4. Screening

4.1 Tone measure spatial dispersion

4.2 Optical methods

4.2.1 Halftoning photo effect in screening

4.2.2 Projection screening

4.2.3 Contact screening

4.2.4 Photomechanic equipment

4.3 Electronic dot generation

4.3.1 Basic principles

4.3.2 Continuous modulation of halftone dot area

4.3.3 Discrete dot area control

4.3.4 Screen function

4.3.5 Form of a dot

4.3.6 Error diffusion halftoning

4.3.7 Halftoning at lacking the resolution output

Summary issues

Tests

4. Halftoning

4.1 Tone measure spatial dispersion

In the certain point of a photograph the value of reflection can vary within its whole range. Similarly, within whole brightness range, the pixel intensity varies on TV screen or computer monitor any of the tone measure quantization levels can be assigned to the certain element of an image file in the so called raster graphics (Figure 4.1 a).

Autotype principle involves the separation of these levels within sampling area, screen mesh or some other print unit portion among the microdots or sub-elements thereof to make their blackening responsible to the difference just between some adjacent tone levels [4.1].

а) b) c)

Figure 4.1. Multilevel tone measure assigned to a pixel in image file (a) is subject to monotonous (b) or random, dither (c) dispersion within some unit area of a halftone copy

Such portion can be discerned on the AM halftones produced with use of the functions similar to that presented in figure 4.1 (b). But it isn’t visually acute on the tint of halftone generated by FM (dither) or error diffusion methods. Nevertheless, some spatial period, within which the reproduction system responses to one level of an image signal, does, in one way or another, exist in any kind of screening.

In the most of the lower discussed digital halftoning methods the levels are distributed as microdots weights of specified screen function. Tone measure can be dispersed periodically with various geometry and law of monotonous increase / decrease (Figure 4.1 b) or in random fashion (Figure 4.1 c). The latter can be uniformly probable, providing the spatial frequency spectra of halftone structure related to the ”white noise”. Filtration by the low band “blue” mask to certain extent excludes the microdots clusters and results in more uniform distribution of print elements and blank spaces.

4.2 Optical methods

4.2.1 Halftoning photo effect

During about a century the halftones were produced by exposure the original through special optical devices – projection and contact screens with the use of so called halftoning photo effect. Along with the variety of masking photographic methods, this, now obsolete technique, had, not too long ago, comprised the basic of prepress image processing. So, it wouldn’t be out of place to briefly describe its essence for further revealing the fundamental differences of the electronic and digital methods currently in use.

Contrary to the usual photographic effect resulting in the exposure dependant blackening of a sensitive layer, the halftoning photo effect divides the image on separate elements their size being defined by the amount of incident light. Within a unit image area (screen mesh) the dispersion is provided optically, by the intensity redistribution to a pyramidal or bell shape inside the light beam reflected from the related element of an original. At low luminance (from the dark area of original) the sensitive film layer responses just to the central part of such beam where the intensity is yet sufficient for creating small dots on the halftone negative.

Mechanism for the halftone effect and two conditions of it realization are explained in figure 4.2 with the use of three interrelated characteristics.

Figure 4.2. Mutual connection the resulting halftone dot size (І), exposure distributions within a screen cells (ІV) and threshold point (D_thr, lgH_thr) on film density curve (ІI)

The first of these conditions is in the use of a high contrast, “line work” film with as possible short density range to operate in bi-level, b/w fashion. Its sensitivity curve is presented in the quadrant ІI as the dependence of resulting optical density D and exposure H = Et, where E and t are its luminance and duration.¹ For high contrast photographic emulsions this curve has a pronounced kink in the marked threshold point (D_thr, lgH_thr) of the transition from the lgH < lgH_thr “fog” area to effective exposure range of film blackening. ²

Another condition is the formation of non uniform, excluding П - shape, exposure distributions of bell, pyramidal or any other profile providing the light dispersion for latent image within each screen cell (quadrant ІV).

