Adjusting Monotone Photoshop Curves for Inkset Control
By Paul Roark
This is written as a tutorial to help people learn to control B&W inksets by adjusting simple grayscale curves in Photoshop or Picture Window. I assume PS CS. I use the UT-FS inkset as an example, but the basic approach is broader. The traditional quad partitioning curves as well as the variable-tone inkset curves are mentioned and compared.
Because I think the monotone curves may also be useful as overlay curves for fine tuning variable-tone curves sets (in layers), I may migrate to a variable-tone layers system that uses a curves set with grayscale overlay included for purposes of matching the print density to the monitor or other reference.
Photoshop Image Adjustment Curves
While controlling an inkset using only the Epson OEM print driver is easy and convenient, more control is possible with Photoshop’s image adjustment curves. To see how this can work, open a grayscale file and navigate to Image>Adjust>Curves. A curve panel will appear.
Curves are used to edit the distribution of information in an image file. They change the relationship between input and output values. With grayscale images this typically includes changes in density (how light or dark the image appears overall) and contrast (the relative difference between tonal values in the image). We can also use curves to control how image information is sent to the individual inks in a printer.
The horizontal axis in the curve panel is the input value of the file; the vertical axis is output value. The default 45-degree line indicates the default relationship each input value has to a corresponding output value. The relationship between values is linear until an edit is made.
Click on the center of the curve and the Input/Output values appear. In the example, these show an input of 127 and an output of 127, corresponding to the edit point. However, if the curve is moved up the output changes to 191 and the file becomes much lighter. All values that were 127 (50%) become 191 (25%), and, importantly, the program smoothes the curve between points. The end points have not changed (black is 0, 0 and white is 255, 255) but the new midpoint position has altered the other input and output relationships along the curve.
These curves are very useful tools for controlling quadtone inksets. One potential problem for some photographers is the high cost of Photoshop. It is, however, the industry standard and most serious printers will want to take the plunge at some point. Fortunately, Digital Light and Color has an alternative image editor for PC’s called Picture Window that is compatible with Photoshop curves. Additionally, Photoshop Elements includes layers and there is a procedure for using curves on layers. These alternatives make it possible to use curves to control inks with or without the full Photoshop program.
Monotone Inkset Curves:
How Photoshop curves are used varies depending on whether the quadtone ink set is monotone or variable-tone. With monotone ink sets, like MIS’s UT-FS (warm) and UT-FSN (neutral), the tone (“colorcast” or “hue”) of the image is fixed by the ink and paper combination. The curves will not change the print tone but will have a major impact on the density and contrast of the print.
Even if the inkset is monotone, however, there are differences between the older quad (4-ink) and newer hextone (6-ink) printers. Due to the relative ease of adjusting monotone inksets on modern hextone printers, they are a good platform for learning about curves. However, because many photographers are still using 4 ink printers I’ll first look at what is required to use them.
Traditional Quadtone RGB Curves
(Some may want to skip to the next section.)
Quadtone ink sets allow us to achieve smooth grayscale prints by combining black ink with three gray inks. Older four ink printers, like the Epson 3000, produce large, non-variable ink dots. Additionally, they lack the light color ink positions (light cyan and light magenta) found in modern hextone printers. The early PiezographyBW and MIS FS quadtone inks used the following ink order: C- dark gray; M- medium gray; Y- very light gray. Hextone printers still use these ink designations but the inks are installed at different positions in the printer.
Ideally, one would like to use CMYK curves to fully control a printer; each ink position would have a specific curve to control its output. However, the Epson driver only accepts RGB image files. If a set of CMYK curves were used the print driver first converts the data to RGB and then converts it again to CMYK. Control is lost in the translation.
To print an older quadtone inkset with curves, the grayscale image file is first converted to RGB (Image>Mode>RGB). This approach is also used for the newer variable-tone ink sets and for glossy papers with monotone inksets. In practice, RGB curves do a good job of controlling the CMY inks. The RGB curves control their complementary colors:
There is no direct black ink control with this approach. However, the print driver limits the color inks and will substitute black ink when it sees the color inks approach the black point. This allows for indirect control of the black ink through the curve authoring process.
