Heliogravure VI – Screen Visibility


Cu_29_65_Gris_96In a previous post, there has been discussed about the methods to prepare a digital screen intended for heliogravure. Although digital screens have been used with satisfying enough results, there is also a common discussion that consider the dust grain and analogue screens as better suited than the digitally prepared. The most often argument is their randomness that avoids the “mechanical aspect” in the print. This argument implies that the digitally prepared incorporate this mechanical aspect to the print. Randomly distributed pattern is desirable in order to avoid the detection of the screen scheme by the observer’s visual system. This detection depends on two basic factors: the size of the pattern elements and its geometrical scheme. The Human Visual System (HVS) has a resolving power or visual acuity limited to a 1min of arc. This angular size means roughly a tenth of millimeter at the common reading distance of 25 – 35cm. Then, any feature smaller than this size ( <0.1mm) is not recognized in size nor in shape. In visual domain, the feature is blurred and the visual perception is of an integration of both the feature and its surround.

As things are always a bit more complex than a precise definition, this HVS resolving power can be increased for a specially favourable conditions. The first factor is the contrast. Isolated points smaller than the theoretical minimum size can be fairly detected if a high contrast is between the ink point and the white of surrounding paper. The second factor enhancing the HVS capability is some sort of geometrical pattern. Black points over a white background and aligned in a row can be sometimes detected by observers in spite of its size smaller than the theoretical limit. Random distributions fight against this HVS extra capabilities.

Trama_Estocástica_Seccio
Figure 1. Randomly distributed screen for heliogravure, created applying a Floyd Steinberg algorithm to a flat image of gray value 135. The black pixels represent the 45% of total surface (click on the image for a larger view).

Conversely, because digital systems are initially formed by squared pixels, all of the same size and distributed in a grid of orthogonal rows and columns, a perfect randomness is not so easy to achieve with white and black pixels. As have been discussed earlier  and attending to the potential risk of lateral etching, the so called Floyd-Steinberg and some derived dithering algorithms are the most suitable options to generate a digital random screen (Fig., 1). Observing the magnified scheme, it is easy to detect some repeating occurrences that help to the observer to be aware of the screen presence, even when the size of those features falls beyond the theoretical visual acuity. Considering the above discussed, dust grain and the so called analogue screens seems to be the better options. Resin or asphalt powders falling down onto the plate surface obey uncertainty law and constitutes a classic example of natural environment randomness. The announced as analogue screens, coming from frosted glass that has been reproduced photographically, encompass the same random properties as the dust grain above described. The only drawback is that its randomness comes from the variation in size of its powder particles or opaque areas in addition of the random distribution. This provokes a variation in the size of the channels between opaque “islands” and introduces different level of risk of channels communication if there is an excess of lateral etching.

Another inconvenience of analogue screens is derived from its own physical size that can limit the image resolution. Being an information transference system, the plate final resolution depends only of the worse of the several steps involved. No matter what is the resolution of the positive film, the final resolution on the plate is controlled by the screen size and the ferric chloride etching. On the other side, digital files have no limit in its resolution. In practice, the high contrast film used to render the screen in a physical form determines the limiting resolution. Supposing normalized conditions of exposure and development in the image-setter, there is not difficult to work with resolutions of 150lp/mm (line pairs per millimeter). This figure means a 7620ppi capability, far beyond the own image-setter performance (≈5250dpi).

In my own experience, it is possible to work with screen resolutions of 900ppi without any drawback during the etching. Beyond that, while the film can achieve higher resolution values, the etching fails in evenness, probably because some kind of difficulties in the ferric chloride penetration into a so narrow grid. Therefore, the only problem in the use of digital screens is this so called “mechanical aspect” of the final print.

Reduced10StepHelio_CMitja
Figure 2. Grayscale step wedge suitable for heliogravure testing.

In order to clarify a bit more all these questions, several trials have been performed. In first place, a step wedge file (Fig., 2) has been printed with the usual method as a positive for heliogravure in an Epson Stylus Photo R3000 inkjet printer. A piece of Dragon Gravure carbon tissue from Cape Fear Press has been initially exposed to a digitally prepared screen of 900ppi with a black coverage of 45%. The carbon tissue has been then exposed to the step wedge. After adhesion to a copperplate and development, the plate has been etched in the normal manner, inked, wiped and pulled on paper in an etching press. The results on the plate and the paper have been reproduced with a photomacrography set up (Fig., 3), in order to better look at the presence of the screen in the different steps. As a first approximation, the resulting images (Fig., 4) have been visually examined. In order to confirm the visual perception, the same images have been digitally measured and analyzed in the frequency domain.

Photomacrography_Plate_MkOf
Figure 3. Photomacrography set up (click on the image for a larger view).

