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Resolving power.

The resolving power - or resolution - of a photographic emulsion defines the ability of a material to reproduce the finest adjacent details (e. g. lines in a grid). It is thus a purely visual criterion for the image quality of a photo-graphic material (nevertheless based on standardized photographic and physical testing conditions).

The maximum resolving power of a photographic material describes the number of lines per millimetre which, at a given contrast of e. g. 1.6 : 1, can be depicted as separate. A direct comparison of the resolving power of various film materials is only possible if the testing conditions are the same for all the materials.

The resolving power is influenced among other things by the range of contrast, the diffusion haloing ( anti-halation), which reduces sharpness and contrast, and by the granularity of the photographic emulsion. One example of the influence of contrast on resolving power is that the resolving power of the CN film AGFACOLOR OPTIMA 100 PROFESSIONAL is 140 lines (l/ mm) at a contrast of 1000 : 1, or 50 lines/ mm at a contrast of 1.6 : 1. Both values apply to the same material and the same outstanding resolving power, but the contrast ratios are completely different.

RMS

Method for measuring the granularity of a photographic material. Granularity is the term used to describe agglomerations of silver grains in the developed emulsion of black-and-white materials, or agglomerations of dye in the emulsion of colour materials. On enlargement, granularity becomes evident as an uneven structuring of grey and colour densities. The granularity of films can either be compared visually (usually with the aid of 12× enlargements of comparative prints) or by measurement, e. g. with the RMS method, which aims to achieve conformity between the measurement and the visual perception of film granularity. The term RMS (Root Mean Square) indicates that this method measures statistical density fluctuations of the film granularity and converts them into figures. Film specimens, exposed and processed under identical conditions, are made to rotate and are then measured with a microphotometer at a density of 1.0 * behind a visual filter. The specimens are measured in a very small range (diameter of measuring area 48 µm; 1 µm = 10^-6 m). From this measurement we get the standard deviation of the local density fluctuations of the specimen. The measurement is multiplied by 1000 to give the RMS granularity. The lower the RMS granularity, the lower the granularity of the film.

For example, the medium-speed AGFA AGFAPAN APX 100 PROFESSIONAL has an RMS granularity of 9 (measure-ment 0.009 × 1000), and the low-speed AGFAPAN APX 25 PROFESSIONAL has an even better RMS of 7 (measurement 0.007 × 1000).

* For black-and-white film: based on measurements at the diffuse density of 1.0. For colour film: based on the integrally measured colour densities of the emulsion dyes at a density of 1.0. S

Secondary colour densities.

This refers to an undesirable property in developed photo-graphic dyes of absorbing light not only in their own, but also in the other spectral regions. Secondary colour densities are the cause of colour distortions in the main colour density (greying, lower colour saturation etc.). Three dyes are needed for the subtractive colour mixture of the three emulsion layers of a processed colour film.

Depending on the intensity of the exposure, each of these three dyes absorbs only one third of the visible spectral region and is transparent for the two other thirds. The emulsion dyes, however, absorb light not only in their own spectral thirds, but also - to a lesser extent - in the others. To eliminate secondary densities, modern colour negative films are nowadays masked ( see : main colour density, masking).

SEM crystals (Advanced Photo System).

Major advances in the field of emulsion technology have led to the development of tabular silver halide crystals (SEM crystals = Surface Enhanced Multi-structured crystals). These have a larger sensitisable surface area and a more uniform size. In addition the halide distribution over four zones with varying iodide contents has been substantially improved. The SEM crystals are tabular and uniformly thin, which makes the emulsion thinner so that the sharpness is much higher.

Fig. SEM crystals

Scanning

A scanner provides a user with the means for making a digital image from a variety of viewable sources. Generally those sources are on paper and contain printed images or text or both. In most applications the viewed source will depend on reflected light providing an optical image for the scanning sensors to see. With the advent of very high-resolution scanning, some scanners can see the image as an optical view through a film (negative or positive film). Thus, a user can make digital images of their photo slides.

A few different end uses of the scanned images should be reviewed so a user may consider the scanner as a kind of “tool” that can assist them in doing a variety of things.

A typical use is for making digital images of pictures – whether these pictures are of photos printed on photo paper or those taken from magazines, newspapers or other print media sources. In a similar vein, the source “picture” may be a line drawing, cartoon, table, graph, fabric, etc. Generally, the end use of the digital image is to be archived, displayed, or shared as a viewable file, although it may get embedded into other software for purposes of making a more suitable presentation.

