Tower of Babel

Cu_29_65_Gris_96

Some days ago its been a discussion in Facebook about the correct word defining different printing techniques. It is a periodically recurrent discussion about the correct word to name a photogravure print, depending on the way it was generated. A paragraph in the discussion said: “… referred to her etchings as engravings–a much different technique.”

Being interested in printmaking and also in the correct use of language, I have performed some search. If we take the former phrase and ask Google Translate on how it translates in several languages, the answers are:

  • Original in english: …referred to her etchings as engravings–a much different technique.
    • Spanish translation: … que se refiere a sus grabados como grabados – una técnica muy diferente.
    • French translation: … appelé ses gravures comme des gravures – une technique très différente.
    • German translation: … Bezogen auf ihre Radierungen als Gravuren – eine viel andere Technik.
    • Italian translation: … cui ai suoi incisioni come incisioni – una tecnica molto diversa.

Note that the three Romanesque languages, do not establish any difference between the english etching and engraving words. Only in German there are two different words. But if we take several dictionaries, the answers are somewhat different and confusing. It is well known that translators like Google establish its answers taking data from dictionaries and also from contextual phrases and/or key words tagged in Internet contents. Then, those translations can often obey to a habitual use of language, better than a correct use of language.

Following with my exercise, I searched in my e-mail. On February 2015, I asked Jennifer Page from Cape Fear Press about how to name a technique with photo-polymer film Puretch used as a resist on copperplate. In her answer he wrote:

  • “In regards to what this etching process is you are doing with Puretch, we call that Photo Etching. I suspect when translating Photo Etching to Spanish the terminology may be somewhat similar to the translation for Photogravure. The problem is when people using photopolymer plates call it Photogravure instead of photopolymer intaglio.”
  • “… Photogravure (gelatin resists), Photo Etching (Puretch) and Polymer Intaglio (Solarplate, Imagon and Toyobo).”

Note that in the first paragraph, Jennifer uses a preventive “we call”, very wise on her part. She also refers to the possible confusion between the English etching and gravure when they are translated to Romanesque languages, as we have already seen in the earlier list of translations. In the second paragraph, she makes an statement about how to call each particular technique. Taking again those definitions in Google Translator, the results are even more confusing than in the early list.

  • English: etching, intaglio, gravure
    • Spanish translation:
      • aguafuerte, huecograbado, huecograbado.
      • ————–, calcografía, huecograbado.
    • French translation:
      • gravure, héliogravure, héliogravure.
      • ———-, en creux, gravure.
      • ———-, en taille douce, ———-.
    • German translation:
      • Ätzen, Tiefdruck, Tiefdruck (no options)
    • Italian translation:
      • incisione, intaglio, rotocalco.
      • acquaforte, calcografica, gravure.

As can be seen in the list, not only there is some difficulty to distinguish between some English words when they are translated to Romanesque languages, but there are also some options not always clear in its conceptual meaning. Even considering that Google translations are more colloquial than normative, the labyrinth is there and it is not so easy to scape.

confusion_of_tongues

Engraving The Confusion of Tongues by Gustave Doré (1865). (Gustave Doré [Public domain], via Wikimedia Commons)

The words heliogravure and photogravure are also used in some areas to distinguish between the classic hand made technique and the industrial printing process, respectively. Also in France is usual to specify héliogravure au grain, referring to the use of a powder aquatint, while if it is not specified, it can be referred both to the use of powder aquatint or analogue and digital screens.

Even taking the classic bibliography, there is not so difficult to find some confusing definitions. Most recently, we can found two Facebook groups which respective names try to establish a distinction. The Copperplate Photogravure group states it is a space of “Discussion of Copperplate Photogravure issues and technique”. Entering the group discussion, the only technique considered is the classic Talbot-Klic , both with powder aquatint or analogue and digital screens. The other group, Photogravure, announces “This group is for sharing techniques, materials and images created using photopolymer and traditional copper plate photogravure.”

The group Photogravure entitled with this word both techniques, qualifying of photo-polymer or traditional copper plate to distinguish between them. But there is no reference separating photo-polymer film used as a resist on copperplate from photo-polymer commercial plates or Imagon thick film. A bit confusing again.

Then, my opinion is that the correct use of the language is very important, both by a simple matter of correction and also for improved communication. The problem is that with this lack in agreement, it is also difficult to say where or when it is appropriate to use each word. On the understanding that this does not excuse those who try to hide the true technique with which an impression has been made.

I’m wondering that in the past, with probably a lot of practitioners but poorly communicated, all that was not so important. Nowadays, with a lot of communication possibilities, it is likely more important than ever to establish some kind of standardization about the names of techniques, specially attending to the differences between english an Romanesque languages. Even taking into account a global view better than a particular use in each region or language. A new (old) theme to discuss.

Posted in Early Photography, Héliogravure | Tagged , , , , , , | 2 Comments

Heliogravure Gallery – Cityscapes

Cu_29_65_Gris_96Gallery with reproductions of heliogravure prints from urban landscape. The images have been engraved using bichromated gelatin as a resist over a copperplate. All prints are hand inked and wiped, then pulled in an etching press. In the Slide Show, click on the “View full size” option to see an enlarged version. Contact for prices.

(Click on any image to start the slide show)

Posted in Early Photography, Heliogravure Galleries | Tagged , , , , , , , , , | 1 Comment

Héliogravure VI – Visibilité de la Trame

(Correction de la traduction en Langue Française courtoisie de Jean Claude Pronier)

Cu_29_65_Gris_96Dans 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).

