Héliogravure VI – Visibilité de la Trame

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Dans un post antérieur, on a présenté des méthodes pour préparer une trame adaptée aux besoins de l’héliogravure par les moyens numériques. Malgré les résultats satisfaisants obtenus avec les trames numériques, il y a des praticiens qui soutiennent que l’utilisation du grain de résine ou des trames d’origine analogique donnent des meilleurs résultats que les trames numériques. L’argumentation la plus fréquente reproche aux trames numériques de conférer un aspect mécanique à l’estampe. Une distribution aléatoire permet d’éviter la détection de la trame par l’observateur. Cette détection dépend principalement de deux facteurs: la mesure des éléments de la trame et leur distribution géométrique. Le pouvoir de résolution (acuité visuelle) du Système Visuel Humain (SVH) est limité à 1min d’arc. Cette mesure angulaire signifie approximativement un dixième de millimètre à la distance normal de lecture (25 – 35cm). Ainsi, tout point plus petit que cette dixième de millimètre (<0,1mm) ne peut pas être reconnu ni en taille ni en forme. Dans la domaine visuelle, on dit que cette point n’est pas perçu car la perception visuelle est l’intégration du point et de son entourage.

Comme les choses sont toujours un peu plus complexes qu’une simple définition, le pouvoir de résolution du SVH peut être amélioré dans des conditions spécialement favorables. Le premier facteur c’est le contraste : des points isolés plus petits que la mesure théorique minimale peuvent être parfaitement détectés si le contraste entre le point et le papier est suffisamment élevé. Le deuxième facteur qui permet de dépasser la capacité du SVH c’est de suivre quelque sorte des patron géométrique répétitif. Par exemple des points noirs sur un fond blanc et alignés dans une rangée peuvent être visibles malgré leur taille au-dessous de la limite théorique. Les distributions aléatoires ont pour objectif d’échapper à ces dépassement des capacités du SVH.

Trama_Estocástica_Seccio

Figure 1. Trame aléatoire pour héliogravure, créé par application de l’algorithme de Floyd-Steinberg a una image plate de valeur de gris 135. Les pixels en noir représentent le 45% de la surface totale (cliquez sur l’image pour accéder à une version agrandie).

Au contraire, comme les systèmes numériques sont initialement formés par pixels carrés, tous de la même taille et rangés dans une grille orthogonal de lignes et colonnes, un parfait aspect aléatoire c’est pas facile à obtenir à partir de pixels seulement blanc et noirs. Comme on a déjà auparavant commenté et attendent au risque potentiel de morsure latéral, l’algorithme nommé de Floyd-Steinberg et leur dérivés sont les plus convenables pour générer une trame aléatoire par moyens numériques (Fig., 1). En analysant la trame amplifiée, c’est facile détecter quelques répétitions schématiques aidant à l’observateur à être conscient de la présence de la trame, même si elle est au-delà de la taille minimale théorique.

En considérant tout ce qui a été discuté, semble-t’il que les trames au grain de résine et les nommés analogiques sont la meilleur option. Grains de résine tombant sur la plaque, ils obéissent aux lois de l’incertitude et donc, constituent un exemple classique du comportement aléatoire dans l’ambiance naturelle. Celles annoncées comme trames analogiques, obtenues à partir de reproductions photographiques de verres dépolis, incorporent les mêmes propriétés aléatoires que le grain de résine. Le seul inconvénient c’est que une part de leur caractère aléatoire provient, en plus de sa distribution au hasard, de la variation dans la taille des grains. Ça provoque une variation de la largueur des canaux entre les “îles” opaques et introduit différents niveaux de risque de communication entre canaux voisins pendant la morsure.

Un autre inconvénient avec les trames analogiques se dérive de leur taille physique, qui peut limiter la résolution de l’estampe. En étant un système de transfert d’information, la résolution finale dans la plaque il ne dépend que du pire élément parmi les étapes impliquées. Quelle que soit la résolution de l’image dans le film positif, la résolution finale dans la plaque est contrôlée par la trame et la morsure. De l’autre coté, les fichiers numériques ont aucun limite dans leur résolution. À niveau pratique, le film en haut contraste utilisé pour élaborer la forme physique de la trame numérique, détermine la limitation de la résolution attendue. En assumant des conditions normalisées d’exposition et développement dans la flasheuse (imagesetter), il y a pas aucune difficulté pour travailler avec résolutions de 150lp/mm (paires de lignes par millimètre). Ce chiffre signifie une capacité de de résolution de 7620ppi (pixels par pouce), beaucoup plus au-delà de la capacité de la flasheuse (≈5250dpi).

Dans mon expérience, c’est possible de travailler en héliogravure avec résolutions de trame jusqu’à 900ppi sans aucun inconvénient pendant la morsure. Malgré que le film pourrait encore monter à résolutions plus hautes, au-delà de ce chiffre la morsure se montre pas uniforme, probablement à cause de certaines difficultés de pénétration du chlorure de fer dans la grille trop étroite. C’est ainsi que le seul problème avec les trames numériques c’est le dit “aspecte mécanique” des gravures.

