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Moving from Density to Spectral Displays in Color Management

Published on December 6, 2013

Dr. Danny Rich, Senior Color Physicist at Sun Chemical, speaks with Kevin Karstedt about how the X-Rite eXact Portable Spectrophotometer.

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By Erik Nikkanen on Dec 06, 2013

One should understand that there is a difference between colour management and process control.

Density is the better parameter to use for process control on offset lithographic presses because it is related to the printed ink film. A density value on press can be reached.

Trying to use Lab values to control the process on an offset press is problematic since there are no control mechanism to use to get the desired Lab values. In the case of density there is, which is the adjustment of ink feed via the ink keys etc..

Even now, one can not hit the target Lab values of a solid print of an ink on press but one can hit the density values.

As I am sure Dr Rich knows, one can not go from just density values to spectral curves and therefore Lab values without knowing beforehand what the spectral curves were that resulted in the density values. The video might give the impression that it is a simple matter but it is a bit more complicated. And I am not sure why one would even want to go back and convert old density values anyhow.

From time to time there has been an effort to go from density to colour as a control parameter on press with the implication that it is a step to more scientific method. I would disagree. For process control, density is the better parameter and therefore scientifically the better method.

Of course, colour is the target outcome but colour is a result of consistent ink films, printed with consistent inks, on consistent paper and with separations that result in predictable colour.

It will be interesting to see if this latest effort to go to spectral values will provide value to the printer and its customer.


By Greg Imhoff on Dec 09, 2013

Erik, you are correct in that press operators can only control Ink Film Thickness or Density on press. We agree there are not XYZ spectral ink (or LAB) keys to move on press. This may be one goal of Xact or X-Rite in converting Densitometric legacy values into Spectral color values. In practice this may appear a bit bit "bass ackwards" as Density values were designed and derived from Spectral, to be an affordable tool for operators to consistently establish color on press. Ftr converting Spectral into Density has always been a math formula at the maximum (Nanometer Bandwidth) light absorption points. Enter Moore's Law.

Spectral is better than Density in defining color I know having led Gretag Color Control sales (now a part of X-rite) when we introduced to print portable spectrophotometery. So technology advances are always useful and we all need to be open yet I too would need to see exact value conversions working for operators.

With Sun Chemical endorsing this may well have deep commercial value so I would be interested to see who pays for the new technology, the average conversion costs and ROI or press room payoffs from now working with Spectral values.

Spectral data is no doubt better for color matching but how does this convert realistically with operators on press, or in legacy job values? What is the goal exactly here?


By Gordon Pritchard on Dec 09, 2013

The notion of the press operator moving from density to color as a way to control presswork suggest a lack of fundamental understanding of offset lithographic printing.

Although color may be the result of the printing process - the presses themselves are not engineered to make color and never have been. Instead the are designed to lay down a film of emulsified ink (of any hue) onto a substrate (also of any hue). The tolerance of ink film thickness is quite narrow. If it is exceeded - the lithographic process fails. That is why press operators measure density. It is a practical, albeit indirect, method of determining ink film thickness.

It's unfortunate that there doesn't seem to be any press operators involved in the development of standards and specifications for the print industry. Their voice is sadly missing from the activities of these groups whose deliberations will impact the pressroom.

What is really needed is more emphasis on the engineering of presses to make them more effective and accurate at laying down films of ink. We also need ink and paper manufacturers to be more involved in developing standards, specifications, and products that are better suited to the various print processes.


By Greg Imhoff on Dec 09, 2013

We agree Gordo as I stated there are no LAB ink keys on presses.

Standards committees would welcome press operators which in the past attended or at least made their general preferences known. Then we began the great "industry shrinkage" and this luxury dissipated...

I do not think Sun Chemical or X-Rite are fundamentally lacking print knowledge and this is one reason why I suggested there be more exact information shared here on end user applications, costs, intended uses, etc.

Moore's Law also relates to engineering in all press processes.


By Erik Nikkanen on Dec 09, 2013

I would not say Moore's Law applies to printing in any way but that is another discussion. Moore's Law was pretty specific to the increase of transistors on chips over time.