Contrairement aux trames analogiques, les trames numériques sont initialement constituées de pixels carrés, tous de la même taille et rangés dans une grille orthogonal de lignes et colonnes, de ce fait un parfait aspect aléatoire c’est pas facile à obtenir à partir de pixels seulement blanc et noirs. Comme on l’a déjà auparavant commenté et compte tenu du risque potentiel de morsure latéral, l’algorithme nommé de Floyd-Steinberg et leur dérivés sont les plus convenables pour générer une trame aléatoire par les moyens numériques (Fig., 1). En analysant la trame amplifiée, Il est cependant facile de détecter quelques répétitions schématiques permettant à l’observateur de deviner la présence de la trame, même si elle est au-delà de la taille minimale théorique.

En considérant tout ce qui a été discuté, il semblerait que les trames au grain de résine et les trames appelées analogiques soient la meilleure option. Les grains de résine tombant sur la plaque obéissent aux lois de l’incertitude et donc, constituent un exemple classique du comportement aléatoire dans l’ambiance naturelle. Les trames appelées 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 ce qui provoque une variation de la largueur des “canaux” entre les “îles” opaques et introduit différents niveaux de risque de communication entre canaux voisins (crevés) pendant la morsure.

Un autre inconvénient avec les trames analogiques provient 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 dépend que du plus mauvais 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. Au niveau pratique, le film 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 n’y a aucune difficulté pour travailler avec des résolutions de 150lp/mm (paires de lignes par millimètre). Ce chiffre signifie une capacité de de résolution de 7620ppi (pixels par pouce), bien au-delà de la capacité de la flasheuse (≈5250dpi).

Selon mes expériences, il est possible de travailler en héliogravure avec des résolutions de trame jusqu’à 900ppi sans aucun inconvénient pendant la morsure. Bien que le film puisse encore monter à des résolutions plus élevées, au-delà de ce chiffre la morsure n’est plus 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 est ce qu’on peut appeler “aspecte mécanique” des gravures.

Reduced10StepHelio_CMitja

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 avec une imprimante jet d’encre Epson Stylus Photo R3000. Une feuille de papier gélatiné Dragon Gravure, de Cape Fear Press, a été exposée à travers d’une trame préparée en numérique avec une résolution de 900ppi et avec 45% de noir. Ensuite, on a exposée la même pièce de papier gélatiné a travers l’échelle de gris. On a ensuite procédé à 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 en numérique dans la domaine de la fréquence.

Photomacrography_Plate_MkOf

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

La simple observation visuelle montre que, malgré le pattern de la trame est parfaitement visible dans tous les zones de gris de la plaque, leur aspect géométrique a été complètement invisible dans le papier. Ce résultat indique que l’aspect mécanique présumé c’est pas présent dans le papier de la gravure final. De plus 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 l’œil nu. Plus encore, l’espace équivalent entre les canaux de la trame est une garantie pour assurer une uniformité dans la morsure. On peut le voir spécialement dans le cadre qui correspond au noir 100%, à gauche. Afin de réaliser l’analyse au numérique, on a pris le pattern du gris moyen 128. C’est dans la zone du gris moyen que l’aspect de la trame est plus visible et donc, c’est là où le risque de transfert mécanique est le plus élevé.

Composite_CompleteScale_web

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 indiqué le valeur de gris moyen réel obtenu dans les reproductions du papier (cliquez sur l’image pour accéder a une version agrandie).

L’aspect géométrique de la trame peut être analysé 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 au pattern 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. C’est seulement au centre de l’image qu’apparaît une périodicité claire, 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).

Afin de permettre une meilleure compréhension, 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 outre, la morsure souffre d’un certain manque d’uniformité de 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 ne peut y 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, ont complètement disparu quand l’encre est transférée 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 le faible 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 visuelle, aucun aspect mécanique n’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, ont été perdus au cours des différentes étapes de la méthode (filtre passe bas), en étant les fibres du papier les pires.

Conclusions

  • Les trames numériques et analogiques, évitent la nécessité d’une boite à grains. Cela économise de l’espace et diminue les risques potentiels pour la santé.
  • Les trames numériques permettent travailler avec résolutions d’image beaucoup plus loin que les trames analogiques.
  • En utilisant algorithmes de diffusion comme le dithering, les trames numériques sont générées avec patrons uniformes. Donc, le risque de la morsure latéral est aussi uniforme sur toute la plaque et plus facile à contrôler.
  • Malgré la disparition progressive des services bureau de pré-impression, qui affecte aussi les trames analogiques, les nouvelles imprimantes au jet d’encre apportent des solutions.
  • Si la résolution de la trame est suffisamment haute, leur pattern peut être complètement invisible dans l’état final.
  • De plus, comme on a déjà expliqué, le pattern de trame présent dans la plaque sera être effacé par la diffusion de l’encre sous la pression de la presse et par la faible résolution des fibres du papier.

À titre de considération finale et malgré que toute la discussion antérieure montre qu’il y a aucune raison pour se préoccuper du pattern de la trame, cela ne invalide pas l’utilité des trames au grain de résine ou les trames appelées analogiques. Dans la domaine de l’Art, les préférences personnelles sont au moins ausi importantes que les vérités scientifiques. Cette préférence, no nécessairement raisonnée, peut être parfois considérée essentiel pour la créativité. La confiance dans les outils est une aide très importante sur le chemin du 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.

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.
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 + or .

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 respective corners. 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 rise the gray value of the bottom-left corner until it reaches an equal value with 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 the image to be modified. 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 future viewers. 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.

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