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Figure 2. Échelle de gris adapté à la calibration de l’héliogravure.

En vue de clarifier un peu plus toutes ces questions, plusieurs tests ont été effectués. En premier lieu, une échelle de gris numérique (Fig., 2) a été imprimée avec la méthode utilisée pour les positifs au héliogravure. L’impression a été faite dans une imprimante au jet d’encre Epson Stylus Photo R3000. Une pièce de papier gélatiné Dragon Gravure, de Cape Fear Press, a été exposée à travers d’une trame préparée au numérique avec une résolution de 900ppi et avec un 45% de noir. À continuation, on a exposée la même pièce de papier gélatiné a travers l’échelle de gris. On a suivi avec toutes les étapes de l’héliogravure jusqu’à l’épreuve finale sur papier. Les résultats sur la plaque et sur le papier on été reproduits avec un appareil de photomacrographie (Fig., 3), pour meilleur observer la présence de la trame dans les différentes étapes. Dans une première approximation, on a examiné visuellement les images obtenues (Fig., 4). Afin de confirmer la perception visuelle, les mêmes images on été mesurées et analysés au numérique dans la domaine de la fréquence.

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Figure 3. Montage pour photomacrographie (cliquez sur l’image pour accéder à une version agrandie).

La simple observation visuelle montre comme, malgré le schème de la trame est parfaitement visible dans tous les patrons de gris de la plaque, leur propriétés géométriques on été complètement perdues dans le papier. Ce résultat indique que l’aspect mécanique présumé c’est pas présent dans le papier de la gravure final. En addition et grâce à la résolution de la trame numérique, leur petit points mesurent seulement 0,028mm ou 28µm. Ça c’est plus de cinq fois moins que les valeurs de résolution obtenus avec le grain de résine (1) et donc, complètement indétectables à œil nu. Encore plus, l’espace équivalent entre les canaux de la trame c’est une garantie pour attendre une uniformité dans la morsure. Ce peut être observé spécialement dans le cadre qui correspond au noir 100%, à gauche. Afin de réaliser l’analyse au numérique, on a pris le patron du gris moyen 128. Il est dans ce cadre au gris moyen où le schème de la trame est plus visible et donc, c’est là où le risque de transfert mécanique est plus élevé.

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Figure 4. En haute, la trame. Au centre, photomacrographies de la plaque à cuivre. En bas, photomacrographies du papier sortant de la presse. Dans ces images d’en bas, on a indique le valeur de gris moyen réel obtenu dans les reproductions du papier (cliquez sur l’image pour accéder a une version agrandie).

En étant l schème géométrique de la trame construit par une série de patrons répétitifs, on le peut analyser dans la domaine de la fréquence. L’analyse fréquentiel nous montre, en plus d’autres caractéristiques, les propriétés périodiques présentes dans une image. En prenant respectivement sélections de pixels équivalentes sur les images de la trame, de la plaque et du papier, elles sont traitées avec la fonction Transformée de Fourier Rapide (FFT) au moyen d’un logiciel de traitement des images numériques comme ImageJ. Les spectres de Fourier ainsi obtenus sont montrés dans la Fig., 5.

FFT_Composite_GV128_PlatePrint

Figure 5. Spectres de Fourier obtenues de: À gauche, l’image de la trame. Au centre, l’image de la plaque à cuivre. À droite, le papier sortant de la presse (cliquez sur l’image pour accéder a une version agrandie).

À gauche, dans le spectre de Fourier à partir de l’image de la trame, on voit la présence de plusieurs pics de périodicité correspondants à la grille de pixels et à le schème géométrique créé par l’algorithme utilisé pendant la création de la trame. Le spectre au centre, correspondant à l’image de la plaque, montre une perte des périodicités en haute fréquence. Il est présente seulement une périodicité claire au centre de l’image, dans la région de basse fréquence. À la fin, à droite, le spectre de l’image du papier ne montre pas aucun pic de périodicité.

FFTRadialPlots_ScreenPlatePrint

Figure 6. Tracés radiaux en moyenne de valeur de gris des pixels dans les spectres de Fourier montrés dans la Fig., 5 (cliquez sur l’image pour accéder a une version agrandie).

Avec la finalité d’une meilleur compréhension du signifié, on a tracé des graphiques radiales des valeurs de gris des pixels dans les trois spectres, en joignant les trois dans un seul graphique (Fig., 6). Comparer en premier lieu les deux tracés correspondants à la trame et la plaque. Le tracé de la trame, en bleu, montre jusqu’à six pics de périodicité clairement définis, tandis que le tracé de la plaque, en rouge, en montre seulement un. Ce pic est placé dans la même fréquence que la première périodicité de la trame. Cette perte de haute fréquence (détails petits) est provoquée par les changements que l’exposition à la lumière UV et la morsure introduisent dans le schème original de la trame. La lumière ne pénètre non plus en ligne droite, mais elle est dispersée et ça provoque que la gélatine durcie suit pas exactement le patron de la trame. En autre, la morsure souffre une certaine manque d’uniformité en leur pénétration dans la gélatine et il y a, en plus, un début de morsure latéral. Le résultat c’est une reproduction de la trame plus arrondie et irrégulière par rapport à son schème original.

