Factors, which are affecting the print qualityАлександр
The only way to achieve the highest quality of each imprint is the understanding of all factors, which are affecting the quality of the large-format printing.
The article is devoted to the main aspects of large-format printing quality management; it is prepared according to the material basis of the Encad firm. Ten key factors and recommendations are given below.
1. The balance of your capacities with the wishes of the customer
When taking orders, advertising agencies often overlook the step of matching the quality of printing output with the wishes of the customer. The customer should have a clear idea of what he or she is expecting, even before the start of the order filling. The ink jet printing is having some certain limitations. Depending on the process parameters and the supplies, ink jet printing produces imprints of varied quality. High-quality prints’ production is more expensive, since more time at all stages of the preparation and output is needed. Why spend extra time for the 600 dpi image resolution preparation and printing, if the 300 dpi resolution would be more than enough for the customer? Is it necessary to scan the image on a drum scanner with the graphic arts’ quality, if the customer is satisfied with the results, obtained on a flatbed scanner? Before filling any order for printing, be sure to discuss with the client what is expected and what could be obtained for the price, which suits the client. A compromise should be reached between the price and the quality with the help of the solutions’ balance. If both the customer and you do clearly understand, what you aim for, you will be able to formulate the order properly and to achieve the desired effect. In any case, the result should not be worse, than what your customer is waiting for.
2. The type of the original image
The original image type will largely determine the quality of the imprint. The best type is the original diapositive (not a duplicate). In that case, the images are drawn quite sharply, and they are having excellent clarity of the contours. Printed photos are not as good, as they are already falling behind «one generation» from the original film. In addition, because of the «wet» photographic prints’ emulsion processing, the clarity of the contours is lost. Yes, of course, they could be used, but they must be of very good quality, to produce acceptable prints. The third and the new type of image originals are the digital files. Most of them are totally unsuitable for large-format printing, as they contain only a small portion of the required information. For example, most digital cameras in the class «below $ 1,000», may form files with maximally 800×600 pixels optical resolution (about 1.5 MB). This amount of information is quite insufficient for the large-format imprints. For the A0 imprints, (with a sufficiently good printing results at 300 dpi), a file with 3200×2400 pixels (about 23 MB) is needed, and for satisfactory imprints at 600 dpi, the number of pixels should be from 3000×4000 to 6400×4800 (36 to 92 MB) (sub voce section “Scanning”). Only the best professional digital cameras can provide such an optical, not an interpolated resolution. The cost of these cameras is more than a dozen thousands of dollars. Digital files from the other sources, such as professional CD-libraries, are usually created by the scanning of slides and therefore may be used as originals. Check the quality of the documents together with the client and make sure that they meet these recommendations. On the best CDs, the images are having approximately 28 MB size and they are stored in TIFF or BMP formats. These types of images are preferred for high-quality imprints. If the original file is stored in the Kodak PhotoCD format, make sure this file is of the highest resolution. The images, which are stored in compressed files such as JPEG, GIF, etc., during the compression process may lose a lot of important information, especially the color information in fine details. When using the higher compression ratios, these details could be even lost. If possible, avoid using such images. If for any reason, still, you need to choose them, use Photoshop LZW compression of TIFF files. As a last resort, you may use the JPEG format with a small compression ratio.
3. The original quality
When taking into consideration the printing quality, the key aspect is the original of the photo, the quality of the slides or the digital files, as they contain the original information. If, for some reason, during the photo shooting, the focus was lost, or the photo shooting was outside the lens’ boundaries and the picture turned out to be unclear, the sharpening with the help of digital image processing will not fix it. The quality of photographic tape and the differences in the photo developing processes could become detrimental to the image graininess and they may affect the quality of the imprints, as well. In addition, if the captured image is too dark or too bright, it could not be fixed with the help of the color correction, as it has lost the difference in the contrast of the components, which are close in their brightness.
By the process of digital image processing, an old axiom «garbage in — garbage out» could be applied. If the original image is obtained in lighting and sharpness optimum conditions, the digital edge sharpening would make it possible to show lots of small details, which actually were in the original image, but were not visible because of the low contrast. After the digital processing, the imprint, that would visually look better, than the similar photoprint, could be obtained from the satisfactory original.
Therefore, many companies, which often use photos for the large format printing, check the focal settings of these photos with a magnifying glass, when taking orders.
4. The type of scanner
The scanning quality of the originals is very important for the entire print production process, when inputting them into a computer environment. The image, which was input by a low-quality scanner, will lose details and the clarity, the color distortion may occur. It is important to understand, which quality of the image is necessary, while printing the order, and to use the appropriate scanner to achieve it. Traditionally, the best type of scanner to achieve the highest results is considered to be a drum scanner. Given above types of scanners are available in modern repro houses, which are rendering the scanning service for the printing and publishing industry. The prices of the drum scanner range from 30 to 200 thousand dollars. Drum scanners are making it possible to scan slides and negative films, photographs and prints.
The latest models of professional flatbed scanners cost from 3 to 50 thousand dollars. Their scanning quality is closer to the one, which the drum scanners are producing. For the process of scanning from the slides, the flatbed scanner should be equipped with an additional slide adapter.
Semipro flatbed scanners, which cost from 500 to 2500 dollars, are making it possible to achieve the quality from middling to satisfactory. Office flatbed scanners, which cost less than $ 500 are providing only the poor or the middling quality, the use of them in the professional image processing is highly undesirable.
It is important to note that if the original image is poor, then the usage of expensive scanner is like trying to crush a fly with a stream-roller, and it could only lead to unnecessary costs.
5. Obligatory digital image correction after the scanning.
The first and indispensable procedure of the scanned image retouching is the technical after touch. This process is necessary for the inevitable dust and the slides’ individual scratches removal.
The second essential step is the color correction. It is necessary, as all the scanners, in one way or another, are adding small colored fog to the image during the scanning process. This is because all photosensitive devices are having a slight offset of the eye point. The easiest way to remove this colored fog is to use such functions like the PhotoShop Auto Levels. This function finds the «whitest» and the «darkest» pixels in the image, converts them accordingly into the «white» and «black» levels and distributes all the interjacent colors into that new brightness-color space. This operation is suitable for approximately 75% of the correctly exposed images. For the dark night scenes or for the scenes with white snow background, Auto Levels does not work properly and, in that case, the correction should be done manually using the histograms.
The digital gain of sharpness is needed for all of the scanned images, even for those, which have been received with the help of expensive drum scanners. The simplest way to do it is to use such functions, like UnSharp Mask at the PhotoShop program. UnSharp Mask is usually used to emphasize the images’ contours by adding a mask, received by the bilateral convolution of the original image with its particular kernel. Most of these tools are making it possible to set the magnitude of the convolution kernel (at the number of pixels) and the depth of enhancement (on a percentage base). It is better to start by using the kernel, consisting of 3-5 pixels and the 75% depth of underscores. Too deep unsharp mask makes the whole image look “tough” and chopped. The contour may start shining whitely or light bluish, so use this tool moderately. When processing images, intended for ink jet printing, deeper unsharp mask, than in the preparation of imprints’ printing, could be used. This is possible thanks to the fact, that, when using the inkjet printing, the color blenders are «blurred» a little more, than when using the offset printing, primarily at the expense of other raster formation principles.
If you are using repro services for the image scanning, its operators typically perform themselves all these operations when filling the order for scanning. However, the obtained images should always been checked before the usage.