The Scanning FAQ

The Scanning FAQ - Color

Color Scanning

First, color scanning is possible on the desktop regardless of what your printer/stripper may have said. It would be possible for me to write a few thousand pages about how to not do it, this FAQ deals primarily with greyscale scanning since it is the most common desktop scanning. But while I have your attention I may as well interject my opinion about color scanning.

For most of us, color scanning is needed for color separations for used in four-color printing. Your UMAX, Microtek, HP, or even Nikon color scanner does a good job if you have a skilled operator and an exacting output service bureau, but don't assume you are going to save any money or time. More often, the years of color experience a good four-color shop has is worth the effort to use traditional methods, and will save you time in the long run. I've worked with some of the best desktop color systems around and choose not to use it for large format and high quality pieces. Instead I send these out to a local Crossfield-based Separator. But a lot of our continuous-tone color work is done in-house.

If your bent on doing color on your desktop, you could try a hybrid approach. Traditional four-color separator scans the art and gives the DTPer a low-res image for placement. Output is then sent back to the four color shop where the high res is used. Hybrid methods have some measure of popularity. Users realize their eye for color and equipment isn't as good as expected. With the broad acceptance of FM or stochastic screening output devices this is, in my opinion, a better solution because the person who knows the strengths and weaknesses of their system best is the person running the high-end imagesetting system, not the desktop publisher.

Color Scanning for Continuous-Tone Devices

Perhaps the most common segment of color in desktop publishing community is the continuous-tone image scan. Continuous tone images are used in lots of "consumer quality" color output. Consumer Quality color output would include desktop color printers, slide recorders, on-screen presentations, and even web graphics. When dealing with these applications it does become cost effective, reasonable, and very practical to use color scanning systems to build files for output to these devices and media.

A continous-tone device is a non-screened system that is capable of applying the entire gamut of its colors to the smallest picture element that it can reproduce. In effect, this means that the device is capable of making any spot on its output any color you wish. This is radically different from a traditional four-color process where color is simulated by creating color rosettes with four different halftone screens. No longer are elaborate screen angles and separation schemes being computed by both the computer and the printer, the printer merely prints pixel-for-pixel the exact color specified on that dot.

As simple as this sounds, however, it creates one major problem for the desktop scanning system, computer, and color printer--FILE SIZE.

In a traditional halftone device (a non-FM system) very seldom are all the output dots represented, in fact, most of the time there is lots of space between the dots. Lots of space translates to no data, and therefore the image files and the output files can be relatively small and efficient compared to a continous-tone or contone device. In a contone device all the output dots have to have data, even if that data is white (or blank).

In a perfect situation, you would like to have one scan spot for each output dot that you would be using, this way you have a 1:1 ratio between spatial and actual resolution. In a halftone device this is no problem, since even a lowly 300dpi scanner can produce enough dots to satisfy the halftone device and its missing dots. But satisfying a contone device is much more difficult.

Compare the following example:

Scan a 5x7 color original to produce an 150% enlargement on 8.5x11 paper on a 133lpi halftone device and a 400x400dpi continous tone device.

The scan file size for the halftone device would only need to be about 300 dpi or approximately 9 Megabyte uncompressed.

The scan file size for the continuous tone device would need to be 600dpi or approximately 37 Megabyte, over four times as large!

Thankfully, most applications of continuous tone are at screen resolution (i.e.- PowerPoint Presentation, or Web Art) where the output resolutions are less than 100dpi. Additionally, the ubiquitous Color Ink Jet printer is actually a halftone device masquerateing as a contone device so we have to deal with them a little different as well.

Color Scanning Formula (Contone)

Just as color scanning is more complex than simple greyscale scanning, so to is the formula for computing the proper original scan resolution for continuous tone devices. Clearly the best possible situation would be pixel for pixel when printing to a continuous tone device, but in reality there are a lot of other factors that come into play when determining the optimum scanning resolution.

Those factors are:

The quality of the output device as determined by its propensity to generate a hard dot. (A hard dot referrs to the devices output dot spread (dot gain) or blooming characteristics. Even though a device is rated at 600dpi or 8000 line resolution, the actual resolvability varies because of a number of opto-mechanical factors).
The characteristics of the original to be scanned. Soft originals--those with compact dynamic range, little fine detail, no hard edges, and sharp contrast changes--can usually be scanned at lower than 1:1 ratio resolution and acheive quality results.
The viewing characteristics of the original. Originals viewed from a significant distance 1:1 ratio resolution becomes less critical than originals designed to be viewed at close distance or under direct scrutiny.

To make this decision as easy as possible, I have boiled everything down to the following formula.

The formula is as follows:

For Color output to a Color Device

Scan DPI = Output Dev. Min. Pixels in X/ Hor.Dim. of Original (ODMP)
multiplied by - % of frame original fills (%FF)
multiplied by - Quality Coefficeint (QC)
or
Scan DPI = OPMP x %FF x QC

Where...
QC = Quality Coefficient

1.0 = Normal Quality -: fine for photographs with little or no hard edges, may show slight pixelization when viewed close.
1.3 = Good Quality -: most commonly used, will reproduce with little pixelization to most output devices. May pixelate on a color copier.
1.7 = Better Quality -: photographs that have higher contrast and edges, will reproduce tightly with little or no pixelization.
2.4 = Best Quality -: practically pixel for pixel scans, used only in extreme situation such as 4x5 transparencies that will be used for large prints or heavy scrutinization
3.3 = Ultimate Quality -: pixel for pixel scans designed for 35mm and 4x5 transparencies, never used except for test purposes.

For example:

5x7 Color original to be placed on a 35mm slide filling 50% of the frame at good quality. The formula would be:

Scan DPI = 1500 / 7 x .5 x 1.3 or 139dpi = 1500 / 7 x .5 x 1.3

Continuous Tone Device Information Matrix

Use this table to select the continuous tone device you will be using and plug the table numbers into the Scanning Formula above to get the right DPI setting for your scanner.

Typical Devices	Res.(dpi)	LPI	Min. Pixels in X Horz. Dim.
Color Laserprinter	400	200	1765 (Ltr) 2800 (Tab)
35mm Slide	2400	200	1200
4x5 Transparency	4000	300	2500
8x10 Transparency	8000	300	4000
360dpi Color Ink Jet	360	n/a	1580 (Ltr) 2520 (Tab)
600dpi Color Ink Jet	600	n/a	2640 (Ltr) 4200 (Tab)
On-Screen/Web Graphic (SVGA)	72	n/a	800 (15") 1000 (17")
On-Screen/Web Graphic (VGA)	72	n/a	640

For file size and time reasons, you should consider converting color scans to an 8-bit (256 color) image if there is no apparent loss of detail. This will save 40-80% of the file size depending on the file.

For best results use a software package such as Adobe PhotoShop, Equilibrium's DeBabelizer, or Inset's HiJaak Pro that allows you to use a diffusion dithering, FM-screening, or stochastic screening process to produce pleasing approximations of all colors.

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