En observant le tracé correspondant au papier, en vert, on y peut pas voir aucune périodicité. Ce qui indique l’absence de structures périodiques dans la gravure sur papier. Toutes les périodicités de la trame encore visibles sur la plaque, on complètement disparu quand l’encre a passé au papier. En fait, ce phénomène est causé par la propagation en largueur de l’encre sous la pression de la presse et aussi par la manque de pouvoir de résolution du papier par rapport au cuivre. Plus sont présentes les fibres de papier, plus on perd l’information de la trame. Alors, comme déjà a montré l’analyse visuel, aucun aspecte mécanique a été transporté de la plaque au papier. Ça signifie aussi que si on utilise un papier plus satiné, le résultat pourrait être différent. Comme dans tous les systèmes de transfert d’information, l’étape la plus faible détermine la quantité d’information que peut être retenue dans le stage final. Dans notre cas, les composants de haute fréquence de la trame, leur petits détails, on été perdus à travers les différentes étapes de la méthode (filtre passe bas), en étant les fibres du papier les pires.

Conclusions

  • Les trames numériques permettent travailler avec résolutions d’image beaucoup plus loin que les trames analogiques.
  • Les trames numériques et analogiques, évitent la nécessité d’une caisse à résine. Ça économise espace et risques potentiels pour la santé.
  • En utilisant dithering algorithmes, les trames numériques sont générées avec patrons uniformes. Donc, le risque de la morsure latéral est aussi uniforme sur toute la plaque.
  • Malgré les services bureau de pré-impression son en train de disparaître, et ça affecte aussi aux trames analogiques, les nouvelles imprimantes au jet d’encre viennent au soutien.
  • Si la résolution de la trame est suffisamment haute, leur patron peut être complètement invisible dans l’état final.
  • Encore plus, comme on a déjà expliqué, quelque patron de trame présent dans la plaque va être détruit par la diffusion de l’encre sous la pression de la presse et par la mineure résolution de les fibres du papier.

À titre de considération finale et malgré que toute la discussion antérieure montre qu’il y a pas aucune raison pour se préoccuper par le schème de la trame, cela ne invalide pas l’utilité des trames au grain de résine ou les nommées analogiques. Dans la domaine de l’Art, les préférences personnelles sont au moins si importantes que les vérités scientifiques. Cette préférence, no nécessairement raisonnée, peut être parfois considéré essentiel pour la créativité. La confiance en les outils c’est très important en la création du chemin conduisant au succès.

References

  1. SACILOTTO, Deli (1982) Photographic printmaking techniques. Ed. Watson-Guptill Publications, New York.
Posted in Early Photography, Heliogravure (english / français), Uncategorized | Tagged , , , , , | Leave a comment

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.

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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.

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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.

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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.

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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.

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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.

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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.
Posted in Early Photography, Heliogravure (english / français) | Tagged , , , , , | Leave a comment

Reproduction of Photogravure Prints

Cu_29_65_Gris_96Introduction. As in other fine art disciplines, one useful aid to the own progress in photogravure is the critical contemplation of the artwork of other artists, successful or not, famous or not. Although the better valuable way to do that is a direct and alive viewing, in those days of Internet connection, we have also the ability to look to a lot of fine art reproductions from other artists. This possibility widely spreads our capability to know styles, paper-ink combinations, finishing techniques and many more aspects about a given artist work. Conversely, other people like artists, curators or potential customers can look at our artwork if it is available on line.

Nevertheless and beside the intrinsic limitations of on screen viewing, there is another drawback compromising our judgement on a given work. The quality of the print reproduction is a crucial aspect that limits the trueness of what we are viewing. Additionally, this original quality level can be modified to a lower range at several different stages:

  • The reproduction system comprising:
    • The lens and camera sensor respective qualities. The lens must be free of distortion or conveniently characterized in order to be fixed by software at raw file level.
    • The type of lighting source. Color temperature and raw file profiling is very important in order to preserve the correct color reproduction. When it is possible, the best option is the flash strobe used in professional photography.
    • The lighting scheme (quality and direction) aiding to enhance or hiding the print textural properties.
    • The processing of the reproduction digital file.
    • The final file format and compression used in order to be able to upload the image to the web.
  • The changes introduced to the properly processed image file by:
    • The system to create the web site where the image is visualized by the Internet users (Dreamweaver, WordPress, Blogger, WebSiteBuilder, etc.). Each of those systems provides different ways to storage and show images in a web site.
    • Internet servers or storage providers. Most Internet providers use some kind of image compressors in order to save server space. This image compression is done after the processing performed by ourselves.
    • The configuration of the web-browser used by the final viewer. Each web-browser (Firefox, Chrome, Safari, Explorer, etc.) uses its own system to show images and not all web site configurations and settings are compatible with all web-browsers.
    • The Video Card of the computer where the image is viewed.
    • The computer display resolution and brightness dynamic range. With computer displays above the mid range level, the most important difference in viewing conditions is the actual size of the image in the screen. As the file in the server has a fixed number of pixels, the image can take a notable difference in size depending on the given display resolution. Because its high resolution rate, Retina Displays from Apple show the images near half size than the majority of standard computer displays. Some web-browsers, in turn, modifies the displaying conditions to fill the screen with the image.
    • The computer display settings. Most users set its display by default, but the size of the contents is very easy to modify with keyboard commands as <Crtl>+ or <Crtl->.

In spite of the steps we have not under control, at least in a complete way, the reproduction work, the digital file processing and the generation of the version to upload to the net, are our responsiveness. These are, therefore, the subject of this post.

Reproduction system. From the point of view of the perspective and given essentially flat subjects as photogravure prints are, it is not important the camera-print distance. The better and comfortable reproduction system is a reproduction column allowing for a correct horizontal levelling of both the camera and the print. This ensures a perfect parallelism between the print and the sensor planes and hence, no perspective distortion. If the print is in a vertical plane, attached to a wall or vertical stand, there is very difficult to ensure the perfect parallelism between the print and the sensor planes. Conversely, the proposed reproduction column allows the easy levelling of both (Fig., 1).

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Figure 1. Photographic reproduction set up. A Nikon D700 camera body equipped with a Micro-Nikkor 55mm f/3.5 lens, a right angle viewer and a two axis level (click on the image for an enlarged view).

For Internet purposes, almost any medium and high end level digital camera can provide a very fine reproduction in terms of spatial resolution and color fidelity. The better option is to capture a raw file and process it in order to obtain a linear tone reproduction avoiding any color bias. It is important don’t forget that the goal of the reproduction is to show the print tone scale as better as possible. With this kind of cameras, there are no special problems because any photogravure print encompasses a dynamic range wider than the camera capabilities.

Lighting System. The lighting system is also of great importance in the way to ensure the realistic reproduction of the paper texture, the ink thickness and the plate embossing. Although the classical lighting scheme for flat artwork reproduction is a couple of light sources, one on each side of the artwork and at 45º of its surface, this system provides a flat illumination that tends to destroy the above listed print properties. Light coming from both sides counteracts each other and the local texture shadows are almost eliminated. The result is that any texture present in the print is prone to disappear.

The Fig., 2 shows a comparison of the results coming from two lighting schemes. At left, the print is illuminated from both sides with two flash strobes equipped with white diffusers. At right, the same print is reproduced using a solely flash strobe following a diagonal axis from the upper-right to the bottom-left corner. The differences in the amount of texture presence are obvious. The reproduction placed at right shows clearly the plate embossing characteristic of gravure prints. This embossing relates immediately to the kind of print we are looking at and provides a sort of volumetric perception in the viewer.

ReproComp_ReedsNo4

Figure 2. Comparison between the result coming from the two lighting schemes described in the text. At left, two diffused lights, one on each side; the plate embossing and the paper texture are almost disappeared. At right, one diffused light at 45º of the print surface and following an axis from the upper right to the bottom left corner; the plate embossing and the paper texture are now clearly visible, preserving the actual aspect of a photogravure print. The series notation, title and signature are also better visible (click on the image for an enlarged view).

As things are never so easy, the second image is not the simple result of the camera capture. While the proposed system, with an unique light source, is perfect in terms of texture reproduction, it lacks in illumination uniformity (Fig., 3).

ReproTexture_ReedsNo4_NoCompensated

Figure 3. Image coming from the raw capture which shows the lack in illumination uniformity caused by the use of only one light fixture. The illumination axis is clearly visible from the upper right to the lower left corner (click on the image for an enlarged view).

Digital Image Processing. As have been previously commented and shown in the Fig., 3, it is necessary a compensation for the lack in illumination uniformity. The good news is that nowadays we have digital image processing to fix this difference in illumination. Photoshop to the rescue! To do that, a Curve Adjustment Layer is added on top of the reproduction image. This layer is adjusted in such a manner to lift the gray value of the bottom-left corner until reaches a value equal to the original upper-right corner. As the Curve affects now the entire image, it is necessary to add a shade in the Layer Mask that controls the area of affectation. The complete procedure in Adobe Photoshop is shown and explained in the following video. My personal preference is do not completely compensating the difference in illumination. I prefer to keep some amount of illumination gradient that, in my opinion, does not compromise the vision of detail in the print, while retaining the feeling of to be viewing to an object and not to a flat sheet of paper (See the comparison in the Fig., 2).

