Answer 1

Its confusing to say the least!
Here's a universal truth that will help aleviate the confusion:
Think about this term: Pixels Per Inch.
Per Inch. You have some things (pixels). A given number of them occupy a given distance (an inch).
How do you do that? By scaling the things (pixels) so that a desired number of them fit within a given distance (inch).
Per Inch is just a scaling value. Understand and remember that universal truth, and it will clear up alot of the confusion.
Once a  raster image is saved, it contains a fixed number of pixels. In order to draw a given number of the image's pixels within a measured distance, the raster image has to be scaled. Change the scale of the image, and you change the number of its pixels which occupy a measured distance.
Now be aware of these universal truths (aka facts):
Raster image file formats are different. Some formats record in the file the intended scaling value (if one is provided) that is in effect when the file is saved. Others don't.
Programs which export raster images are different. Some programs write the scaling value into the raster image file (if the raster image format supports that). Others don't.
Programs which import raster images are different. Some programs pay attention to the stored scaling value (if any). Others ignore it.
Someone would like a small 1 inch by 1 inch logo...
First issue: Is the logo square? Is the design's height equal to its width?
...for invoices...
This suggests that "someone" intends to import the raster image into some program used for invoices. That could be any of a kazillion different programs, from Word to Excel, to FileMaker Pro, to QuickBooks, to....you name it (but you didn't). You always need to know your customers' intended usage and be at least somewhat familiar with the target software's capabilities regarding importing artwork.
...at 600 DPI....
This suggests that "someone" wants a raster image with a color depth of 1-bit. That means a raster image in which each pixel can be either black or white, and nothing else. This is suggested because that is a commonly-used scale factor for 1-bit raster images that are intended for use in common programs like office applications which have very limited graphics import capability, and because it is the most common hardware resolution of most office laser printers. In other words, 1-bit raster images scaled to 600 PPI is a kind of "lowest common denominator" for commonplace office environments.
But the above suggestion  is based on the assumption  that "someone" knows what he/she is talking about, which is never a safe assumption. If the image is of any color depth higher than 1-bit, then 600 PPI is ridiculous. In other words, if the image file contains any pixels of any color at all, then 600 PPI is absurd for use as an invoice imprint. This is because any higher color-depth image will have to be halftone screened by the printer in order to render it with any decent-looking fidelity. That means rendering the image as a grid of variable-size dots. But the printer cannot actually print variable-size dots. It only prints fixed-size printer spots. To render variable-size halftone dots, it "fakes it" by  "building them up" out of its fixed-size printer spots.
Moreover, even if the logo (the actual design) contains nothing but black and white, but you save the raster image file at a higher color-depth anyway, then 600 PPI is still just as ridiculous. Grayscale, RGB, and CMYK raster images are saved with a color depth of 8 bits per pixel per channel. When the printing device receives an 8-bit image, it is going to try to screen it (print it as halftone dots). That means that the printer will not use one printer spot for each pixel, but several. So there is no reason to provide as many image pixels as printer spots that the printing device can physically print in one inch--unless the image is not going to be rendered by halftone (or stochastic) screening (i.e.; a 1-bit image).
However I understand that illustrator doesnt work in DPI or PPI since it's vector.
But Illustrator's raster export filters do indeed work in PPI, because they are exporting raster images, which are nothing but pixels. In order to export a raster-based rendering of the vector-based artwork (which is all a raster image is), Illustrator has to write some number  of pixels into the raster image file.
Your vector-based path is a mathematical description of a curve. In order to render it as a raster image file, or even to print it on a printer, or even to display it on your computer monitor as you work in Illustrator, it has to be rendered as pixels. In effect, your vector-based paths are "overlaid" onto a grid of pixels. The pixels which land inside the path are turned on. The pixels ouside the path are turned off. This is basically what occurs when a vector path is printed, when a vector path is exported to a raster image, or even on-the-fly while you view your vector paths on your monitor.
That's what vector-based paths are: Mathematical instructions to determine which squares of a raster grid  to turn on.
In other words, all vector-based artwork is eventually  rasterized; it's just a question of when. The scaleability advantage of vector graphics does not derive from the myth that it does not involve raster-based imaging; it derives from the fact that the rasterization is postponed until output time.
In still other words, all your vector-based artwork is going to be  overlaid onto a raster grid, the squares of that grid will be colored accordingly, and that finished raster image is all anyone is ever going to see of your vector-based formulae. It's just that which  grid is to be overlaid is left undetermined until the "overlay" is performed.
So when you export your vector-based artwork to a rastter-based image format, that's what you're doing: You're telling Illustrator to overlay the vector paths onto a particular raster-based grid (so many rows, so many columns of pixels, at some scale).