I would say that the industry is basically clueless in understanding its own processes at the fundamental level that is needed in order to make significant advances. This includes Sun, X-Rite, press manufacturers, standards groups and graphic arts institutions.

This does not mean that they know nothing but only that they don't know enough to be able to put together a rational agenda for what is required to move the process forward. Because they don't understand some of the simplest rules that govern the process, they can not innovate suitable solutions.

The scary part is that they don't really want to know and have no curiosity about finding out. This is an historical and cultural problem and it will not be solved.

I think what can happen is that some companies, who want to understand and put some effort into getting that knowledge, can develop technologies that will work well and make money for them. The rest of the industry can just follow as they have done for so long before.

But that is only my personal opinion and only time will tell if that is true.


By Guy DuHamel on Dec 09, 2013

This is a very positive move and one that all ink providers should jump on. It has long been accepted that if you control the ink film thickness on a press, you effectively control the press. Control of the ink film thickness allows an ink manufacturer to have input into ink water balance and...within reason, the emulsification of fountain solution into the ink.This optimum ink film thickness must be defined on press and is not arbitrary. By defining colour in spectral terms at press side we gain input back up the chain so that decisions can be made at ink manufacturing point to allow a specific target density to by achieved while still satisfying the more complex requirements of spectral analysis. This is a win win, but I maintain that a press operator can still be "running to the numbers" on his densitometer providing the back work has been done to insure that his target densities relate to l*a*b* definition of the colour to be reproduced.


By Gordon Pritchard on Dec 10, 2013

Guy, IMHO, you are correct...however thiat is not the direction the standards and specifications folks are taking (according to the good doctor).


By Erik Nikkanen on Dec 10, 2013

If it has been long accepted that the control of ink film thickness on press is critically important then why is it that the industry is clueless about how to do this? The reason is that the industry does not understand the problem.

Measuring density and then having an operator or closed loop control to adjust the press is not controlling the process but is an action for making corrections to a process that is not under control. After 100 years, the industry is still thinking in terms of measure and adjust to cope with problems instead of correcting the fundamental problems and designing them out of the process.

"Wrong thinking" is what exists with most of the efforts to deal with colour control problems in this industry. Simple problems are made complicated by wrong thinking about what is required.

There is a need for some kind of leadership in the industry. My view is that it needs to start from the press manufacturers, where it can be demonstrated that consistency and predictability of density control can be obtained at low cost using the "right thinking" methods.

After that, colour management can be addressed more easily when applied to a very stable process. More direct, simpler and lower cost methods to obtain consistent, predictable and repeatable colour can be obtained that do not require an army of consultants.


By Larry Goldberg on Dec 10, 2013

Erik is STILL correct after all these years about closed-loop control not "solving" the problem of unstable inking systems.

Greg is still making fruit salad of the apples and oranges of more laws, Moore's Law, and language in general.

Gordon has the luxury being an armchair explorer, having retired from the trenches, but still trying to "move the graphics arts towards the graphics sciences", for which the industry is, and should be grateful.

With the lack of consistent training in the industry, machine operators are being asked to take greater responsibility for processes that they may have little understanding of.

Prepress is better than ever and some very clever people are developing excelllent tools to create separations that should print correctly when the following conditions are met:
- proper ink with regard to hue, strength, etc.
- proper substrate with regard to how the job was proofed and the anticipated stock, etc.

THEN, the operator should be able to achieve the desired result by.... running to density and verifying that dot gain is within bounds.

Guy is correct about "we gain input back up the chain ..." but that's not production. That's diagnostics, or a post mortem if the job has already failed.

The old X-Rite "BEST MATCH" function in the 500 series was good at determining how close you could come to the target color by adjusting density only, the only tool in the pressman's tool box.

Research and deeper understanding is great, but not at press side while the clock is running.


By Erik Nikkanen on Dec 10, 2013

Larry, good to hear from you again.

Yes, prepress is better now than ever IF the inks are the same, the paper is the same, the screen set is the same, etc. for everyone.

