The Value of Digital Halftone Proofing

William L. DeMarco
Worldwide Product Line Manager, Digital Halftone Proofing

First published for PIRA Proofing Conference held in London, November 1999

Benefits of Laser Thermal Digital Halftone Proofing
With the onslaught of digital technologies and the precipitous drop in the cost per computation, the impact on electronic imaging for prepress is staggering. The drive to find totally digital solutions is fueled by the desire to step on to the digital cost curves in which the cost per computation is still dropping by approximately 50% every 18 months.

The glue that holds the design-to-press workflow together is the proof. It is the linchpin between the color space of electronic files and the hard copy color space of the printing press. Given the expense and color variability of the press, off-press proofing has become an essential centerpiece of the workflow.

The proof has taken on several roles in the process. It is a contract between the budget holder and the service provider. It communicates the desired end result to those who must create it. Most of all, it functions to reduce the risk to all players in the process.

Print buyers and designers rely on good proofing to assess and represent the aesthetics and accuracy of the future printed piece. Prepress service providers rely on the proof to make the judgements required to product a superior set of color separations that will print in harmony with other images on that press form. Press owners rely on proofing to verify that the prepress work has been done in a way that (1) facilitates high quality color reproduction (2) will not waste valuable press time and (3) will not cause spoilage of materials.

Preference for Laser Thermal Digital Halftone Proofing in the US
It is interesting to note the differences between the various geographic regions with regard to the chosen solution for digital proofing. Much of this is based on the historical developments of the market with regard to the implementation of digital technologies and the degree of dependence upon industry standards for color. Although hundreds of Kodak Approval Digital Halftone Proofing Systems have been sold in Europe, more have been sold in the US, where color printing standards have carried over from the pre-digital era. There, the development of printing color standards called SWOP (Specifications for Web Offset Publication) enabled the use of proofing, particularly off-press proofing, to be used as a common currency of color separation exchange. The SWOP organization was founded by publishers and advertisers who pulled together significant prepress service providers, as well as equipment and materials suppliers to better understand the variables involved in the design-to-press workflows. Color standards were established based upon practical pressroom operating conditions.

Much of the credit for the vast improvements in printing quality that have resulted are directly attributable to the work of many great industry players. Felix Brunner introduced the industry to the concept of dot gain and dot gain control. Kodak provided the tools by which a large number of press production runs could be analyzed and statistics gathered. The political process of determining a common color space was navigated by some of the industry's greatest leaders. These findings were so significant that they forever changed the face of printing in the US.

It is against this background that Laser Thermal Digital Halftone Color Proofing has been successful. Prepress service providers are free to compete as suppliers to virtually any printer, since they can operate in the same color space and their color separations will print in any plant that can print to SWOP standards. This has therefore created a very efficient market. The fact that the prepress proof is used as a currency of exchange between prepress houses, printers, and their customers has inevitably led to the selection of Laser Thermal Digital Halftone Proofing as the trusted successor to analog and press proofing for serious professionals. This is especially true in facilities in which CtP is eliminating film.

Verifiability
A primary benefit of Laser Thermal Digital Halftone Proofing is that generated proofs are verifiable. Since the Cyan, Magenta, Yellow and Black colorant sets match the process colors used on press, simplified color control is possible. Dot gain and density are the only tools required, providing an excellent match to the printing conditions. Once densities are set to match the printed result, it is easy to determine the validity of the color in the electronic file and digital proof since the conditions of the proof can be verified using familiar and widely used dot gain and density measurements. To validate the proof, it is necessary to take readings on only a limited number of color patches.

It is this simple verifiability that is one of the primary drivers which have led to the establishment of laser thermal digital halftone proofing as the most relied upon contract level digital proofing system in America.

Other technologies, such as ink jet, process the image through four-dimensional look-up tables that cannot be verified by measuring the proof. It is quite possible, for example, to use an ICC profile that adjusts the amount of cyan in a red without adjusting the pure cyan scale.

This means that the proof is not verifiable in any practical way since it is possible to interact with the color in ways that are not detectable on the proof.