The RGB curves are used to partition the quadtone inks; they instruct the printer to use the light inks – and only the light inks – for the light tones, the medium gray inks for the medium midtones, and the dark gray inks for the darker midtones.
RGB curves set for Epson 3000 and a traditional quadtone inkset
To get the black ink to turn on all curves must converge to the black point. The light and medium gray curves start to resemble roller coasters. Producing smooth “crossovers” where the curves overlap, to achieve smooth print density transitions, can be tricky.
Authoring these curves is complex and many users do not learn how to make or modify them. This limits their appeal considerably.
Grayscale Curves for Hextone Printers
With modern hextone printers monotone ink sets are much easier to control and use. Between their smaller, variable dots, light color ink positions, and automatic driver crossovers, the complex RGB quadtone curves are not needed. A simple grayscale curve will do the job. This is an approach that most will find easy to deal with.
UT-FS curve for 1280 and EEM
The monotone MIS UT-FS and UT-FSN ink sets I’m supporting for the Epson 2000P, 1270, 1280, and 1290 hextone printers use the traditional ink densities, but in a different order than the traditional quadtone printers. The hextone ink order I use is referred to as “CMCMY.” The letters refer to the original UT-FS quadtone ink densities, but they are loaded into the as follows: the dark gray C ink is put in both the Cyan and Magenta positions; the medium gray M goes into both the Light Cyan and Light Magenta positions; and the lightest gray Y stays in the Yellow position.
With modern printers the ink dot size is so small that even medium gray ink will print essentially dot-less. The light and medium inks can be used in the highlights. With the dark and medium gray inks in positions where the driver automatically partitions them, the print driver handles all the crossovers.
Relatively minor adjustments through a grayscale curve are still needed to fine-tune the output densities. Because this is the easiest curve to make for controlling ink sets, it is a good place to start.
21-Step Test Files and Target Densities
Before getting into the details of how a curve is built one needs to understand the test files and target densities that are indicated on them.
I use 21-step test files to make curves. These files are sometimes referred to as “step wedges” and can be downloaded from my website. (See http://home1.gte.net/res09aij/Test-files.htm) The test strips will have a set of increasingly dense steps in 5% increments from 0% (white) to 100% (black). I also label the steps with the equivalent 0 – 255, 8-bit luminance code values. On this scale, 255 represents white and 0 is black.
A typical 21-step test file.
Papers print with different maximum black densities. As such, the target densities vary depending on what paper is used. I’ve made a series of test files that have different target densities for the papers I work with. When printed the 21 density steps can be read with a spectrophotometer or visually judged to see if they are evenly spaced.
There is some disagreement among black/white printers as to what the density distribution of target files should be. However, most rely on the work done by the CIE (the Commission Internationale d’Eclairage). This international body did extensive research in the 1930’s on how the eye responds to color and different light values.
Among other things, the L*a*b* color space came out of this research. The “L” stands for luminance. CIE defines a Luminance scale from absolute black, L = 0, to totally white, L = 100. The spacing of the scale is based on how the average human eye sees apparently equal spaced differences in luminance. The CIE luminance scale accounts for the fact that our eyes see logarithmically, not linearly. We can more easily distinguish between light tonalities than dark, for example.
If a graph were drawn of what we perceive as evenly spaced densities it would not be a straight line. It would be a straight line if the CIE Luminance values are used because the nonlinearity of human vision is built into the Luminance scale. When one speaks of “linearizing” a printer’s output it should mean that the output densities match the relevant CIE Luminance values. This is not always the case.
To put this in more concrete terms, the typical output from a number of Epson printers using MIS FS inks on Epson Enhanced Matte paper ranges from a paper white density of 0.03 to a Dmax of 1.68. This can be seen on the 21-step test strip for a paper with Dmax of 1.68 that can be downloaded from my website.
These density values cannot, however, be simply divided equally into a series of steps that will appear equally spaced. For this, we use the CIE Luminance values. The EEM paper white density of 0.03 equals Luminance 97. The Dmax of 1.68 equals Luminance 16. If one divides this range in half, the middle is Luminance value 56.5, which equals density 0.61. That is where the 50% patch is often placed. The other steps basically follow the Lab luminance spacing.