This is true but it is also the main and critical flaw in thinking. It means that this colour management approach only works to obtain accurate colour when the process converges to having the same process conditions. It is not a general solution. It would be great if everyone had all the same equipment and raw materials and was willing to only print in the same limited gamut, but that is not what is going to happen.

For a general solution one should have the ability to use any ink sets, any paper and any screen set and have a practical and straight forward way to obtain target colours in an image if they are obtainable.

The misuse of standards has worked against the goal of having capable systems that can guide the printer so that they can produce stunning effects that are consistent and predictable and easy to manage.

The job of the printer is not to print to a standard but to predictably print awesome images.

Also I think Gordon is still stuck in some trenches from time to time. :-)


By Gordon Pritchard on Dec 10, 2013

It's a pity that the good Dr isn't contributing his thoughts on these comments.


By Greg Imhoff on Dec 10, 2013

Spectral values are calculated electronically so the instrument optics driving this may also fit under Moore's Law ie: ..."The capabilities of many digital electronic devices are strongly linked to Moore's law: processing speed, memory capacity, sensors and even the number and size of pixels in digital cameras.[5] All of these are improving at (roughly) exponential rates as well (see Other formulations and similar laws). "...

Best Match http://www.xrite.com/xrite-exact was likewise originally popularized in the Gretag SPM Series. Basically this spectro feature analyzes any Print Color in Delta E, to the goal and gives to the operator Density move suggestions for "best color matching" to the job goal under the press conditions.

In August this link http://whattheythink.com/video/65020-patrice-aurenty-sun-chemical-announces-their-endorsement-x-rite-exact/ essentially states that Sun Chemical endorses eXact to Thousands of Packaging Printers globally.

In the end this means the ink maker, instrument maker, printers and packaging clients will all work from the same platform for best color matching communications.

I agree with Gordo that Sun may need to be more specific. The question simply put may be who pays for the newly endorsed technology?


By Mitch Bogart on Dec 11, 2013

As director of R&D for a ripping system, Rampage, I'd like to add an additional perspective to this fascinating, and rather classic kind of discussion, if I may.

One of the features of a prepress system is to handle newly defined spot colors, either keeping them separate, or simulating them with process inks. These new colors are defined as either CMYK percentages or Lab values. The former can be converted to density, via the Murray-Davies equations, and the latter relate to perception and readily produced from spectral data.

How would one measure colors of an object or photograph and tell a rip or design program to print with them? If the desire was to define new spot ink channels for each color then ok, spectral and Lab values would be fine. But suppose one wanted to just set some objects' colors to that. It sure would be great to get the measured colors as CMYK percentages and design or paint with them.

This eXact technology would allow that. A printer knowing the densities of his printed inks could take the densities from the eXact and put the two densities into the M-D equations and produce dot area or cmyk percentages and tell the designer. The printer or designer shouldn't do that of course, but either the design program or the spectro device could, should, or perhaps even already does, do it.

As artisans know, it's not totally that simple. There is considerable and varied dot gain, or dot area gain from Pdf values to press, and so the converting entity should also have a place specifying assumed dot gain.

I see this therefore as a bridge or gateway product between two worlds. Perhaps as more bold individuals try out the bridge and share their experience, the bridge will be become well-traveled, saving everybody time, which as we all know is money.


By Erik Nikkanen on Dec 11, 2013

Mitch, it is great to have someone comment from a development group that works on ripping systems.

I do think there is potential in this area. My view is that density and dot gain are not directly related to colour and also the offset lithographic printing process is non linear and non independent with respect to the CMYK channels and therefore should not be involved with any method to determine the separations.

The use of tone curves and similar methods are what cause a lack of predictability of colour in a ripping system. So any effort to work in that direction will be problematic.

I believe the solution is fairly simple. Use LUT but design their use to take advantage of modern computer and memory capabilities and develop suitable colour measuring technology potential. What is required is the need to measure easily thousands of patches or small spots on a form and algorithms to interpolate between measured values to map the total potential combinations of what the press can print for a given paper, ink set, screen, density target combination.

This kind of approach can be easily automated, does not require experts or adjustments, because the LUT would tell you what the print will be.