Moiré and Image Artifact Detection
Halftone proofing systems can detect a moiré pattern that is caused by the interaction of the printing screen frequency and an image subject. To the degree that the halftone proofing system actually represents the critical screening parameters, success will be obtained in this effort. A screen angle and screen ruling match between the proof and film or plate is critical. Dot shape is a somewhat lower order requirement.

It is for this reason, that the second generation Laser Thermal Digital Halftone Proofing System, the Kodak Approval XP System, images the actual dots from the host environment in which the system is integrated. Most laser thermal proofing devices sold today can be ordered at either 2400 or 2540 spots per inch.

It is interesting to note that the final proof result is often closer when a separate proofing device is used instead of the plate or film-exposing device. This is true if the separate device is designed to match the dot structure of the press rather than the dot structure of the film or plate. This is because, if the dot structure of the proof exactly matches the film or plate, it is possible to print dots on the proof that will not print on the press or throughout the press run.

Dot Utility in the Color Separation Process
Users of the Kodak Approval XP Digital Halftone Proofing System have found it is extremely useful to be able to assess dot structures in the proof. When adjustments must be made to color in the electronic file, it is critical that clear communication to the computer operator is facilitated. By looking at the proof with a loupe, the reviewer of color who is showing proofs to the customer can instruct the prepress operator to adjust a specific dot area of the tone scale by a specific amount. This precise communication between knowledgeable professionals reduces proof cycles.

In addition, an Kodak Approval proof will also demonstrate the proper construction of other elements of electronic files. The proof structure is like the printed sheet and demonstrates that spot colors are assigned to a spot color plate and not built with tints. This may not be true of other laser thermal digital halftone systems.

Dot Utility in the Press Room
By looking at the halftone dot structure on the proof, the press operator has more information to analyze and compare, The operator can assess print quality differences between the proof and the press, and what might be done to solve a particular color problem.

Confidence in the proof is an essential ingredient for press operations. The verifiability of digital halftone proofing is, therefore, extremely critical. Press operators must be sure that the problem that they are solving on press is not generated in prepress.

Visible Trapping
The proof is also used to check that accurate trapping has been performed. With the Kodak Approval XP4 System, customers are able to observe the trap between process color elements. In addition, the trap between process colors and spot colors is visible, even without the use of separate colorant sheets to create spot colors.

This is possible because of the unique nature of the Kodak Approval media architecture in which the laser power level determines the amount of colorant transfer. By using a second exposure for spot colors, additional colorant is driven to the proof, thereby displaying traps accurately. Since the donor transfers approximately one third of the colorant with each exposure, the same donor can be used for spot and process color and still display traps.

Color Stability
Although several stunning examples of continuous tone lithography have been demonstrated, all these processes were ultimately abandoned for one simple reason. Continuous tone printing processes inherently have more color variability than halftone printing processes. Simply put, halftoning creates more color stability.

This can be demonstrated. We can calculate the tone value change in an image that results from a change in density by using the Murray/Davies equation (Fig. 1). In a halftone image, wide swings in solid ink density will result in smaller density changes in the image in areas that are screened. A density change of .3, from 1.45 to 1.15, results in a .02 density change in a 50% dot area (from .29 to .27). A change from 1.35 to 1.25 results in a density change that is so small that it is lost in the rounding error of the table. In screen areas below 50%, even wide swings in density result in undetectable color shifts.

Simpler Process Control
The stability of Laser Thermal Digital Halftone Proofing Systems and the similarity of color creation with the press has made color control simple to understand and manage. Automated density scanning tools have made calibration and process control a task that entry-level employees can master.

No Calibration between Similar Papers
Although halftoning is an important differentiating factor, the Kodak Approval XP System has been designed to overcome many of the deficiencies that plague processes such as Ink Jet. Both processes can be imaged to a wide variety of paper stocks. The Kodak Approval System, however, has been designed so that the calibration does not have to be changed if similar papers are used.

A small change in paper surface can create a very large change in the color result using ink jet. This same small change in paper surface on a printing press results in a much smaller degree of change. This has forced users of ink jet to calibrate for each and every paper or to standardize on a special paper.