Another use for making digital images is to prepare a suitable file to send to your computer’s printer.

Yet another use is to make a file of primarily printed text that can be converted into a file in which the text appears as a formatted text file. This process requires the use of Optical Character Recognition sofware. Varying degrees of detail in the layout of the scanned text file may be represented in the formatted text file, and some degree of maintaining the layout of a page of text and images, as well as the elements of a page consisting of a printed form is also a possibility with the right software and its judicious use. In most cases, some editing will be necessary, as the compiled image is not likely to be letter perfect, with everything properly positioned.

Many reasons are cited for making a word-compiled file from a printed page. The resultant file is likely to be much smaller, the file is often more “crisp” in its viewing qualities, it may be integrated with other compiled text prepared by the user, etc.

Before a scanned image is produced, the user should account for the type of useage being planned, so that proper settings can be made for the scanner to acquire the kind of image desired. There are about four major determinations to be made:

• Is the image to contain color or black-and-white?
• How much spatial resolution (dot per inch (dpi), or fineness of the scanning grid) is needed?
• How much color (or black-and-white) resolution is needed?
• Is the source a photo image obtained from print media which used a half-tone screen?

More advanced users may find that some other settings may enhance the scanned image.

Choices of scanner settings :

In most cases, a scanning resolution of 100 dpi will be okay for scanning photos, unless you expect to use the digital image as an enlargement of the source photo.

For black-and-white images, a few settings of “color” resolution may be made, and the manner of specifying these settings vary among the companion scanner software. In some cases, descriptive terms of end-use of the image file will guide you, such as for filing, faxing, or copying, in which case this choice selects one bit per pixel (no shades of gray, only black and white). For getting photo-quality reproduction capability, you select photo, which provides for 8-bit grayscale resolution (this is the equivalent of 24-bit in color, which requires 8-bits of resolution for each of three colors). For converting to text, select OCR, which usually provides a default setting of one bit per pixel (i.e., black-and-white only), as well as a spatial resolution of 300 dpi. These parameters meet the requirements for OCR software. Most software will provide for custom settings which vary from the defaulted settings.

For color images, you get 24-bit color resolution, and you usually can select between “Normal” and “Best”, or some similar choice. The default values here are often 100 dpi and 300 dpi, respectively. Recognize that the 300 dpi setting in color will require a lot of bites of data per square inch, and that the resulting image may be about three times larger in linear dimension than the source image. Also note that there will be nine times as many bites in a file of the same source photo when scanned with 300 dpi rather than 100 dpi. Again, custom values of dpi which vary from these default values may be chosen to obtain a result which meets your objective.

It can be shown that a color image that fills an 800 x 600 pixel screen often contains about 1.4 million bites. This assumes that the image file is not shrunk by a file-reduction process called compression, and that the image is presented at the normal size of 96 dpi. The temporary image developed in the scanner software meets these criteria, and the presented image will be 8.3 x 6.3 inches. If scanned from a source that would produce a color image of that size, the scanned file when using a setting of 300 dpi would be about 12.6 million bites.

Thus, one should use discretion in making high-resolution settings for their scanned images, as they may make a very large file that prints a picture that won’t fit on a piece of printer paper or on a monitor screen unless it is modified by image modifying software. Further, more resources of file size and compiling time will be used that won’t be beneficial to the end result.

Software associated with scanner image creation :

Besides the scanner driver software required to be installed for the scanner to operate, some comments will be made about the software used when operating the scanner.

The “Twain” software provides the abilty to make the settings mentioned above, and (typically) to call for a low-resolution Preview of the scanned object. Then the user can select out (“crop”, if you will) the areas outside of the area of interest (a custom setting of the positional and size requirements for the area chosen from the previewed image). In the case of Hewlett-Packard scanners, they may by-pass the preview, proceed directly to a high-resolution image, and then allow you to crop out the non-useful areas before exporting the image within the selected boundary.

Scanners also provide some additonal “Advanced” features such as “De-screen”, which should be used when copying from a half-tone printed media image. And, it provides the Scan button for starting the scanning process.

