Finding the Best Input Resolution for Printers

[Steve Everist]

I stumbled across the website below while searching for information concerning the best resolution input (ppi) relative to a printer's rated output (dpi).

This has become an issue as many offices have switched over to digital capture of their latent prints, and evidence in general. A common question seems to be, "What is the best printer for printing out 1000 ppi latent prints at 1:1?" With the current crop of printers, this just isn't going to happen. Although printers advertise outputs in excess of 1440/2880/etc... dpi, they aren't able to resolve, with the eight or nine inks available to them, the color and clarity of an image containing as many as 16.7 million colors with a resolution of 1000 ppi or greater.

So the issue to consider is what resolution at input creates the the most accurate output from your printer. I've heard different ways of figuring this, including elaborate math problems with length and width dpi of the printer and the square root. I think the solution is more simple than this after using these test images with our Epson 2200.

I printed both of the images (one vertical lines and one horizontal) on our Epson, using the same printer quality settings that I use for printing out high resolution images. And it turns out that 180 ppi and 360 ppi seem to be the best. I would guess that 720 would also be good, but the printer doesn't appear to be able to resolve this level of detail as the lines are too fine to see that they're lines. The Epson 2200 lists an output of 1440x2880 dpi and both 180 and 360 are divisible into both numbers (as is 720). The clarity of the lines at 180 ppi are actually better than those at 200, 240, 260, etc... until you reach 360.

So if you'd like to test your printer to see which input resolutions seem to hit the sweet spot, go to http://www.inkjetart.com/tips/ppi/ and download the two PDF files. Remember when printing to not use the "fit to margins" setting, otherwise it will modify the image for printing. Also use the settings and paper that you use for high quality prints.

I think the diagonal lines are very telling. With the Epson 2200 at 480 ppi input, there's a block pattern created that was very interesting to say the least!

[Gerald Clough]

I found one guy who did extensive testing of the 2200 and found 288 ppi to be optimal. http://www.rags-int-inc.com/PhotoTechStuff/Epson2200/

Good discussion, too, and a good starting place for those setting out to analyze their own hardware.

[Steve Everist]

Thanks for that link Gerald!

It looks like my understanding was on the right track, but I failed to consider multiples of 5 and 10 (288 and 144). I went straight to considering numbers that represented halves 1/2, 1/4, 1/8, and 1/16.

Unfortunately the person who created the PDF files in my original link didn't think to include them either. It is odd that a multiple of 5 would be where he found his best resolution with a 4x24 cartridge.

I'm wondering if the banding effect has something to do with overlap of the cartridge nozzles or even a flaw in his printer.

I'm in the process of getting his images to print and look at. Unfortunately they aren't the same as those in PDF test sheet that I posted in my first post.

[Gerald Clough]

That's another question, and I don't know enough about the printer hardware to know if there's any loss of precision as it ages and wears or even from changes in the dot nozzles. I would expect native conflicts between pixels and head configuration to present regular banding across the image and maybe aging defects to produce isolated bands.

From what I can tell of the mechanisms, it always looks to me like they would be subject to all sorts of problems of wear and environmental crud.

[Steve Everist]

Gerald,

I printed out the image from your link at 288 and also ran into the banding of the dots in both the 288ppi and 144ppi grids.

I then converted it (no resampling) to 360ppi and the one that was considered 144ppi (assuming it converted to 180) looked better than the original, and there was no banding evident. The dots from the higher resolution grid (288 - 360) aren't as clear, although there is no banding as there was when printed out in the 288ppi configuration.

The text was clearer at 360 than at 288 also. with the 288ppi image showing a yellow fringing that wasn't visible in the 360ppi version. As far as the photograph portion of the image, I'm not seeing anything that sends me really one way or another. Since it wasn't resampled, the physical size at 360ppi has been reduced. This does appear to give it the perception of increased sharpness, but it may be mostly a difference in the physical size.

[George Reis]

Your initial post on this topic, Steve, really has two parts to it - the scanning resolution; and the printing resolution.

I have never been a proponent of the 1000 PPI "standard" and find many reasons to consider other resolutions. One issue is that many scanners have to resample to scan at 1000 PPI - scanners generally don't have a continually variable resolution, so when setting a scanning rate that is not an even factor of the maximum optical resolution, there is a good possibility that the scanner is resampling the image. There are other issues involved, such as then corresponding the scanning resolution with the printing resolution, and an implication that sub-1000 PPI settings are somehow unusable, etc.

Another issue is the whole concept of printing latents 1:1. I have never had to do this in my career. I began using digital imaging with latent prints in 1992, and I've spent a lot of time helping agencies do this, but still haven't seen anyone with a good reason for doing so. Potentially, the original latent has the most information. Then, the digital file is next. The printed image is never going to equal the image quality on one's screen. And, why would want a tiny, little print anyway?

But, your main topic is printing, and it's great to see your post on this topic. It is unfortunate that printing is such a complex issue. I've seen so much bad information out there that it's hard to sort out the good stuff. Then, some police IT departments won't load the proper printing profiles for printers, so the analysts who do the printing are per-ordained to get bad images.

Anyway, good discussion. I tested my printer when I got it several years ago and find that I get excellent quality with good sharpness at 240 PPI, and a little more sharpness at the cost of color fidelity at 360 PPI (this is on my Epson 1280 - yes, I am overdue for an upgrade - I plan to convert this to a grayscale printer using four shades of black/gray inks eventually).

George

[Steve Everist]

George,

I'm glad you weighed in on this topic. I was just about to email you regarding another issue relative to scanning lift cards that have been processed with silver powder.

I agree with your thoughts about the 1000ppi issue. I usually scan at 2400ppi as it's the highest native resolution of our scanners. It's not always needed, but I'd rather have more and need less than vice versa. The Hayden case, which most digital imaging courses reference, was much less than 1000ppi.

For some reason, after the SWG's included 1000ppi in their guidelines, it became a target to reach instead of a sort of theoretical safety net. I even remember being taught about how to get your camera/lens to capture an image as close to 1000ppi as possible, and then being told to mark on the lens that spot so that all photos could be captured as close to 1000ppi as possible. I'm glad the current trend is to just train to 'fill the frame.'

[Charles Parker]

Great discussion. This may solve the problem with one of our printers putting out crap.

In discussing this and other issues today, we were told that if you scan a latent print in it is best to put it in the center of the scanning glass instead of the corner. And if you are having problems with a light print that you cover it with a gray sheet instead of using the normal white backing of the scanner lid.

I can rember using that concept on some photography years ago but it has slipped from my mind.

I have to try out everyones suggestions.

I like this kind of thread----keep it up.

[Mike French]

Here's another useful chart to calibrate your printer:

http://www.graphics.cornell.edu/~westin ... schart.pdf