The simple visual analysis shows as although the screen scheme is clearly visible in all the gray patches on the plate, their geometrical properties are completely lost on the paper. Simply this verification informs us that the supposed “mechanical aspect” is not present on the final print paper. Additionally and because of the capabilities of the digital screen, the tiny dots the screen have an equal size of 0.028mm or 28µm. This is more than five times smaller than the published resolution values obtained with dust grain (1) and completely undetectable by the naked eye. Furthermore, the equal spacing between screen channels is a warranty to achieve an also uniform behaviour respecting the ferric chloride penetration and therefore, a better etching evenness all over the plate surface. This can be specially observed in the patch corresponding to the 100% black, at left. For the digital analysis, the patch corresponding to the gray value of 128 has been taken. This is the patch where the screen scheme is better present in the etched copperplate and therefore the more delicate if there is any mechanical transfer to the print.

Composite_CompleteScale_web
Figure 4. Top row, digital screen. Center row, photomacrographies taken from the copperplate. Bottom row, photomacrographies taken from the printed paper. In this bottom row, there is indicated the actual average gray value measured in the reproduction (click on the image for a larger view).

As the screen geometrical scheme is constituted by a series of repetitive patterns, it can be analysed in the frequency domain. The frequency analysis shows, besides other features, the periodic properties of an image. Taking equal selections from the original screen, the copperplate and the paper respective digital images and filtering them through the Fast Fourier Transform (FFT) with a digital image processing software like ImageJ , the respective power spectrum are obtained. Their are shown in the Fig., 5.

FFT_Composite_GV128_PlatePrint
Figure 5. Power spectrum obtained from: Left, the screen image; center, the copperplate image; right, the printed paper (click on the image for a larger view).

At left, on the power spectrum coming from the screen image, it is clearly present a lot of periodic peaks corresponding both from the pixels grid and the geometrical scheme caused by the dithering algorithm used to prepare the screen. The image at the center, coming from a thresholded version of the plate image, shows a lost of the high frequency periodicities, being present only a clearly periodic occurrence around the center of the spectrum. Finally, at right, the spectrum corresponding to the printed paper image do not show any clear peak of periodicity.

FFTRadialPlots_ScreenPlatePrint
Figure 6. Radial plots of the pixel gray values taken from the Power Spectrum images shown in the Fig., 5 (click on the image for a larger view).

In order to better understand what it means, radial plot profiles have been taken from the three power spectrum and are plotted together in a single graph (Fig., 6). Compare in first place the two plots corresponding to the screen and the plate respectively. While that of the screen, in blue, shows up to six periodic peaks, the resulting from the plate image, in red, shows only one isolated peak. This peak is of the same frequency of the first in the screen power spectrum. This lost in higher frequency components is caused by the changes that the exposure to the UV light and the etching introduce on the screen scheme. The light do not penetrates the gelatin following an straight line, but scatters and causes a hardened pattern less precise as the screen pattern is. Thereafter, the etching suffers of uneven diffusion of the ferric chloride during the gelatin penetration and of a more or less important amount of lateral etching. The result is a more rounded and irregular aspect of the pattern present in the plate relating to the original pattern in the screen (Fig., 4).

Looking at the plot coming from the printed paper image, in green, there is no one isolated peak. This indicates any periodicity in the final printed pattern. All periodic occurrences yet present in the etched plate are completely lost when the ink passes to the paper. In fact, this phenomena is caused by the ink spreading under the etching press pressure and the small resolution capability of the paper surface. The more present are the paper’s fibres, the more screen information is lost. Then, as the visual analysis has been previously verified, no mechanical aspect is transferred from the screen to the printed paper. This also states that using smoother papers with high resolution capabilities, those results could be different. As in any transferring information chain, the weakest step determines the quantity of information retained at the end of the process. In our case, the high frequency components of the screen periodic properties (tiny details) are lost (low pass filtered) by the different system steps, being the paper fibres the worst.

Conclusions

  • Digital screens allows to work with resolutions far beyond those of the analogue screens.
  • Digital and analogue screens avoid the need of a big dust grain box, saving space and potential health risks.
  • Using dithering algorithms, digital screens are generated from a uniform pattern and then, the risk of lateral etching is equalized all over the plate.
  • Although pre-press service bureaus are quickly disappearing and this affect also to the analogue screens, the capability of new inkjet printers comes to the rescue.
  • If the screen resolution is high enough, the pattern can be completely unseen at the final stage.
  • Moreover, as the above explained shows, any pattern present in the copperplate is destroyed by the ink spreading under the pressure of the etching press.

As a final thought and although all the above discussed shows that there is any reason to be aware about the scheme present in the heliogravure screens, this does not eliminate the usefulness of both the dust grain and the analogue screens. In the fine art domain, personal preferences are almost as important as scientific statements. This not necessarily reasoned preference can be considered sometimes an essential part of the creative work. The confidence with tools is of great importance in creating the own path to the success.

References

  1. SACILOTTO, Deli (1982) Photographic printmaking techniques. Ed. Watson-Guptill Publications, New York.

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