A companion image viewer, image modifier, image exporter/converter software is used to acquire the image file from the Twain source, to present the scanned image for review, to make modifications to it, and to “Save As”, or “Export” it to a folder as a file having a name and a file format extension of your choosing. In most cases, a scanned photo would be exported or saved in the JPG format. Initially, this software allows you to gain access to the Twain software to make the needed settings and the Preview of what’s on the flat-bed glass screen prior to scanning the desired area seen in the Preview. Many of the image-modifying programs (e.g., Irfan View, Photo Deluxe, Paint shopm Pro, Visua, and Ulead Photo Express) as well as Optical Character Recognition programs provide the means to access the Twain software for purposes of acquiring the image and then refining it to obtain the desired result.

To get the most from the use of their Twain devices (scanner and camera), a user should take care to review the acquired images with a critical eye and determine if they want to make improvements to that image. If a scanned image could benefit by re-selecting settings on the scanner software and performing another scan, the user may choose to do so.

Suppliers of Twain devices bundle image modification software with their products. The user should be aware of the various kinds of improvements that can be made, and have familiarity with the image modifying software and the processing steps that they have at their disposal to achieve their objectives. In the case of a Visioneer scanner, a user does not need a separate image modifying program, as they have an integrated software package (PaperPort) to acquire the image from the Twain software. However, the image-modifying capabilty is limited, and a user should consider having a separate image modifier to improve upon the quality of a picture, and to crop and re-size the picture, if that is needed. The PaperPort software makes provision for exporting the image in a number of standard image formats. A user will ofter choose JPG and a quality setting around 70 for most scanned photos.

After having done this with an indivudual scanned image, the image file is redy to sent as an attachment, or may be archived on a hard drive or CD.

Many of the comments made about acquiring and modifying an image obtained from a scanner will apply to an image obtained from a digital camera.

The software functions described above may be separate items or may be combined into one, as has been done by the PaperPort software offered by Scansoft (often bundled with Visioneer scanners). The image modifier software acquires a (temporary) file of the scanned image from the Twain software. In the case of PaperPort software, individual images are retained unless the user deletes them, and a “thumbnail” of that image is a reminder of those images that it retains. A user should purge these files after the data from these images is exported to a file which can be used elsewhere. In a typical stand-alone image modifier, the acquired image is truly a temporary file which is not retained when the user closes that application. Because the image conversion/modification capabilities are integrated with the “Twain” software, PaperPort (bundled with Visioneer scanners) may be one of the easier software packages to use when scanning. There are some custom settings that determine whether a page of scanned text will appear as a single column after the Optical Character Recognition process, or whether it will place the text in the separate columns as they were on the original.

An Optical Character Recognition software package may be needed or chosen as an option. This may also be part of an integrated package, as in the PaperPort software.

Various separate OCR programs are available, including Text Bridge and Recognita, for example. Another type of specialty OCR program which also retains page layout details and captures a mix of text and images exists. A popular program of this type is ReadIRIS. Most OCR programs require that scanner text be 300 or 400 dots per inch, and be one-bit color (that is, balck-and-white). The process of commanding optical character recognition from within the OCR software is often referred to as “recognition.” The result of this process is often a compiled text character file with an RTF (Rich Text File) file extension. Usually, there will be a few typos, and the text may need to be re-formatted to suit the user’s desires.

In some cases, an OCR program will only function well if the image is taken directly from the scanner. In other cases, a 300 X 300 black-and-white image file may be opened in the OCR application, and be processed for OCR text compilation. The user must make sure that the image file is strictly black-and-white for the OCR application to accept it. One exception is ReadIRIS, which will accept an image file with 24-bit colors.

A few words on scanning from a newspaper :

A few things should be noted about newprint that makes it a unique challenge for scanning. The paper is thin and some of the ink from the reverse side will be visible on the front side. To suppress this factor, a user should have a black sheet of paper available to place over the sheet that is laying on the glass bed of the scanner. This sets a more uniform black background over the entire area being scanned, and the ink pattern on the reverse side will be less noticeable in the scanned image. A good paper to use for this purpose can be found in some of the 3-ring transparent plastic protectors which are made to hold punched paper sheets. A black paper of that weight from any source is convenient for this purpose.

Although text and cartoons are printed with no grey content, the scanner should be set for grey scale to get a good image.

Because the newsprint paper is far from being white, the image will often show prominent amounts of grey background that should be white. A user can compensate for much of this by increasing the brightness, prior to scanning and/or after the image has been moved to an image processor. Along with the increased brightness, the user should also increase the contrast, and perhaps the gamma – the amout of each to be determined by observing the image quality. If the newsprint is on a portion of the paper that has wrinkles or creases, it is very difficult to do much about seeing these effects in the image. You may get a far better result of you do the copying on a commercial copy machine. The media which is they use to capture the image is more capable of providing an image that doesn’t show as many effects of the wrinkles and creases.

