Evaluating Inkjet Print Quality

With the onset of inkjet technologies into the commercial printing market one of the biggest questions to arise is, how do we define quality of the print. Historically, one may have used densitometric properties to define quality, but with multi-color inkjet, this method of measurement does not provide enough control. This method of printing requires more advanced techniques like spectrophotometers and complementary visual methods of evaluation.

Spectrophotometric control of inkjet prints in a production environment requires simplified methods of measurement that provide quality data points for assessing the accuracy and consistency of the print. Based on experience if measurement space and time is limited, focus should be placed on gray balance. Ideally, one should control primary and overprint colors, as well as gray balance. Focusing on current industry standards and guides is highly suggested especially if attempting to match other printing processes.

With the dynamics of inkjet heads, common print quality issues arise because of head alignment, jet blockages and resolution. If the heads of the inkjet are not aligned properly, bleed occurs between hard breaks in color. These bleed areas produce lines that cause visual disturbances. For example, say there is a cyan solid next to magenta solid. If the inkjet head was not aligned properly, a dark blue line may appear between these solids (Figure 1).

Figure 1. Inkjet Bleed

One of the underlying factors in defining the quality of inkjet has to do with the intent of the print. Quality expectations based on resolution or image quality are somewhat different if the print has come off a production inkjet press or off a grand format device intended for long distance viewing, such as a billboard. These different quality expectations may may require specialized inkjet test targets to compliment standard test targets, as has been done in different solutions including the wide format inkjet test form from Printing Industries of America.

The importance and use of these targets is critical in ensuring the optimal performance of the inkjet device. These targets will be discussed below using the wide format inkjet test form as the basis for the discussion. One of the key targets required for the inkjet market is tone scales. Tone scales are available for CMYK and overprint colors of which include RGB simulation of the CMYK in order to evaluate color separation techniques and accuracy (Figure 2). There is also a gray balance tone scale, which is composed of percentages of CMY that, when rendered correctly, produce a neutral gray.

Figure 2. Tone Scales with CMYK and RGB Patches

Analysis of this target should include measurement and visual examination. If a box appears in the upper right-hand corner, the conversion from RGB to CMYK renders color differently. It is imperative for the effective viewing of images and color that the lighting conditions are within the ISO 3664 standard. Measurement of the CMYK, RGB, and gray patches with a spectrodensitometer will provide solid ink density, CIE L*a*b*, tone value, and gray balance data and information.

Mechanical state of the inkjet device has a direct impact on image quality. The line and bleed target is ideal for inspecting condition of the inkjet heads (Figure 3). This target is composed of solid colored blocks with transitioning positive and negative lines, which vary in thickness. Each element is composed of solid cyan, magenta, yellow, or black 1-in. box. Within each set of boxes are six line structures composed of the opposing colors. The six negative and positive lines running through the boxes range in size from 0.007-in. to 0.056-in.

Figure 3. Line and Bleed Target

These lines are positioned both in a vertical and horizontal position. The horizontal lines will show when two colors are placed adjacent to each other if the colors will merge together producing a third color. This third color will typically produce a visual effect of a second color line. Horizontal lines will also show if the inkjet heads are not registered or spaced correctly. When this target is imaged with a head out of register, white space between the colored box and the colored line will appear.

Adjacent to the horizontal lined boxes are the same cyan, magenta, yellow, and black boxes containing vertical lines of the same colors. These refer to the line quality portion of the target. The lines of this target run 90° to the imaging head, meaning the lines are rendered in multiple passes instead of a single pass as would occur with lines rendered in the direction of head travel.

If these lines are not imaged as a single continuous line, there could be limitations of the inkjet device. The limitation of the line quality will show the edges of the lines to be broken or appear as stair-stepped. If the coarse lines appear smooth and the fine lines appear banded, stair-stepped, or broken, this indicates the inkjet system has a resolution limit and may require a higher resolution setting or highlight that line quality is acceptable and a lower resolution can be used to increase productivity.

Finally, if it appears that the lines or the boxes seem to bleed past their intended borders, there may be a compatibility issue between the ink and the substrate. This can even appear if the inkjet head is correctly aligned.

Consistency of the print as the head travels across a wide-format device can be problematic based on design and upkeep of the unit. For this reason, inkjet test forms should also include vignettes and solids across the width of the target (Figure 4).

Figure 4. Vignette

The printed vignettes can be visually evaluated for any signs of blotchiness and banding, which is an objectionable, abrupt change in the tonal gradation commonly caused by plugged-up inkjet heads. The vignette target should appear smooth from the highlight through the shadow. Blotchiness may be due to the incorrect profile for the media. Banding in the opposite direction of the head travel may be due to low resolution at the RIP. Banding in the direction of the head travel may only be visible in certain sections of the tonal range in the vignette target, corresponding to the tonal range in the printed image.

In the example shown, between each vignette is a solid bar of black, cyan, magenta, yellow, red, green, blue, and gray. These bands of solid color allow for the determination of inking consistency across the width of the substrate. Common methods of measurement include density and CIELab as a determining factor of consistency.

The bottom line is that process control is important with inkjet devices, just as it is with conventional printing presses. Devices should be checked for limitations and problems with resolution, plugged heads, head alignment, and other issues. The routine use of a test form designed for inkjet printing, complete with a variety of targets, will allow printers to determine whether a particular device can meet certain quality requirements and if maintenance is required.

Published on Thursday, February 10, 2011 (updated 01/12/2015)

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