On line file. The preparation of the file that we will show on line can be accomplished attending to several aspects. One common decision to be taken is the size the image will measure at the display of the on line viewer. Few years ago, this was an easy calculation. Taking an average resolution for the majority of computer displays of 72ppi, we need so many pixels as the number of inches in size we have decided multiplied by this resolution number. As an example: If our print image must measure eight inches wide when viewed in a computer display, we need a 8inch x 72ppi = 576pix wide file. In this way, we can control  the size the image will be seen.

Nowadays the decision is more complicated. Current available displays exist in a variety of sizes and resolutions, from little, medium and big mobile phones to professional digital image processing displays, with a lot of in between tablets, laptop and desktop computer displays. Additionally, this variety of sizes is combined with several spatial resolutions. Beside that, the exhibition platforms and the web browser offer even more possibilities of displaying the images in sizes that can vary from the original spatial resolution up to several zoom-in or zoom-out, controlled or not by the user.

All that drives to a complete uncertainty about which is the size our print image will take when viewed by an Internet user. In order to constrain this uncertainty, we can prepare the print images for a given Internet media and a given display or displays. My own experience shows that the better option is to show the work in a platform under control, as WordPress blog, Blogger blog, etc. After choose a platform, we can experiment with this platform and a given web-browser in order to analyse how the print image looks with the several options the platform offers. After this analysis, we can decide how many pixels must the image contain to be seen at a given size under well defined conditions. At least, we can show our work in a concrete way when the actual art work is not available. It is important too to realize that all these criteria will change as the technology changes.

Conclusions. In spite of the difficulties that can be derived from the previous discussion, do not forget that, in some cases, a judgement about our work is a direct consequence from the Internet viewing. It is not matter if we are not interested in to acquire new photographic skills, if we do not feel able to acquire it or do not have the necessary equipment. Those recommendations can help someone to commission the work to a skilled professional photographer. Even more, for a given professional assignment and because we know the actual possibilities, we can also judge if the photographer’s work is at the level our artwork deserves.

Posted in Early Photography, Heliogravure (english / français), Photography, Photography Technique | Tagged , , , , , , , , , , , , | Leave a comment

Perdre el tren

SocialNetworks_UserInterfacesFa dies que hi dono voltes. Des de que vaig arribar a l’edat de la jubilació, he tingut un seguit de propostes de retrobament amb gent que, per diverses circumstàncies, feia temps i fins i tot molts anys que no veia. Reps un e-mail i al cap de poc dies estàs sopant amb companys del primer treball als quinze anys. Es comença per l’inevitable reguitzell de “te’n recordes quan…?” Després cadascú posa als altres al dia de la seva trajectòria vital i al final de tot, hi ha casos en que els cinquanta anys que han passat pesen massa com per tornar a connectar, mentre que amb altre gent sorgeixen noves complicitats.

També hi ha situacions no tant agradables. Aquesta vegada l’e-mail és d’un company del Servei Militar. Quedes per anar a sopar i abans de les postres estàs fins al cap de munt del relat fatxenda d’algú que si mai havia tingut escrúpols, els ha ben perdut pel camí. Ha fet calés i compra pisos a base de saldar el deute amb el banc al que alguns no poden fer front. Quan insinuo, tant educadament com puc, que potser hi ha un conflicte d’ètica en aquesta mena de negocis, em respon que “Hi ha massa gent que estira més el braç que la màniga” i es queda tant ample. En aquest cas i per tal de no arriscar-me a una altre proposta de trobada, li faig saber que, per mi, els més de trenta anys que han passat sense saber res l’un de l’altre han de significar forçosament alguna cosa. Així que fins d’aquí a trenta-cinc anys més, en el cas improbable que tinguem salut i esma d’anar a sopar plegats.

Altres retrobaments son forçats per persones amb qui hi va haver algun desavinença i d’aquí el distanciament. Que algú vulgui escurçar aquestes distàncies és una prova de generositat i sovint, un alleujament d’allò que sabem que hauríem de resoldre però no trobem com. També en aquests casos et pots trobar amb una persona ben diferent de la que recordes. No només pels canvis que imposa l’edat, sinó sobre tot per les vivències que cadascú ha tingut fent el seu camí. Quan el canvi no t’agrada del tot, et consoles pensant que l’altre també et veu diferent a com et recordava i no necessàriament en positiu.

Tot plegat és un exercici de socialització que mai va malament. És gent que en no pertànyer al teu cercle més habitual, et parla de coses que no et son tant properes i moltes vegades t’explica realitats que no coneixes perquè no formen part del teu paisatge. Així doncs, benvinguts e-mails, que a tornar a tallar la comunicació sempre hi som a temps. De totes formes, no és ben bé d’això del que volia parlar. El que fa dies que em volta pel cap és que totes aquestes persones, per edat, son d’una mateixa generació, la meva. El cas és que al menys en el meu cas, hi ha un percentatge força alt d’ells que tenen reticències a tot el que s’anomenen les TIC (Tecnologies de la Informació i la Comunicació). Tot i que atenent a la mida reduïda de la mostra, l’estadística és poc significativa i segurament no extrapolable a la població en general, un noranta per cent d’ells utilitzen les TIC gairebé com una imposició i sense cap entusiasme.