In the Kodak Approval process, the paper is sealed. This provides a consistent color result. By sealing the paper, the operator can control dot gain and density through digital control.

It should be noted that not all Laser Thermal Digital Halftone Proofing systems have this capability. One of the competitive systems, for example, proofs directly to paper and has similar problems to ink jet in this regard.

Throughput
The throughput of Laser Thermal Digital Halftone Proofing systems is significantly faster than most ink jet applications today. By using many ink jet nozzles, ink jet manufacturers are moving to attain the throughput speeds of these Laser Thermal devices; however, the inconsistency of the nozzles greatly reduces the reliability of these systems.

One might ask if the lower cost allows multiple ink jet devices to compete with the throughput of a more expensive Laser Thermal device. Though the cost of the devices is lower, the user is faced with the complication of having to calibrate several machines so that output is identical. This complexity significantly increases the probability of poor proofs.

Look and Feel
The look and feel of a halftone proofing device is closer to the printed sheet since it too is halftone. In our experience, customers are more likely to be satisfied with the printed job when a more literal representation of the job was provided in the proofing process.

True Metallic Colors
True Metallic Colors can be proofed using the Laser Thermal Digital Halftone Proofing process used in the Kodak Approval System. Metallic flakes can be used in order to create the same specular appearance as ink on paper. This material, identical to that used in press inks, is problematic for ink jet nozzles.

Drying Time and Susceptibility to Damage
Laser Thermal Digital Halftone Proofs do not need to dry. They are delivered with a protective layer as a surface and can therefore be handled immediately without fear of damage.

Accuracy of Text and Graphics
Current Laser Thermal Digital Proofing devices are imaging at platesetter and filmsetter resolutions. This provides the ability to accurately assess the aesthetics of text and graphics.

True Cost Effectiveness
True cost effectiveness of a proofing process must be assessed by its impact on the wider chain within which it must perform. In the final analysis, the cost of a proofing system is equal to the sum of the cost to make the proof plus the cost of poor proofs. The resulting cost of a poor proof is often under estimated in the selection of a proofing system. Poor color quality, wasted paper, wasted press time, scrambled schedules, rework and all the associated costs of possible customer dissatisfaction must be assessed.

Indeed, proofing must be viewed by its ability to reduce the risk of incurring spoilage and downtime. Over one thousand owners of Digital Halftone Proofing Devices protect their most valuable resources by investing in the no-compromise proofing that Laser Thermal Digital Halftone proofing can provide.

Future
It is clear that ink jet, as an imaging technology, is very capable. However, when comparing Laser Thermal Digital Halftone Proofing to ink jet, it is clear that many advances to ink jet technology must be in place before it can challenge the superior performance of Laser Thermal Digital Halftone Proofing.

Many have said that the resolution of ink jet will make Laser Thermal obsolete. This focus on the resolution issue is misleading to many users, especially those who expect accurate halftoning in the near future on these devices. Building an adequate halftone system requires accurate spot placement as well as resolution. This is a significant challenge yet to be fully overcome.

It is my belief that Digital Halftone Laser Thermal Proofing will experience several years of dominance as CtP evolves. In most current workflows, in which ink jet is used for proofing, a set of films is made and proofed just prior to press. This is the last opportunity to check the work before valuable press time and materials are committed. Once films are eliminated, a proof of adequate quality to ensure a successful press run will be needed. The most cost-effective proof is one that creates certainty that the press will run successfully.

The ability to represent a greater number of special colors will improve. ICC profile capability is already available for devices such as the Kodak Approval XP. It is likely that judicious use of these profiles will become more widespread in the near future.

Summary
The ability to measure proofs, to accurately reflect traps, to proof true metallics, to detect moiré patterns, to provide color stability, and to achieve high throughput speed lead cost conscious professionals to choose Laser Thermal Digital Halftone Proofing. They understand that true cost effectiveness means eliminating the risks in the pressroom.

The no-compromise match to the printed sheet will continue to drive the penetration of Laser Thermal Digital Halftone Proofing. This is especially true in CTP environments where the use of film and analog proofing is being eliminated.