That's the difference between vector-based artwork and raster-based artwork: A raster image is a pre-determined  rectangular grid of a fixed number of color values. A vector graphic is a set of mathematically-described shapes which can be overlaid onto any rectangular grid, so that the number of color values is dependent upon the particular grid used, be it the grid of your monitor's pixels, the grid of a printer's spots, or the grid of a raster image file that you export.
Would I have to transfer the small image over to photoshop.
No, you can export directly from Illustrator to any of several common raster formats.
I did try and save a 1 inch by 1 inch logo at 600 PPI, however when I saved the image it was no longer 1 inch by 1 inch. It had increased significantly in size regardless of what file format I saved the image as.
It "had significantly increased in size" where? When viewed in what program?
When this occurs, the raster image is simply not being scaled  so that its pixels measure 1/600th of an inch. As explained above, some raster image formats do not record the scaling value. Even if the raster image file does record the intended scaling value, some programs into which you import the raster image will ignore it.
So since you can't really control whether the scaling factor is going to be ignored, to meet the stated requirements (600 PPI when scaled to  1 inch), you simply need to concern yourself with providing the correct number of pixels and it's up to the recipient's software to scale them to the desired measure.
For example, if your vector-based artwork measures 1 inch according to Illustrator's rulers, then export the raster image at 600 PPI. If your vector-baed artwork measures 2 inches according to Illustrator's rulers, then export a raster image at 300 PPI, and you'll be exporting the same number of pixels. You can either do the math, or you can simply export a raster image by specifying the desired number of pixels directly.
But do not forget to consider the question of color-depth. 600 PPI at final rendering scale is not necessary for anything other than a 1-bit image.
If the logo in question consists of nothing but a single solid color:
In Illustrator, scale it to 1 inch.
Do not color it. Use only solid black and white fills and strokes. No grayscale. No graduated fills, strokes.
Export it to a raster image format at 600 pixels (in the measured direction), with a color depth of 1-bit. You can do this by either specifying the nubmer of pixels, or by specifying the scale factor, 600 PPI. Understand, this means using a raster image format which supports 1-bit (for example, TIFF) and  which the recipient's program can import. (Most common office productivity programs can import TIFF.)
When the recipient imports it into his invoice program, if it appears enlarged, it just means his program is ignoring the embedded scale factor. He can just scale it to 1" and it will then be 600 PPI.
The above scenario is common. A typical 600 SPI (printer Spots Per Inch) laser printer will turn on one of its printer spots for each black pixel in the image, thereby rendering the image as accurately as it can.
But even if the logo contains no graduated tones, and you apply some color to it (the recipient's logo spec color, for example), then you'll have to export it to a higher color depth in order to include that color. Even if you export it as grayscale, you will still be exporting 8-bit color values for each pixel. In that case, it may look fine on the recipient's monitor, but when he prints it, the printer will have to screen the image in order to render it using halftone dots which, again, are larger than actual printer spots.
The same applies, even if you use only solid black in the design but still export it to a higher color depth (grayscale, RGB, or CMYK) raster image. The recipient's printer is going to recognize the incoming raster image as 8-bits of data per pixel and assume that it has to halftone it; so again, it will not be printing one printer spot for each image pixel.
All this is why Illustrator's so-called "Save For Microsoft Office" feature is really nothing but another interface for its PNG export filter. Many AI users (mostly beginners) want to do the impossible: export their Illustrator drawings to something that ordinary office programs can import, display, and print with fidelity.
But that's really a pipe-dream. Office applications don't understand the kind of vector-based curve math which Illustrator (and all similar programs) generate. So the Save For Office interface resorts to a raster image export.
Office users don't understand the caveats and limitations of CMYK color mode, lossy JPEG compression, or color depth and don't want to have to muck with them. They just want something that they can plop into their word processing, spreadsheet, and presentation programs and have it "look good" on screen and when printed on their low-resolution office inkjets and laser printers.
Too many Illustrator users also don't want to have to think about color depth.
So the Save For Office interface defaults to RGB PNG, turns off transparency (even though PNG supports it) because some specific Microsoft Office apps before a particular version have a problem with it, and does not present the user with much in the way of other PNG-capable options (such as 1-bit grayscale).
Why am I bringing this up? To point out that Adobe's own default and explicit  "recommended" export for office-type programs is an 8-bit RGB format which, again, obviates the appropriateness of 600 PPI. In RGB, 300 ppi provides more than enough pixels for the office-using recipient to scale the image to 400% without evident pixelation because even 150 PPI is higher than the screen ruling of most office printers.
So 600 PPI at the actual final on-page scale is sensible for simple office uses like ploping a black logo onto a form like an invoice that is goingto be printed in one ink (or toner) assuming 1-bit color depth; but not otherwise.
JET