If a photo is present in the scanned area, you should select “de-screen” or its equivalent prior to scanning to suppress moiré effects.

Comments on the Moiré effect

When a color or grayscale image is printed in a newspaper or magazine, they make use of a “half-tone” screen technique. This permits the offset press to hold a block having the ability to transfer ink to the paper in a manner that permits light to heavy deposits of ink of the desired color. The surface of this block has a regular grid of dots that vary in size and in closeness that they can provide most of the desired variation in ink deposition. Thus, if you were to take a microscope and view the printed image, you would see the grid of dots.

When you do scanning, the scanner samples the “color” (saturation, intensity, and hue – including shades of gray) in a regular grid also. In many cases, the half-tone screen and the scanner will have a similar spacing of these grids. In this case, there is a real likelihood that you will see an interference (or Moiré) pattern in a resultant scanned image. The visual appearance is similar to that which is seen with sheer curtains that are gathered and have two or more overlapping layers through which to look.

It is important to learn the means for overcoming the Moiré effect so it will not have an adverse affect on the scanned file. Two primary means are available at the time of scanning, (1) setting the scanner’s dots per inch to a value of 200 or higher, so the scanner grid is much finer that the half-tone grid, and (2) selecting de-screen in the scanner software, a feature that suppresses the Moiré effect. Other secondary means are available after an image is scanned, and they consist of using the various image software modifiers that can reduce the Moiré effect. Some users will have access to an Adobe software package that can make some slight improvement to reduce the Moiré effect, but the net improvement in the picture is usually not great, nor is the end result.

Additional scanner/scanning topics

At this point, the reader may want to have some concluding comments about scanning, or a few simple statements about what to do next. The fact is that working with images is too broad a topic to sum up in a brief conclusion. The detailed manner in which you obtain a suitable Twain image, and what you do with it, will depend on the steps that are specific to the software item available to the user, and the comfort they have with being able to operate the individual software items.

Some of the steps required for saving the image files will depend on the user’s ability to perform file management. Some of the choices of file formats to select for saving a given temporary image may be poor choices if the user does not have a fair working knowledge of the benefits of the more common image file formats.

Each of the topics which address what to do with a file obtained from a Twain device can be treated as separate elemental pieces of expertise, and a user should develop a working knowledge of enough of the steps used to perform these additional tasks as will be needed for their purposes.

If a user is interested in retaining the layout of a multi-column article, they may individual scan and OCR operations of each of the columns or other segments of the original (e.g, images, captions, etc.). Then the individually-constructed text files may be re-assembled in the chosen layout software.

If the user wants to make a digital replica of a form (or of an article layout) they may use Readiris to their advantage. In it’s normal operational usage, Readiris will scan an entire 8 ½ by 11 inch area and retain the positional and size details of the text and images, but may not do an accurate job of performing the OCR function, or of obtaining a good collection of the images that are present on the original. Thus, a user can take whatever steps at their disposal are best suited to re-construct the layout. This may include doing a batch of smaller scans of segments of the original for performing OCR (perhaps on a more accurate OCR software than Readiris), or for collecting appropriate image files, to gather the elements best suited for the re-construction.

Sensitization.

This refers to the sensitivity of a photographic emulsion to light of a certain spectral wavelength ( spectral sensitivity).

All photographic emulsions have, in principle, an inherent sensitivity to blue and ultra-violet light. Increasing the inherent sensitivity or general sensitivity of a photographic emulsion by adding gold salts or sulphur compounds is called chemical sensitisation.

Orthochromatic emulsions are also sensitized to green light, and panchromatic emulsions to all the colours of the visible spectrum. The use of suitable emulsion additives (sensitizer dyes) can extend the blue sensitivity of photographic emulsions to cover the other colours of the visible spectrum. This is called optical sensitisation, which was discovered in 1873 by H.W. Vogel, and is one of the principles of colour photography.

Sensitometry.

Sensitometry originally meant measuring and determining the sensitivity to light of photographic materials. In its broader sense, sensitometry also includes determining all the characteristic properties of a photographic emulsion, taking into account the characteristics and intensity of the exposure under defined processing conditions. Sensitometry is thus the principle behind the reproducible manufacture of photographic materials. The criteria applied in sensitometry are standardized (e. g. DIN standard 4512, Parts 1 - 5 and ff.). Sensitometry is also very important for photographic processing ( see : process monitoring).