Frases com “Avui encara no he mirat el correu”, indiquen que mirar el correu és per a ells un acte que potser es fa una vegada al dia i no necessàriament cada dia. Un acte en el que cal pensar-hi, dedicar-hi un temps i també un espai, el de l’ordinador. Això vol dir que encara que tinguin un smartphone, no hi tenen el correu configurat o no saben com utilitzar-lo. Correus contestats quan ja quasi no recordes que el vares enviar. Son molts els que no tenen perfil a Facebook i tampoc a Twitter. Uns pocs han traspassat el llindar dels SMS per aventurar-se al WhatsApp. Algun cas extrem es sorprèn quan el banc li comunica que a partir d’ara, haurà de consultar els moviments del compte a la pàgina web de l’entitat o a la corresponent aplicació per telèfon mòbil.

No vull fer cap mena de judici. Molt menys criticar les persones i les seves dificultats per adaptar-se a les TIC. Però les TIC han vingut per quedar-se. Entre moltes altres propietats, les TIC tenen una gran capacitat de mutar en versions millorades cada vegada més de pressa. I el que també és cert és que aquestes persones de la meva generació tenen, tenim, una esperança de vida que a molts de nosaltres ens ha de permetre veure encara canvis més substancials. Sobre tot en la forma com els avenços tecnològics afecten i seguiran afectant les nostres vides. Cal doncs fer-se a la idea i estar receptius.

El tren de les TIC va passar per l’estació de les nostre vides ja fa uns quants anys. Alguns el varen agafar per simple curiositat. Uns altres perquè la feina els hi va obligar. Uns més perquè sempre hem estat interessats en els avenços i el dia que els ordinadors varen passar a dir-se “personals”, ho vàrem interpretar com una crida a cadascun de nosaltres. Això no ens converteix necessàriament en un addicte ni en un freaky. Els fem servir, ens ajuden a treballar, a distreure’ns i també rebutgem aquells aspectes que o no ens convencen o no ens interessen. Però pels que no varen agafar el tren, aquest cada vegada és més lluny. Potser al principi no anava gaire de pressa i encara hi haguessin estat a temps amb una corredissa i la corresponent esbufegada. Però ara el tren ha agafat velocitat i segueix accelerant.

El problema de tot plegat és el risc d’acabar adquirint, sense voler-ho, un cert grau d’analfabetisme social. En el sentit d’Albert Camus, convertir-se en un estrany. Ciutadà de ple dret, però desplaçat, aïllat. Gabriel Jaraba parla al seu blog, en un magnífic article titulat El éxito de Pokemon Go o el punto de no retorno en la vida móvil, sobre la recent aparició al mercat de la Cacera de Pokemons i el reguitzell d’opinions i comentaris, positius i negatius, que ha provocat als mitjans de comunicació. A banda del tema central de l’escrit, una observació que m’ha semblat especialment significativa és la supervivència de la Ràdio en fer-se mòbil. Ja fa molts anys amb els aparells portables basats en la tecnologia dels transistors i ara amb els smartphones.

La mobilitat és un dels trets més importants de les TIC. No només en el sentit que els dispositius son mòbils, també en que ens podem moure virtualment. Ens podem sentir a prop de gent que estimem però que viu a centenars de quilòmetres. Els podem veure i parlar-hi en temps real. Podem saber el que ells volem que sapiguem del que fan. Els podem fer saber el que vulguem que sàpiguen de nosaltres. Els podem felicitar. Podem fer-los sentir bé. En definitiva, ens podem sentir menys sols, menys estranys, menys aïllats i podem fer que els altres també estiguin més acompanyats. La socialització és també un exercici de solidaritat.

En la meva opinió, no val l’excusa que les TIC ens esclavitzen. Ho poden fer si deixem que ho facin. Però si la nostre capacitat de tria preval per sobre de tot, com ha de ser, ens fa més propers, més ben informats i per tant, millors. Companys de generació, doneu el primer pas i entreu a les xarxes socials. Potser vareu perdre el tren directe, però encara us queden els rodalies. Passen per la vostre estació cada dia. Potser amb el retard habitual de Renfe, però això juga al vostre favor.

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Heliogravure V – About Etching

Cu_29_65_Gris_96In photogravure on copperplate, etching  is commonly done by immersing the plate in several solutions of ferric chloride of different concentration. As the pigmented gelatin is thicker where the image tone is higher, the trick is to allow the ferric chloride to penetrate before the thin gelatin in the shadows areas  and progressively do the same up to the highlights. It is usual to put a grey scale step wedge aside the image on the same plate. This serves to control the etching progress. General advise is to change the plate to a more diluted ferric chloride solution when it is not more apparent etching. In an ideal situation, the plate will show equal etched steps for more or less equal periods of time. This supposes that the copper etching is constant once the ferric chloride had passed through the corresponding gelatin thickness. For a given carbon tissue, there is also a recommended group of ferric chloride dilutions that will perform a correct etching.