People might think that with all the dot gain compensation and G7 efforts that some how the press prints differently, but basically all most of that does is just change the stated value of the apparent dot size from one value to another in the data base. The actual dots on the plate will print the same no matter what they are called in the data base.

I don't know the details of developing a ripping system but I do know that if one uses methods that do not have a mathematical connection to colour, they will not work well. Dots and tone curves have not direct connection to colour. That is why they fail.


By Danny Rich on Dec 20, 2013

So I participated in the interview but I do not normally visit the video site. However, I was contacted by marketing to say that there were a large number of posts here. At least one asked if I would add something. Actually, I think that Guy and Mitch have been very eloquent in their reponses.

Greg - I am surprised that you did not mention the Colorimetry section of the latest IDEAlliance Guide to Print Production which you, I and Ray Cheydleur wrote as it addresses many of the issues raised by the posts. Also Ray and I did a webinar which can be found in the IDEAlliance webinar library, titled Press Performance Metrics: Colorimetry and Best Practices.

I do agree with Eric that the industry as a whole does not tend to understand the technology underpinning graphic reproduction. There is a comment that dots and tone curves do not have a direct connection to color but we compute tone value and tone value increase from the Murray-Davies equation and that equation is derived from the work of Neugebauer whose equations directly relate the dot area to the colorimetric tristimulus values.

The real distinction lies in the history of press control measurements. Spectral instruments were very slow, large and expensive in the early parts of the 20th century but instruments based on combination of Wratten gel filters were much more affordable and usable. Modern instruments, like the eXact offer the best of both metrologies.


By Greg Imhoff on Dec 20, 2013

Danny you are right and here is the article http://www.ipabulletin.com/?xml=IPA_Bulletin&iid=59553&startpage=20&crd=0&searchKey=colorimetry In later IDEAlliance conference calls Print Buyers clearly stated they found reading this a bit too technical. Of course this may not be so to this crowd so here it is for those interested.

Danny I admire your work and my posts were to support this which is why I added in Patrice Aurenty's earlier Video link:>>
"In August this link http://whattheythink.com/video/65020-patrice-aurenty-sun-chemical-announces-their-endorsement-x-rite-exact/ essentially states that Sun Chemical endorses eXact to Thousands of Packaging Printers globally."

And explained a bit more in layman's terms, for example :>>
"Best Match http://www.xrite.com/xrite-exact was likewise originally popularized in the Gretag SPM Series. Basically this spectro feature analyzes any Print Color in Delta E, to the goal and gives to the operator Density move suggestions for "best color matching" to the job goal under the press conditions."

eXact does what you say Danny (best of both methodologies) and most know that new ways may sometimes be difficult to explain at least to everyones individual satisfaction.


By Mitch Bogart on Dec 20, 2013

This is an excellent discussion as essentially no one seriously disagrees.

Many features enabled in a modern rip system are enbaled and only possible with correct color science backing it up. The device color space for printers is almost always cmyk, with the exciting exception of the LumeJet, excitingly reviewed here by our elder statesman of print, Frank Romano.

When a user specifies an addition spot color, not ot be converted to process, proper proofing before imaging requires a colrimetric definition of that spot color, preferably Lab, but with cmyk common, but prone to inaccuracy because of differences in assumed dot gain, and ink sets.

Also, when one wishes to convert cmyk data meant for one printer and conditions to a totally different one, the rip system can do it if both printers have ICC profiles.

This being said, one has to acknowledge that there is a plethora of artisans out there who think and adjust "color" in cmyp space. They amazingly just know what values for good skin tones should be. I just hope not too much of their subliminal brain power is used up for this internal CMM!

In any case this is what Dr. Rich, Guy, and the eXact system are acknowledging and appreciating - the use of layman's terms.

How great that the colorimetric "DeltaE" has migrated to become one of them!


By Greg Imhoff on Dec 20, 2013

Thank you Mitch for pointing out this as an excellent discussion. The goal is to simplify and I neglected to add in one more sentence from my Dec 10 posting:>>

"In the end this means the ink maker, instrument maker, printers and packaging clients will all work from the same platform for best color matching communications. "

Where needed or used this may save some brain power and simplify color for all.


