*A color proof is a printed or simulated printed image of each process color using inks, toners or dyes to provide a simulated impression of the final printed reproduction.
In general, proofs will be required at several points throughout the design to print process before the customer signs off a final proof prior to printing.
HISTORY OF PROOFING
Historically press proofing using a rotary or flatbed proofing press was the most commonly adopted method of visualising a design. This method which requires actual printing plates, materials and press time is extremely expensive and time consuming. Although replicating the printing process exactly, this type of wet proof is an inefficient method for initial viewings and for making minor alterations.
Early in the 1970’s film based analog proofing systems emerged, with the development of Cromalin proofing (from DuPont), a toner-based off-press proof. Cromalin proofs are still used today together with dye-sublimation and digital proofing – each of which is described below.
A Cromalin is an off-press color proof for four color process printing, using separations to construct an image, by the successive exposure and application of adhesive polymer-dry powder toners.
A Cromalin proof is created by exposing a carrier sheet to ultraviolet (UV) light, applying a toner within the cyan, magenta, yellow and black (CMYK) color model and then laminating to a white material.
Cromalins were originally used by the plate-maker to make color adjustments, but are now more widely used by printers, designers and customers as an alternative to mechanical or on-press proofing.
This type of proof will give a close representation of what to expect for color, but it does not replace a press/wet proof. Variations in inks and material substrates used on press will slightly modify the final product (see Figure 4.1).
Figure 4.1 - Approval of proofs to agree color with customer – Color calibrated (Cromalin)
By the 1980s, film-based analog proofing had largely displaced press proofs. A range of manufacturers besides DuPont further advanced the technology so that matching proofs to press became even more accurate.
In the late 1980’s came the digital revolution, during which digital proofing devices began to appear. At this stage dye-sublimation proofing took hold.
Dye-sublimation proofing uses heat and solid dyes to produce photo quality images. The printers lay down color in continuous tones, one color at a time, instead of dots of ink.
Dye-sublimation printers contain a roll of transparent film made up of panels of color. Solid dyes in CMYK are embedded in the film, whilst the print head heating elements vaporise the inks, which then adhere to a specially-coated paper.
As the color is absorbed into the paper (rather than sitting on the surface), the output is considered more photo-realistic and more durable than other ink technologies.
By the mid to late 1990’s low cost inkjet proofing solutions emerged with both desk-top and large format ink-jet proofing solutions becoming available.
A digital proof is an off-press color proof produced from digital data without the need for color separated films.
Using digital technology the requirement for film to create the image is no longer required. Eliminating the need to make proofs from film was a significant advance for the industry, helping to reduce costs and increase efficiencies.
The job is printed from the digital file using inkjet, color laser, dye sublimation or thermal wax print technologies to give a good approximation of what the final printed piece will look like.
The digital proof is generally less expensive than other proofing methods and can often be produced on the actual face stock of the job thereby adding another element of accuracy.
SUMMARY OF THE ADVANTAGES AND DISADVANTAGES OF DIGITAL PROOFING
The pros and cons of digital proofing are highlighted below;
True half-tone reproduction
Not adequate as a contract proof when metallic inks or phosphorescent inks are involved
Screening process - does not show possible moiré problems
Cromalins still have advantages
Availability of substrates limited using digital proofing
A “wet proof” is a proof that is produced using the graphic processes, embellishments and conversion methods used in the “actual” print manufacturing process.
There are no “compromised” processes with this type of proofing and it can be used for all methods of packaging and product decoration. The benefit of wet proofing is that it gives a result that is identical to the actual production run and can be used for sheet-fed, web-fed or direct printing operations.
Equally it can include embellishments and can be converted to a profile shape.
Wet proofing is the most expensive method of proofing.
With improvements in monitor accuracy and monitor-based proofing software, soft proofing first became viable around 2002.
The term soft proofing is used to describe the technique of previewing a page on a monitor, rather than taking a physical hard copy proof.
The challenge is to achieve accurate color representation on the screen proof that will compare with the actual printed job (bearing in mind that print uses additive color and monitors use subtractive color).
Professional monitors and systems are now usually sold with calibration capabilities to improve the color match between the two. When a monitor at the press and another at a remote customer can be verified simultaneously and display exactly the same output as each other, the need for hard copy proofs is potentially eliminated.
STAGES OF PROOFING
The proof stage is arguably the most critical within the pre-press process, since it is at this point that the customer is provided with a visual of how their final printed piece will look and upon which contractual agreement is made.
A comparison of the process steps involved in producing a wet proof versus a digital proof is featured in Figure 4.2 below. With digital proofing no film output or plate making is required, with digital data directly generating the digital proof.
Figure 4.2 - Stages of proofing – wet proofing versus digital
The proofing system utilises the press characterisation data to build a color profile which is applied to incoming files and to this profile. If generated correctly, it will allow the resultant proof to closely resemble the final printed piece.
As inkjet printing devices are commonly used to produce these final proofs, the original ink sets need to be manipulated to take into account the ink hue, grey balances, dot gain and overprints found on press.
The incoming file format for these proofs should be in a form that will allow the final inkjet proof to accurately represent the final printed piece. It is for this reason that a Raster proof format is favoured.
A dedicated flexo proofing system using raster data* allows the customer to see how the dot structure and appearance can influence the visual effect of the final print. For example it allows the same image printed at 65 lpi, 100 lpi and 150 lpi to be considered and how much of an impact this screening has on the resultant print.
*A raster data structure is based on rectangular or square-based cells.
The proofing system should also faithfully reproduce other elements that may impact the original design: elements such as rosettes in images and other tonal areas, the appearance of trapping and how this affects color and how colors interact when overprinted.
Accurate spot color reproduction is also sought on the final proof. With the latest ink sets available in devices such as the Epson Stylus Pro series with its additional orange and green inks, this is now achievable for the vast majority of target colors.
With the advances made in inkjet technology, proofs can now be created on multiple substrates such as film and metallic media with the added appeal of printing with a white ink, surface or reverse, to further mimic the final printed piece. A control strip printed automatically on each proof allows the operator to monitor and maintain the output quality.
Apart from the final print, the proof is often the only part of the production process that the client will see. Once the final design is agreed, an accurate color proof may be prepared by ink-jet, dye sublimation, or other suitable procedure. This must be as close to the colors that will be produced on the press as possible.
Only after this approval stage has been completed should the final pre-press procedures be carried out either to the plate making or to a digital press.
The use of press proofing, analog proofing and expensive proprietary digital proofing systems is on the decline.
Inkjet proofing however, operating with high end Raster image processors (RIPs), has become very accurate and flexible and will continue to satisfy those customers who require a hard proof.
Soft proofing, using color management and software clearly represents the future of proofing in the label sector.
The barriers of high prices of soft proofing systems and more complex associated workflows, however, will need to be overcome before it becomes the industry norm.