In quadrant I there are shown the density distributions within the halftone dots on a negative after film processing. Dotted lines with arrows explain the formation of a pair of points for each of these distributions.³ It’s seen that these positions are set by the threshold coordinates D_thr, lgH_thr on the film curve II and exposures shape within a screen cell (IV). Such shapes were provided in camera prepress by means of projection and contact screens.

4.2.2 Projection screening

Projection screen is comprised of the opaque stripes produced by inking the grooves engraved on a glass plate. The screen giving not linear, but dot structure of printed and blank elements was obtained by gluing two such plates so that their strokes passed at an angle of 90° to each other.

The non uniform light distribution behind the screen cell can be explained by diffraction and shading theories. According to the latter, its formation is illustrated in figure 4.3, which schematically presents: the camera diaphragm 1, one of screen cells 2 and film layer 3. Relationships of these elements sizes and distances there between define the result of halftone photoengraving and are used in its settings computation. There are also shown below two exposure distributions 5 and 6 in the latent image for utmost illuminations, i.e. for the lightest and darkest areas of an original.

Figure 4.3. Range of halftone dot size variation depends on the camera diaphragm dimension D

It’s seen in figure 4.3 (a) that the maximum amount of light, which has passed through the diaphragm 1, falls on the central part of cell 2. With the transition to the edge of the cell, the number of light rays reaching the photo layer 3 decreases linearly and in the limit there can be specified the point where only one beam 4 falls. From a comparison of schemes a), b) and c) it also follows that the form of light distribution depends on the diaphragm size D.

For elaboration of the whole range of tone values, for example in the range of 5 to 95%, the film was exposed in two steps. The first "partial" exposure was made with a relatively large "light" diaphragm to obtain the greatest dots for “white” of an original in the shadows of a negative (Figure 4.3, b). The second exposure with a small "shadow" diaphragm gave the smallest dots in the light areas of a negative (blanks for “black” of an original), without significant increasing the size of the large dots provided by the first exposure (Figure 4.3, c). This mode of exposing allowed for providing the entire effective range of tone variation on a transparency and thus the proper contrast of a halftone.

The dot area, as seen in figure 4.3, depends on the sizes of a screen cell d and diaphragm D, as well on the distances: diaphragm - film (R) and screen – film (r). R and d sizes are specified by the screen ruling and image scale. Therefore, only the diameter D and the distance r can serve as variable shooting factors.

From the ratio of these variables should follow:

4.1

In practical calculations of the aperture value and exposure duration, a number of empirical coefficients were also used.

It is appropriate to note here that in addition to adjusting the copy contrast, there were no other options for gradation curve (TRC) form variation within effective range in the projection screening [2.5]. Therefore, the gradation correction was carried out previous to screening by continuous tone photographing of the original with the appropriate selection of the photo layer characteristics, the use of gradation masks, etc.

4.2.3 Contact screening

In the contact method, the photo layer was exposed through superimposed on it film transparency - contact screen (Figure 4.4). In each of screen cells was fixed the image of weight function as two-dimensional periodic distribution of the optical density D_CS (x, y), which sets the law of light dispersion. Its one-dimensional (x-axis) slice is shown in figure 4.4 (b), explaining the mechanism of screening effect realization in this method. Blackening (in the form of halftone dots) was obtained on the contrast layer of the negative only where (D_ORG + D_CS) < D_THR.

Figure 4.4. Formation of the variable area halftone dots in contact screening

To implement the effective density range, the exposure value, as can be seen from the same scheme, was chosen so as to ensure the formation of dots on halftone negative, for example, with values of 5% and 95% respectively for the black and white levels of an original. Important was also the close proximity of the screen density range ΔD_CS to that of the original ΔD_ORG. Otherwise, differences in the details brightness were transmitted either with low contrast or not in the entire range. More universal in this sense were, in contrast to the neutral (gray), the colored, in particular, purple screens. Their optical density, and hence the range ΔD_CS, were different for light of different color and can be adjusted by the filters selection when screening black and white originals.