By Erik Nikkanen on Dec 20, 2013

If Danny wants to imply that there is a direct connection between density or the M-D dot area to colour, I will let him argue that with other colour scientists. I still say there is NO direct connection between density or the tone curves with colour.

The M-D equation is based on the relationship between the reflectances of the non print area, the dot area represented by the reflectance of the solid and the total reflectance of the screen. The equation is straight forward in describing this. There is no connection to colour.

There are also many issues with the M-D equation where it fails to do what it was hoped to do. The solid density is used to provide the density of the dot but dots are NOT small solids. Dots will have different ink films than solids and different ink films reflect differently. The hue can change with changes in ink film and therefore change the colour.

We all know of optical dot gain and related to this is the fact that light goes through the dot at the edges of the dot with only one pass. One pass through an ink film will result in a different colour than the normal two pass.

It is known, that even if one prints two samples, with the same paper, the same single ink, at the same solid density, but with different screens such as AM and FM, the visual colour will not necessarily be the same for the same tone values.

For these and other reasons, even the Neugebauer model is too simple and fails to be predictive enough.

When I say that there is a lack of understanding in the industry about the requirements to reproduce colour, I am not only thinking of the everyday practitioner but also the experts. The use of overly simple models by experts do not describe what is actually happening. Errors that experts sometimes think are small are actually critical.


By Greg Imhoff on Dec 20, 2013

Yes Erik I agree knowing true dot properties in Optical Dot Gain (or not) are each important to know which Density Dot formula you are calculating. Curves matter too and so on all this the following may help http://www.linkedin.com/profile/view?id=23629550&trk=nav_responsive_tab_profile to visually see how one may adjust curves and read dots accurately if you fully read both (2) two full pages.

Spectral values (Solid or Screen) are still spectral values which is a key distinction in this thread. Spectral values are more "reliable" than just Densitometric values so, when these "2 tools" are each used together, as in "Best Match" then the operator is better off than not.


By Erik Nikkanen on Dec 20, 2013

Greg, people can use curves if they want but in principle, I am not in favour of them because with offset lithography, the CMYK channels in the process are non linear and non independent and therefore curves can NOT solve the problem of reproducing colour with accuracy. It is just my view.


By Greg Imhoff on Dec 20, 2013

Erik people do use Curves usually at the RIP to adjust accurately for their Dot Gains which exist not in a uniform manner, through tonal ranges for example: 10, 20, 30, 40, 50, etc. to 100.

Curve calibration enables printers to have better Press matches to their Proofs and also helps to stabilize the entire process by balancing the (1) RIP - to the (2) Laser - to the (3) Substrate.


By Erik Nikkanen on Dec 20, 2013

Sorry Greg, but I think that is flawed thinking, even if it is used often.


By Greg Imhoff on Dec 20, 2013

Erik, the printers doing the curve adjustment print better more efficiently and profitably than those that do not.

I am not saying it is best and, you may have better ideas...


By Gordon Pritchard on Dec 20, 2013

Erik noted that "It is known, that even if one prints two samples, with the same paper, the same single ink, at the same solid density, but with different screens such as AM and FM, the visual colour will not necessarily be the same for the same tone values."

That is very true - and adjusting curves (tone reproduction) does not bring the color of the two screens closer together. Doesn't that indicate a disconnect between dot gain/TVI and color?


By Greg Imhoff on Dec 21, 2013

Gordo, the statement is true in AM vs. FM because the dot area of FM is totally physically different from AM. Each screening method is totally different in shape and size so the dot gain % #'s, and curves applied or adjustments made are likewise totally different.

If however this statement is applied to the same file being printed either AM or FM (only) then the statement may be true, only if applied to Two (2) different presses. This would be rare but possible, as each press has its own unique print characteristics.

If applied to one press printing with all the same variables the difference if any, will be inconsequential.