(Note that the 50% density of this approach does not match the 50% density of the Lab space in absolute terms, and it arguably should. The Lab L = 50 translates into density 0.73. The Kodak 50% gray card does match Lab L = 50. The 50% = 0.61 density is a historical standard I’ve been following, but it is arguable not the best, and I may change my target. )
I currently deviate from the “linearization” approach that places the 50% density “half” way between the dmax (balck) and dmin (paper white) because I want to print on any paper with the same file and have the prints look like they are the same density and contrast. This seems to work best if the same 50% point is used for all papers. No matter what the Dmax and paper white are, I have stayed with the same 50% density (as would a system that put 50% at Lab L = 50). Many RIPs do not use a standard 50% density and files may need to be altered for each paper type used.
Making a Grayscale Control Curve for a Monotone Inkset and Hextone Printer
The basic process involves printing 21-step test strips, evaluating the print densities of the steps, making adjustments to the curve used for the test (or a new one if no curve was used for the first print), saving the new curve, and printing a new test strip with this curve. After several iterations the test print will have density steps that are close enough to the target densities to be called a final curve.
Keeping track of what printer and paper the curve is for can be accomplished in the name of the curve. The exact settings also need to be noted. I sometimes put this information in the name of the curve as a reminder. It is also important that the test file has not been previously altered by a curve or profile. Save the files before applying a curve that alters their values.
21-Step test file as the top layer in an 8x10 image
Grayscale curve for 1280, UT-FS, and EEM
Advanced box for 1280; settings for UT-FS ink on EEM paper.
a. For the new test strip open curves and load the curve used to print the test strip.
b. Highlight the first point. This will show 12 in the Input box. This is 95% black.
c. Adjust the first point up or down as needed.
(1) Click on the Output box.
(2) To make the step darker, use the arrow keys to move the point down.
(3) To make the step lighter, use the arrow keeps to move the point up.
We can estimate how far to move a point by looking at how the last curve printed. For example, the 50% patch should have printed with a density value of 0.61. However, if it turns out that, for example, the 60% patch printed with the density of 0.61, then we know that the output value that the previous curve used for 60% is what we should use for 50%.
(4) Note on the test strip how far each point is moved up or down. In the next iteration see how much difference that move made. That will indicate how much of a move is needed next time.
d. Repeat the above steps for each point.
e. Add a new point if the test strip is off on a step for which there is not an existing point.
(1) Highlight an end point – (0, 0) or (255, 255) – and hit the Delete key.
(2) Place the cursor along the graph line until the Input box shows the step number that you want to adjust.
f. Save the new curves file with a new name. I click on the name of the existing previous version and add a new number to the end of the name.
Glossy Paper Curves
For glossy papers there are two options with the UT-FS or UT-FSN monotone inksets. First, Photo black can be loaded into the printer and simple grayscale curves will control the system as described above. However, the matte black can be left in the K spot for printing on both matte and glossy papers. Printing on glossy paper with matte black (MIS Eboni) works because the two dark gray inks used together can produce a very good black Dmax. To do this, the matte black must not be used on the glossy paper.
Keeping the matte black ink from printing on glossy paper is accomplished by converting the file mode to RGB and using an RGB curves set. The lightest gray ink in the Y position is controlled by the Blue curve. This curve is pulled down a bit at first, but then is pushed back up to the top of the graph, thus turning off the yellow ink and preventing the printer from using any black ink. The printer only uses black ink when it sees that all the “colors” are being used and the densities are very high. With the Y channel turned off at the 100% point, the black ink is turned off as well. The combined RGB curve can contain the adjustments just like the single grayscale curve would for matte papers. Making adjustments to a curve for glossy paper when Eboni matte black ink is installed is just like for the matter papers and outlined above. The Red and Green curves can be left in their default positions.
Curves for the 1280, Epson Premium Semigloss, and MIS UT-FS inks.
The original MIS VM ink set was created for quadtone printers using two gray inks and one bluish toner ink to cool the print tone. The system worked quite well but some photographers could see dots in the highlights of a print when viewed very close up.
More problematic was the fact that most users seemed unable to adjust the curves themselves to profile new papers or to adjust for different ink batches or printer variances. Additionally the inks were light and tended to flood glossy papers, and the range of the MIS VM ink set was limited to a medium warm to cold print tone. The next generation of variable tone ink sets attempted to cure these shortcomings.