The exposure distributions in the latent image on the photo layer, similar to those shown in Fig. 4.4 (d), was also possible to receive in two steps. First the film was exposed through a projection or contact screen without the original. Thus obtained pre-screened film could be delivered centrally for mechanicals producing in cameras and contact-copying frames without the use of optical screen.

TRC form in that kind of screening is defined by the density distribution within a screen unit cell. Example of three exposure values for three different type of screens in figure 4.5 conditionally explains how at the same utmost dot sizes the different form of such distribution provides the unlike gradation (halftone dot area) in the midtones.

Figure 4.5. Effect of the optical density distribution in a contact screen cell on the TRC form.

Contact screens were widely used for the halftone transparencies recording by xenon lamps and lasers also in the first generations of electronic scanners.

Their nomenclature, in contrast to projection analogues, was wide enough to allow more flexible control of reproduction parameters. They differed not only in ruling, but also in the range of densities and their distribution in cells (the form of TRC), the substrate color (gray and purple), the structure geometry and orientation, the dot shape and other parameters. There were also the screens with the specific geometry aimed on achieving the special effects as shown in figure 4.6.

Figure 4.6. Image from Polychrome pamphlet for the contact screens with special effects.

4.2.4 Photomechanic equipment Central to the range of photomechanical equipment was large format cameras. According to the location of the basic optical elements they were vertical and horizontal. In the first of them, the table holding an original was in the horizontal plane, above which the lamp, lens, filters, screen, film cassette and other components were located (Figure 1.5 a).

The optical channel of the second type cameras was horizontal (Figure 1.5, b). "Light room" camera was equipped with a lightproof cassette filled with film in a dark room. The lamps and original holder of the "two-room" cameras were located in a bright room, while the exposed part with photographic material was in a dark one, where the film photochemical processing was carried out.

The latest models of these devices were comprising high-performance systems, equipped with light meters, digital means for exposure calculating, automated control for diaphragms and shutters, color filters replacement, focus adjustment at given magnification, as well as with vacuum suction of film and contact screen. A number of such complexes were equipped with systems of automatic supply of photographic material and its output to the developing machine with control systems of such parameters of chemical solutions as their concentration, temperature, mixing intensity, as well as means of densitometric control of film processing.

In most cases, the fast development technology (Rapid Acsess Process) was used, in which the "from dry to dry" time was 1.5 - 2.0 min., due to the increased temperature of the solutions and the use of films with a heat withstanding base [4.2]. To improve performance, processors were linked in a production line with cameras as it’s nowadays done for imagesetters and CTP devices.

Auxiliary photomechanical equipment includes a contact-copying frame for image transfer from one film to another with change of polarity (negative to positive and vice versa), accompanying such transfer. This additional operation, caused, for example, by obtaining a positive from the first (negative) halftone in relation to the production of litho plates of positive copying, was accompanied by a double consumption of expensive (silver-containing) film.

Contact-copying was also widely used for the selective addition or subtraction of the CT transparencies densities in the procedures of gradation and color correction masking, as well as for images cropping and combining.

Obtaining halftone transparencies of different polarity by such copying was necessary for their manual local retouching. Reduction of the dots size was provided in it by etching, but for their increase the etching solution has to influence a negative.

The mechanism of etching of halftone dots on the film is explained in figure. 4.7. "Proportionally" acting solution reduces the optical density at approximately the same value over dot area. The potential of such retouching was estimated by the so-called correcting effect as a tone value by which the dot could be reduced. For un-sharp, “soft” dots such facility is higher and therefore conflicts with the further discussed copying properties in plate making. The “hard” dots of a П - shape density distribution, having the perfect copying properties, do not at all possess the effect of such correction.