But, what is a correct etching? If the goal is to achieve a complete tone scale, as usually is, the answer implies that what we want to complete is an etching  inversely proportional in depth to the tone in the image. The low (darker) the tone, the more deep the etching. This ensures a high charge of ink for the shadows and smaller quantities for the highlights. Additionally, a correct etching should provide the darkest and deeper black avoiding open bite caused by lateral etching. This determines a maximum total etching time. It must be previously determined using the same screen that will be used with the actual images. Once the maximum etching time is found, there is also necessary to find which is the first dilution of ferric chloride concentration capable to penetrate the shadows in a reasonable time (usually 2min) and the last dilution that preserves the white highlights. The departures of those indications would be corrected changing the positive density scale and/or the respective screen and positive exposure times. Following the instruction sheet of the Dragon Gravure tissue from Cape Fear Press, this would be accomplished by a sequence from 45ºBé to 40ºBé.

In practice and even following all above recommendations, the results are not always the expected. Even with a deep and rich black without open bite and a clean and brilliant white, the final aspect of the print can show a certain departure from the original image. It is common that, even with those correct ends of tonal scale, the relationship between the steps of the medium grey values is different of those of the step wedge. Another common problem is that following the etching progression by visual inspection, it is difficult to observe the previously determined total etching time without risk of over etching the highlights. Attending to those difficulties and looking for a better controlled etching procedure, there are some questions that must be taken into account.

From a text published by H. M. Cartwright in 1961 (1), it can be derived that the etching action of the ferric chloride is highly dependent of its concentration. This indicates that the concentration not only determines its ability to penetrate the gelatin resist, but also acts with a different speed when finally is in contact with the copper. The Fig., 1 shows a plot taken from this text representing the depth achieved by several ferric chloride dilutions on a copper plate for a given time.

Etching_Depth_10_50Be

Figure 1

As can be observed, the etching power of the ferric chloride on copper rises up as a function of the concentration. This occurs up to 33ºBé approximately, from where the activity goes backwards decreasing again. If we take a portion of this plot, that corresponding to the concentrations between 39ºBé and 45ºBé (Fig., 2), we can realize that is almost linear. In addition to its capability of resist penetration using gelatinized paper, this is a good reason for the use of a group of dilutions highly recommended in many texts.

Etching_Depth_39_45Be

Figure 2

In spite of this linearity, the figures in the depth units indicate that the activity of a 45ºBé solution is of only the 73% than that of 39ºBé. This suggest that if we want to progress a constant depth into the copper along the total etching time, the plate must remain more time in the concentrated solutions and less as it is immersed in those more diluted. Following this hypothesis and for a given total etching time, it can be divided in as many parts as dilutions will be used. The weighted individual time for each is derived in turn from the activity shown in the plot of the Fig., 2.

As an example, we suppose that our total etching time producing a rich black is 25min and the dilution that penetrates the shadows resist is 45ºBé. Following with the example, the last dilution that still avoid an over etching of the white patch in the step wedge is 41ºBé. A weighted etching sequence derived from the upper plot is shown in the Table 1.

Table1_EtchingSequence

Table 1

There are still two more questions that might be taken into account. The first, related with the influence of the temperature in the etching progression. The second, the swelling of the resist and the subsequent ferric chloride penetration. The influence of temperature can be derived from a graph in the same Cartwright (1) text cited above (Fig., 3). The paragraph says textually that “There is a practically straight-line relationship between temperature and the rate of etching, at least within the ordinary working range… Depth increases about 2% per ºF rise” (≈0.66% per ºC).

Etching_Temp

Figure 3

The same text explains that although a higher temperature improves the uniformity of results, around 90ºF (32ºC), the ferric chloride tends to weaken the resist. In my own experience and using stochastic screen, this occurs before, around 26 – 27ºC. Probably the strength of the resist of hardened gelatine is different as a function of a given carbon tissue, that in turn depends mainly from the wavelength of the UV light source output. If the resist weakens, the highlights are quickly over-etched. In any case, maintaining the ferric chloride solutions temperature as a constant, say to 21ºC, is very important in order to control the etching sequence and how it affects to the tone scale. If this is not possible, the actual temperature should be read and the general scheme correspondingly modified. The total penetration depth achieved, is responsible for the correct black density and, if exceeded, risks the apparition of open bite.

Conversely to this linear behaviour, when the plate is immersed in the ferric chloride baths the swelling of the resist is not linear at all. Justifying the need of more than one ferric chloride bath, the same Cartwright explains in a text of 1930 (2): “It is assumed that, when a solution of a salt such as ferric chloride is applied, two separable processes take place. The first of these is the swelling of the gelatine, which goes to a maximum and then the diffusion of the ferric chloride solution begins. The maximum swelling is not proportional to the time, but is rapid at first and then slows down as the maximum is approached”.