The UT2 inkset for the Epson 1280 was designed to print with the OEM print driver’s slider controls. Photographers uncomfortable with the use of curves, did not have Photoshop, or wanted to print from another application could use the inkset because control was in the print driver, not in Photoshop curves.
The Epson driver sliders can control the UT2 inkset.
Using curves, however, gives more control.
Because hextone printers are much better than the older quadtone machines, the very light inks were not needed to achieve smooth highlights. The lack of the lightest inks helped printing on glossy papers and allowed an ink position (Y) to be used for a sepia toner.
The 1280 was designed for light and dark cyan and magenta inks; light and dark pure carbon, (warm gray inks) are put in the magenta spots, and carbon pigments that are toned cold with blue pigments are put in the cyan spots. The printer driver handles the crossovers between the light and dark gray inks.
While sliders can control the tone of the print somewhat, RGB curves give the most control. With the driver handling the light-dark ink cross-overs, the curves are easier to deal with than the old quadtone monotone curves. Nevertheless, they are more complex than the monotone UT-FS grayscale curves.
With the UT2 ink set the warm inks are controlled with a single Green curve. To print with just carbon the curve is much like a grayscale monotone curve; until it is time to turn on the black ink. Then the other two curves must be pulled steeply down to the black point at the bottom left of the graph.
For neutral prints the cool gray inks use a second Red curve. The control of how cool or warm an input value prints is a function of the relative amounts of cyan and magenta position inks are put into the image. So, the relative positions of the Red and Green curves control the tone of the prints if tones cooler than pure carbon are used. Loading a UT2 neutral curve and looking at the Red and Green curves will help.
UT2 curves for EEM and neutral tone.
The Red and Green curves for UT2 generally have points at 25%, 50%, and 75%. No intermediate mid-tone points are needed; sometimes no shadow points are needed at all. However, most will have points at the usual 5% steps from 75% to 100%. Additionally, the Blue curve (controlling the yellow position sepia toner) must be dropped steeply to turn on the matte black ink.
To make a curve cooler or warmer the Red and Green curves can be edited in an offsetting manner. While some fine-tuning may be needed, starting with offsetting moves usually produces a reasonably good tonal ramp. For example, to make the highlights cooler the Red curve at the 25% (191 input value) point would be moved down, say 15 units, and the Green curve would be moved up that amount.
The sepia curves look a lot like the carbon ones but use the Blue curve.
While PK can be used with UT2 for glossy paper printing, I don’t recommend it unless the ink set needs to be printed using only the sliders. This might be the case if a non-Photoshop application is used. As with the monotone ink sets the two dark gray inks, in the C and M spots, produce a good black Dmax. This allows the ink set to print on glossy and matte papers without having to change inks. To generate the black with the two gray inks the Blue curve is pushed up to the top at 100% to keep the black ink turned off and the Red and Green curves are pulled all the way down.
The general procedure for modifying curves is the same as with the grayscale inksets, except that the tone of the test patch also has to be evaluated and adjusted. As such, the process is more involved, and a spectrophotometer is much more important for doing a good job.
Alternatives to the Sepia Toner
With the UT2 and UT7 ink sets, the sepia toner isn’t required unless one wants to print with sepia. If one only prints matte prints in the range from carbon warm to cool a second, light carbon ink (UT2 LM) can be loaded into the yellow position. This makes the range of prints that can be made with the sliders slightly cooler. It also makes slider prints slightly more lightfast, since the sepia toner is not as lightfast as the pure carbon ink.
Another option is putting Gloss Optimizer (“glop”) in the yellow position. For those who do not print sepia and do print glossy prints, this is what I now recommend. With glop the bronzing that is usually associated with glossy paper and pigment inks can be largely eliminated. The worst papers will still bronze to a small degree but the best papers can be virtually bronze free.
The Blue curve becomes the glop curve. To eliminate bronzing, the Blue curve is pulled down in proportion to the bronzing seen on the 21-step test print. It is strictly a visual adjustment. None should be put in the 100% black spot. The Eboni still needs to be turned off by having the blue curve all the way up to 255 at that point. Also, in the white spaces, there is little reason to put any. If there is a gloss differential there, glop can be used to help this. It does have some density, however, so pulling the curve down to no more than 230 of 255 is recommended. UT2 glop curves can be downloaded from my web page or MIS.