By Danny Rich on Dec 21, 2013

As Greg stated, the link between density and color is the spectral reflectance factor. When I want to determine density I read the reflectance curve, apply an ISO Spectral Product function (light source x filter transmittance - Blue for Y, Green for M or Red for C) and integrate across the entire visible spectrum. When I want to determine color I read the same reflectance curve, apply a CIE illumant and CIE standard observer (Z in the blue, Y in the green, X in the red) and integrate the products over the entire visible spectrum. From a spectroscopist's view point there is no distinction. By focusing on the fundamental spectral data we have the ultimate tool for our analysis. The original model envisioned by Neugebauer was very simple - but it has been modified and extended over the years to the point where it is almost good enough to predict real world printing. There is the Yule-Nielson modificcation and the Clapper-Yule modification, both apply math that attempts to quantify the effects of dot edge and hardness. These nonlinear spectral models apply different coefficients for the prediction of the dot area reflectance to be additively mixed with the substrate reflectance to produce the estimate of the measured reflectance. This is exactly what the M-D equation does, it used the reflectance of the substrate, the reflectance of the solid and the reflectance of tone to predict the percentage of the solid ink reflectance is reproduced in the reflectance of the tone. This is pure Neugebauer theory and it has seeming worked adequately for many years. There have been proposals for modified Murray-Davies equations using the Yule-Nielson N-factor or the Clapper-Yule PSF but the additional work required to derive reliable estimates has not been considered worth the effort in terms of the improved accuracy.

So how good are these more complicated models. Well RIT has reported and I have been able to duplicate, that such models can take the colors of the primaries and secondaries and reproduce all 928 colors in the TR001 standard offset IT8 chart to less than 1 CIELAB unit. In the ANSI CGATS documents, process control aims for offset printing have been between 3 and 4 CIELAB units.

I do not attempt to tell people how to run a press, you can use curves or LUTs or models or whatever. But all of those need reliable data and the modern handheld instruments, like the eXact, provide the data required to manage and control your printing better than ever before and using the same measurement data (spectral reflectance factor) you support both density and color and track what is happening on press.


By Erik Nikkanen on Dec 21, 2013

No argument that if one measures the whole reflectance curve, one can determine density and colour. But one can not go from density back to the spectral curve and then determine colour. That seemed to be one claim for old data.

I would argue that the M-D equation does not predict the solid ink reflectance in the tone but that the equation uses the measured solid ink reflection and assumes that the reflectance from the dot is the same, which it is not. The M-D equation results in a value that is apparent dot area. That is its intention.

When so much fiddling is needed with multiple coefficients to try to make an equation workable, it kind of tell one that the approach is flawed and needs some rethinking done. The idea of coefficients has always bothered me. For them to be predictable, one needs to know the result before one can determine the coefficient that will give you that result. Not so good for new, unknown printing conditions.

Maybe things have changed but in the past, the density of an ink or the apparent dot area was determined by a measurement using one of the filters in a traditional densitometer. I am assuming the spectrophotometers now used, duplicate that filter function via calculation. I am also understand that those filters, such as using a green filter for magenta, are different from the Y function and that is why the values from the densitometer can not be used to determine colour. The filter functions for density are not colormetric functions.

The original complaint about using colormetric values at the press instead of densitometric values was specifically based on process control. If XRite can prove it provides benefits to the operation of the press by using colormetric values, then that is great. They have to show it.

As far as control at the press goes, control is a result of capable systems. Even if one applies the best measurement methods to a process that is not capable, there will still lack of control.


By Greg Imhoff on Dec 21, 2013


M-D was intended to calculate (estimate) the effect of Optical Dot Gain or light trapping and casting a shadow under the corners of the Dots.
Y-N was used (pre Dotmeters) to measure physical Dot Area by applying a "N" factor (aka "Nudge" Factor) but the issue today is this is only applied to the 50% and all dots (ie: 10% or 90%) are not relative to a 50%...

Densitometer Filter sets are derived from Spectral (Nanometer) values where maximum Light Absorption actually takes place, in a given CMYK Ink. So the idea was to develop an affordable tool that was handy for press operators where ink film thickness could be measured and managed. = Handheld Densitometers.