Silver chloride emulsion.

Most colour negative printing materials (e. g. the AGFA-COLOR PAPER TYPE 11) need a special process with a wet time of only three minutes and a developer temperature of 35°C (in AGFACOLOR PROCESS 94 which is compatible with Process RA-4).

This short processing time is made possible by the use of photosensitive emulsions based on silver chloride. In emulsions of this kind, the concentration of silver ions in the aqueous phase - it is these which accelerate the development process - is 500 times greater than in the conventional silver bromide emulsions.

Spectral sensitivity.

The sensitivity ( sensitisation) of photographic materials to certain spectral components of visible light is known as the spectral sensitivity. In their optically unsensitized state, photographic emulsions have the property of being sensitive to only one part of visible light (up to approx. 510 nm). Their inherent sensitivity lies in the shortwave region of the spectrum, making them sensitive only to blue and violet light (AgBr) or to UV radiation and violet light (AgCl). The silver halides can also be made sensitive to green and red light by means of special organic dyes (spectral sensitizers), which are adsorbed on the surface of the silver halide crystals. This is an important condition for the black-and-white rendition of all the colours with the right tonal values (panchromatic black-and-white films) and for colour photography. In other words, the speed of the unsensitized emulsion is optically extended by the region in which the spectral absorbability of the used sensitizer dye lies (optical sensitisation). Use is made here of the fact that the excitation energy required for the sensitizer dye is smaller than the excitation energy for the pure silver halide, which means that light waves of correspondingly lower energy - in other words also green and red light - can become photographically active. Every photographic material is sensitized for its specific job in line with this principle.

The following nomenclature is used to differentiate between the types of optical sensitisation of photographic materials:

Fig. : Two forms of optical sensitisation. The sensitisation to green and also to red light is increased by the adsorption of dyes.

• unsensitized: The emulsion layer is sensitive to ultra-violet, violet and blue light.
• orthochromatic: Sensitive to blue and green light.
• panchromatic: Sensitive to blue, green and red light, i. e. to the entire range of visible light.
• hyperpanchromatic: Panchromatically sensitized with increased sensitization to red light.
• Infra-red sensitized: Sensitive to the non-visible infra-red radiation.

With black-and-white films, panchromatic sensitisation is used to transform the radiation components of the entire visible spectrum (colours of the subject) into grey tones graduated according to the visual impression. Black-and white photo papers are generally not panchromatically but orthochromatically sensitized, since they are intended for the rendition of the grey densities of black-and-white films. (The rarely used panchromatic black-and-white papers are an exception because they are able, due to their extended sensitisation, to convert colour densities of colour films into grey densities with the right tonal values.) In the production of multi-layer colour materials, every individual layer is sensitized only for a certain region, i.e. for one of the three main spectral regions of the visible spectrum. The curves included the AGFA Technical Data brochures for the spectral sensitivity of colour films and colour printing materials depict the respective emulsion sensitivities to blue, green and red light. The spectral sensitivity of the emulsion layers is in inverse ratio to the radiation energy which, as a result of exposure, produces a given spectral density in each of the three emulsion layers (with colour film the density = 1.0 above minimum density; with colour paper the density = 1.0 in reflection).

Fig. : Sensitisation maxima of the unprocessed photosensitive emulsion layers of a colour negative film.

The spectral sensitivity is measured with special instruments (see: grating spectrograph). According to the sensitivity of the layer after processing and as a function of the wavelength, we obtain a density profile that can be measured with a densitometer. In order to make the measurements of different layers comparable, the energy distribution of the lamp spectrum is taken into account, and the resultant spectral sensitivity converted to an equienergy spectrum. Unlike the overlapping sensitisation of photographic materials, the sensitisation of printing materials is selective and not overlapping. This means that the spectral sensitivity curves of the three photosensitive emulsion layers do not intersect or, if they do, only slightly. The sensitisation maxima of the unprocessed emulsion layers of printing materials must match the absorption maxima of the emulsion dyes of the processed colour negatives. (The emulsion dyes of the colour negative act like filters. For example, the blue part of the printing light is modulated by the yellow dye density of the negative. This modulated blue component specifically exposes the blue-sensitive emulsion of the printing material.) Because of this colour-selective exposure, the problematical spectral overlap areas of the emulsion dyes (with the unfavourable influence of secondary densities on colour rendition) can be largely dispensed with.

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