UT2-Glop neutral curve for glossy paper.
16 bit Grayscale Files
I recommend scanning and editing in 16-bit grayscale mode. However, even with 8-bit files, the print will be smoother if the file is converted to 16 bit before the curve is applied. This is also true if the file is then converted back to 8-bit after the curves are applied.
When starting with a color original, even if it is edited in color, the image must be converted to grayscale. Any residual color information will throw off the RGB curves.
When using curves the settings in Photoshop and the OEM print driver are critical. I begin with Color Settings (Photoshop>Color Settings). Select Abode RGB (1998) for the RGB working space and Gray Gamma 2.2 for the Gray working space.
The exact combinations of print driver controls will change with the ink set used, but for working with curves you’ll always select No Color Adjustment in the print driver’s Color Management panel. This deactivates the sliders, allowing the inks to print at full strength. This makes the curves cross-platform; they will work with Windows and Mac.
Once the grayscale image file has been edited and is ready for printing, the final should be saved. Then the file in converted to RGB mode. Then a printing curve is applied. There are different curves for different papers and tones. They can be downloaded from my webpage or that or MIS Associates.
The Photoshop image adjustment curves act like profiles but they are accessible, easily modified, and can be used without expensive software or hardware. When RGB curves are applied the image on the display becomes brightly colored. If it doesn’t, the image mode is wrong. Check the file and convert to RGB if necessary. These colors simply tell the printer how much of each ink to put onto the paper.
The printer settings are critical. The combinations you should use are listed by ink set and paper on my webpage. Follow these recommendations carefully. It is also important that no other profile is tagged to the image file. In Photoshop CS use Print with Preview so that all of the settings can be seen. In general, the following driver settings are used for the UT2 inkset:
· Print Space – Same as Source
· Media Type – "Photo Paper" works well for most, but this can vary with paper type.
· Print Quality – 1440, High Speed unchecked works well for most prints; 2880 gives marginally better quality but is slow. For glossy papers some might see the difference.
· When Glop is used, 2880 is recommended.
· Color Management – “No Color Adjustment” is always used. Because this setting is used, the curves should work equally well with both Windows and Mac computers.
Once the image is printed close the false-color file without saving it. Avoid saving this file over your Master grayscale file.
Profiling the Display to Match the Print
Most photographers doing serious color printing prefer to use profiles that calibrate their displays and printers to achieve consistent results. These profiling systems, however, are not made to work with grayscale files and ink sets. (Or, vice versa – the traditional grayscale approaches have not been made to be consistent with the standard monitor calibration systems. One could argue this has been the real problem, and I may alter the approach outlined here.)
Photographers who are accustomed to the sophisticated color display profiling systems will often say that, at a minimum, one must have a display calibrated with special software using a sensor. I have one of those systems, and I’ve compared it to using Adobe Gamma, which is a simple visual system included with all Mac and many Windows computer systems. My conclusion is that the free visual systems can be quite adequate for B&W monitor profiling. My workflow does not attempt to match the tones (hues) of the prints. I approach B&W as being a system that does not worry about this in the context of working up a print. It is a separate decision that is based on actual print, hard copy viewing in the context of their display conditions.
To obtain a better match between my monitor and prints, I currently adjust the monitor view. (But, as noted before, one could also decide to match the print to a standardized monitor view.)
To adjust the monitor image to match the print, I use the Photoshop preview feature/procedure described below. This procedure uses the Edit>Color settings to make a custom dot gain curve, and saves the curve in a manner that it can be used in a View, Proof Setup preview system that does not alter the working space and affect the RGB values when the image is converted to RGB for printing. Here is the procedure I use:
You can also record a Photoshop Action so that the preview/display profiling steps become a simple, single-keystroke function (for example, function key 2 on a PC can be designated to apply the preview). To record an Action click the Actions pallet tab. Then click the upper right arrow to start recording. Select the function key and click on View>Proof Setup, etc. as above, then click the Actions pallet upper right arrow and stop recording.
That’s it for now.
Enjoy the journey.