Color Printing
The
19th century witnessed a rapid expansion in the science of light and color.
Early in the century the French government funded color research at the behest
of the textile industry. Soon the new science of chemistry begat a host of new
pigments and dyes. Meanwhile, researchers studied perception, focusing on the
interaction of thin patterns of colored thread. Finally, chemistry and physics
mated to give birth to photography.
Light
became the most studied phenomenon by scientists and artists alike. Most noted
of these was the French painter George Seurat.
Using
his knowledge of color optics (Chevreul), Seurat invented pointillism, a method
of painting that broke light into minute points of color.
Seurat
and his fellow impressionists linked art and modern physics through light and
color. Subsequent artists tightened this link by portraying light and energy to
symbolize the changing scientific concepts of these phenomena. Both quantum
physics and Einstein’s theories of energy in matter enter these two examples
of Italian Futurism.
Understanding Digital Camera Resolution
Representing images on Computer Monitors
Computer monitors use a physical device to display the pixels (picture elements). High quality monitors have pixels which are about 0.25mm apart. Such a measurement is called the dot pitch. A dot pitch of .25mm means that there are about 100 pixels per inch of screen. It is crucial to realize that, no matter how fine the grid we apply when capturing (scanning) an image the computer screen resolution is fixed.
So if an image which is 10 inches wide is displayed on the screen, then the maximum number of pixels that can be displayed across the 10" area is 1000. If the image data has more than 1000 pixels of information across the width (i.e. it is scanned at a high resolution, probably for greater clarity in printing), that information is redundant and discarded in the display process, and therefore unnecessary.
For video imaging the criteria are: how much of the screen you want to be taken up by the image.
So, what is important for determining scanning resolution for video and computer display is:
And the obvious answer is: to select a resolution that will scale that input size (in inches) to that desired output size (in pixels).
Example: if we intend to scan a 8 inch wide picture and need an image with 400 pixels of width, then we scan at (400 pixels / 8 inches) = 50 dpi. Remember that "dpi" is Dots Per Inch.
Color Printing and Photography Glossary
Color cast An unwanted overall discoloration or color tinting, of the original, proof, or press sheet, by the trace of some color hue, resulting in colors that look too blue, green or red. Always use standard lighting conditions to view color (in the original transparency, on a proof, or on a monitor) in order to avoid a color cast introduced by the light source.
Color gamut / color space The range of colors captured by (or reproducible on) a device. The CMYK color gamut of a printing press is relatively small compared to the RGB color gamut captured by transparency film or digital cameras. If you're preparing an image for print, make sure you convert the file to CMYK before retouching or proofing. Otherwise, you might be viewing colors that lie beyond the capabilities of the printing press. Also, color spaces within RGB and CMYK vary. Adobe RGB 1998 has a greater gamut than sRGB, for example, and SWOP CMYK is different than EURO CMYK.
Compression Image compression reduces file size. To gain the benefits of compression without losing image quality, the DISC organization recommends the JPEG file format, at a compression level of 8 or higher. A word of caution: repeatedly saving JPEG files as JPEGs can result in degraded image quality.
Density range Also called dynamic
range, is the difference between the lightest and darkest tones as measured by a
densitometer. The density range of a scene captured on transparency film or with
a digital camera is much broader than the density range of a printed sheet. On a
printed sheet, density readings gauge how much ink is on the sheet.
Densitometer In photography, an instrument that measures the density of photographic images or colors. In printing, a reflection densitometer is used to measure and control the density of inks on the substrate.
Dot gain A fact of life in the printing process is that dots print larger than their original values. Normally, prepress compensations eliminate unwanted effects (shifts in colors and tones, "plugging" of dark tones resulting in loss of detail. Influencing factors include screen ruling, paper stock, ink density, ink characteristics and press settings.
Finished file Finishing is a process that standardizes the raw image files captured by digital cameras, in order to make them usable for display or print. JPEG and TIFF are examples of finished file formats. However, some raw file data (dynamic range and color gamut) must be discarded during the finishing process. Deciding how to finish files is an important step.
Gray balance On a printed sheet, gray balance is the perfect combination of cyan, magenta and yellow dots required to produce a neutral gray at a standard density.
Halftone The reproduction of continuous tone images through a screening process, which converts the image to dots of various sizes with equal spacing between centers.
High key image An image that has mostly highlights.
Low key image An image that is predominantly dark areas.
Megapixel (MP) value Indicates the resolution capability of a digital camera, and is a function of image sensor size. A sensor that is 4500 pixels by 3000 pixels rounds up to 14 MP, and can deliver a maximum print size of 15" x 10" at 300 ppi. Go to www.disc-info.org for more information about reproduction quality, image size and MP recommendations.
Metadata Data about data, which is encoded within the digital file. It can include the date of shoot, and photographer name and location, for example. Metadata is becoming increasingly important as a way of tracking image usage and controlling copyright.
Normal key image An image that has equal amounts of highlights, midtones, and shadow.
Raw file A raw file contains all the image information captured by the digital camera, including the full dynamic range and full color gamut of the original scene. Raw files are proprietary to camera manufacturers.
Registration The precise alignment of colors on a printed sheet. Registration marks help position plates in register.
Sampling rate The number of pixels per inch scanned from an original. Converting pixels to halftone dots on a printed page requires oversampling of the input - in other words, more pixels than dots. As a general rule, oversampling of digital input data is done by a factor of two.
Screen ruling The number of lines per inch on a halftone screen.
Sensor Digital cameras capture images by exposing their image sensors to light. With image sensors, size matters: larger sensors can capture more image information (resolution) and deliver a larger digital image file.
Slur On a printed sheet, slur is a printing defect that results in blurred or distorted halftone dots and fine lines. It is caused by movement of the sheet, blanket or plate on press.
Standard viewing conditions Eliminate the influence of ambient light in evaluating color. The color temperature of the light source should be 5000 Kelvin. The viewing environment should be a neutral gray. For complete specifications refer to ISO 3664, available from NPES at www.npes.org.
Tonal compression The reduction of an original image's tonal range to one that is printable with ink on paper.
Trap There are two different types of trapping. Color trap refers to prepress positioning of adjoining colors to avoid fine white lines from appearing between them. Ink trap is the ability to print one ink on top of another ink to achieve the desired color.
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