All color comes from RGB light so Spectrophotometers capture color in spectral (Nanometer) data which then enables math (Chips on board to now Portable Spectros) for equations to take place to match the CMYK Filter sets used in Densitometery. The Best Match goal of a X-Rite type Spectro with eXact is as a press tool to simplify all these steps for the best color match in Delta E, then converted into Density moves for that press, ink set, substrate, etc.

So by beginning at the beginning (RGB) and ending in (CMYK) values that are used and managed press side, is the key. Think of this as custom process control for the job on press, based in color science but communicated in terms that are usable.


By Erik Nikkanen on Dec 21, 2013

One can not go from RGB to colour values. One needs the whole spectral curve. Colour does not exist in nature. It is not in the light. It is a perception we humans have. Functions applied to the whole visible spectrum were developed to quantify those perceptions. Which is amazing.

M-D was intended to determine dot size and when it was not good for that, the factor was applied to cover the error due to optical dot gain and other issues mentioned earlier. The equation is quite clear about being used to determine dot area.


By Erik Nikkanen on Dec 21, 2013

Also, the press is not a place to manage colour. That should be done in prepress. At the press one should manage the process. One should not be trying to change colour at the press to any great extent.


By Greg Imhoff on Dec 21, 2013

The tool works to simplify color match repro communications. In Additive Color Theory White Light (RGB) is found in the visible spectrum ~ 400nm to 700nm sometimes referred to as XYZ (400 to 700 nm).

In Subtractive Color Theory (CMYK) filters are found in the Nanometer bandwidth areas of maximum light absorption aka complementary filter sets eg: Green for Magenta Inks.

Using a Spectro and Densi in 1 step simplifies the color match process goal scientifically yet with Density values for on press Ink Key moves.

The data is Spectral so it is far more accurate and expressed in Density and so is far more usable. The tool works to simplify color match repro communications.


By Gordon Pritchard on Dec 22, 2013

Greg wrote:
"If applied to one press printing with all the same variables the difference if any, will be inconsequential."
That is wrong, and has been shown to be so in many press tests. If the tone values are the same and the AM and FM printed side by side on the same press at tye same time the color will by very different - very cosequential. If you read carefully what you wrote in the rest of your post I hope hope that the self contradictions will be clear.


By Greg Imhoff on Dec 22, 2013

Gordo you misinterpret my statement where I am separating AM from FM in this thread.

I am not stating at all that by adjusting curves you can match AM to FM At all. The reason for this is due to the differences in AM & FM dot structures and frequencies are each so physically different.

Stated another way my intention was to NOT compare the two (AM and FM) at all. If however one is taking one file type (AM OR FM) only and then applying a curve then results after will be seen.

Applied curves do make a difference. The following was intended to help people understand curve adjustment for AM or FM (alone) is beneficial...

" Gordo, the statement is true in AM vs. FM because the dot area of FM is totally physically different from AM. Each screening method is totally different in shape and size so the dot gain % #'s, and curves applied or adjustments made are likewise totally different.

If however this statement is applied to the same file being printed either AM or FM (only) then the statement may be true, only if applied to Two (2) different presses. This would be rare but possible, as each press has its own unique print characteristics.

If applied to one press printing with all the same variables the difference if any, will be inconsequential."


By Mitch Bogart on Jan 29, 2019

Even though it's been 5 years, this is still one of my favorite discussion threads. It left us on AM & FM color significance. My question is this:

How close in perceived color will these two jobs on the same press, with same inks, environment...be?

A - AM screened with ink curves linearized and a 20% dot gain curve added.
B - FM screened with ink curves linearized and a 20% dot gain curve added.

Both screenings are not extreme, i.e. required compensation for linearity was not excessive.

The Neugebauer equations are independent of screening. As Wiki says,
"The equations estimate the reflectance (in CIE XYZ ...) as a function of the reflectance of .. the 16 combinations of CMYK inks weighted by the area they take up on the paper"

Put another way, do two patches having exactly the same measured CMYK dot areas, but very different screenings, for the most part have nearly the same perceived color?


By Gordon Pritchard on Jan 29, 2019

1. I don't know what you mean by "ink curves linearized" If you mean linearizing the plates - then that is not needed. What matters is the tone value on the press sheet not the tone value on the plate.