If all those interlaced relationships between the resist hardness, the resist swelling, the ferric chloride penetration and the etching depth on copper were linear in response, it would be possible to etch a plate with a unique solution of ferric chloride. It is important do not forget that the goal, beyond a rich black and a clean white, is a linear grey scale in between. May be because my reduced experience, I have found that this necessary linearity is hardly achieved only by visual inspection over an step wedge. Scanning the printed results for an step wedge, shows that the visual inspection drives each time to a different results with only a common characteristic, the lack for a linear response. Wondering about that, it seems to me that a simplified method would be more effective if the above given non-linearities are come into play. Since a video of Fanny Boucher (3) and after an email dialogue with Jon Goodman (4), I have noticed that both are using only two ferric chloride solutions. Then, I have decided to try with this procedure and take into account the early explained causes of non-linearity.

The sequence of steps for testing the suggested etching method is as follows:

  • Exposing a tissue sequentially to the digital screen and then to a black transparency printed with the usual settings in the inkjet printer, is useful to determine the minimum exposure times that produces as thin as possible resist layer. The thickness of the resist is better seen transferring the gelatine on a transparent glass with the same method used with copperplates (5). A transmission densitometer reading or a digital image reproduction both with camera or scanner, can help to distinguish this thin resist layer from the glass background.
  • Once this minimum exposure times are determined, what is necessary now is to find out which is the etching time that produces a rich black without open bite risk. This is done preserving strips of the copperplate with packaging tape during the etching with a very diluted ferric chloride solution, that is to say, 39 – 40ºBé.
  • Take, for instance, four correctly exposed tissues using the digital screen and a step wedge printed with the same method that will be used with actual images and transfer them on an equal number of copperplates.
  • Prepare four ferric chloride baths of decreasing concentration. That is to say, for example, 45º – 44º – 43º and 42ºBé.
  • Following with the previous example, consider four couples of baths, (45º – 44ºBé), (44º – 43ºBé), (43º – 42ºBé) and (42º – 41ºBé). Taking the total etching time previously determined and the values shown in the Fig., 2, it is possible to calculate how many time the plate must be etched in each bath for each couple of baths.
  • Etch the four plates following this schemes and print it with the usual inking, wiping, paper and press.
  • Scan on and measure the grey value of the step wedge patches. Plot a graph that shows the tonal range progression. The goal is an as straight as possible line.
  • Take the best result in terms of linearity and fine tune the grey scale applying a Correction Curve to the test target image in Adobe Photoshop or any other digital image processing software.

The Fig., 4 shows the linearity of grey tone scale achieved (in red) after to apply a Photoshop correction curve to the best result in the previous assay (in blue).

Helio_46_Plot_BeforeAfter

Figure 4

The Photoshop correction curve follows from the actual reading values for each step in the printed target. In order to calculate the necessary correction, we can use an spread sheet and create a column with the actual read values in a column (J in Fig., 5) and then use an equation to calculate the corresponding target values (column K in Fig., 5). If we take the example of the calculated target value 220, it is the result to apply the equation =K3-((J$3-J$13)/10) in the cell of the spreadsheet. The procedure to plot the correcting curve can be done in Adobe Photoshop setting the actual and target values in the correspondingly input and output windows of the Curves Tool. This correction curve can be saved with the available commands in Adobe Photoshop and then, applied to all images to be printed as positive transparencies.

Actual_Target_Corrected_Values

Figure 5

From the comparison between the two plots in the Fig., 4 it can be derived that although both show correct shadows and lights endings, the progression of the intermediate grey scale is linear only in the graph obtained from the corrected positive transparency of the test target. In this way, we can not only simplify the etching step, but also preserve the image visual appearance as we have edited it in the computer screen. A bit closer to a WYSIWYG system.

REFERENCES

  1. CARTWRIGHT, H. M. (1961) ILFORD GRAPHIC ARTS MANUAL Volume 1 Photoengraving. Ed. Ilford Limited, Ilford, Essex.
  2. CARTWRIGHT, H. M. (1939) PHOTOGRAVURE A Text Book on the Machine and Hand Printed Processes. Ed. American Photographic Publishing Co., Boston.
  3. FANNY BOUCHER Héliograveur (2013) Réalisation d’une Héliogravure du photographe Georges Saillard. On line https://www.youtube.com/watch?v=FsRZcEVCXAI. Last visit: July 12th 2016.
  4. GOODMAN, Jon. Jon Goodman * Photogravure. On line: http://jgoodgravure.com/index.html. Last visit: July 12th 2016.
  5. MORRISH, David & MacCALLUM, Marlene (2013) Copper Plate Photogravure, Demystifying the Process. Ed. Focal Press, New York/London.

 

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