2. The impact on color is not because of how the dots are arranged (i.e. AM or FM). It is because of the size of the dots making the tone on the press sheet. A 350 lpi AM screen will show a similar effect.

3. If the tone value of the patches on the press sheet is the same - i.e. a 50% tone request in file results in a measured 68% tone on the press sheet for both screens then the single color and two color patches will exhibit different hues. Primarily in the tone range from about 25%-75%.

4. The difference in color can be measured and is not subtle. Perceiving the difference in color is a different matter since that's based on human perception which has a number obstacles in making color comparisons. If the AM and FM patches are placed side by side then the perceived difference will be very obvious.


By Gordon Pritchard on Jan 30, 2019

You can see the gamut difference mentioned in my post here: http://the-print-guide.blogspot.com/2009/03/am-and-fm-gamuts-compared.html

Creo/Kodak used to hand out a demo sheet that made a side comparison of AM/FM tone patches that folks could measure for themselves that clearly showed the difference.


By Mitch Bogart on Jan 31, 2019

That is exactly what I was looking for. Thank you not only for the answer, but for the excellent visual evidence and illustrated technical explanation as well!

(Yes, in CTP and of course digital we linearize the press sheet and build in any desired dot gain response)

From experience I had a feeling for the visual improvement, but it's better to know exactly why.

Thanks for the great paper and for keeping it at your finger tips even after 10 years!


By Gordon Pritchard on Jan 31, 2019

If you use a curve linearize your plates - or to "linearize the press sheet" (a different thing) and then apply a dot gain curve then you are more than likely wasting your time and adding an unneeded variable. The explanation is here: http://the-print-guide.blogspot.com/2010/10/to-linearize-your-ctp-plates-or-not.html


By Mitch Bogart on Jan 31, 2019

I actually have relevant experience with this. At Rampage Systems I implemented 3 curves (1,2, or 3 user selectable) of compensation,
Film, Plate, and Press, with an added innovation called "Actual" and "Desired for the press response since many CMYK scans and input files assume a typical 20% dot gain, admittedly the single number being an oversimplification.

Convolving of functions enabled one to null out the actual and perhaps aberrant day to day response and build in the desired (less frequently changed) Press response. With CTP, most people did use two curves, but not Plate and Press. The two curves people used were 1) Actual press response and 2) Desired press response. Actual would change with environment, paper, screening, day, hour... and the myriad of variables. The Desired would usually not be a linear identity curve, but rather an idealized dot gain curve that maxed at about 40% to 50% input tone (like the old System Brunner analysis).

Some people tried to use a single curve for both (which is what the math produces after convolution), but most pressmen appreciated the simplicity of simply taking some patch measurements of the Press and using that for the current Actual curve, without fear of having an overall linear press response, which produces washed out photos and improper cmyk-simulated Pantone colors, which assume typical dot gains of coated and un-coated stocks.

Inversing a [response] curve is not easy for everyone and besides, that's what computers are for!

There are some RSI users still out there, perhaps one can chime in.


By Gordon Pritchard on Jan 31, 2019

@Mitch Bogart

There may be some confusion in term usage.

RE: "With CTP, most people did use two curves, but not Plate and Press. The two curves people used were 1) Actual press response and 2) Desired press response."

That's the correct method. The "actual" press response should be based on uncurved plates. The desired press response is a tone curve target (shop or industry defined)

RE: "Actual would change with environment, paper, screening, day, hour... and the myriad of variables."

Correct. But not an issue since most shops use one screening type and build tone reproduction curves for two general paper types - coated and uncoated. That's usually sufficient.

RE: "Some people tried to use a single curve for both"

Not sure I understand what you are referring to and what you mean.

RE: "Inversing a [response] curve is not easy for everyone and besides, that's what computers are for!"

If you mean that inverting a press response curve in order to determine a dot gain compensation curve then that is not the correct method. It is neither science-based nor does it achieve the goal.

Building curves for a desired press response is covered here: http://the-print-guide.blogspot.com/2010/02/principle-of-dot-gain-compensation.html


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