Chapter I, Digital Imaging Fundamentals: Lesson V Output http://www.kodak.com/country/US/en/digital/dlc/book3/chapter1/digFundOutput1.shtml

In the Capture Module we saw how the image sensors in the scanner capture the brightness values of a film image one-line-at-a-time.

In this module we will see how, at the other end of the digital imaging chain, print heads use this digital information to output digital images one-line-at-a-time to paper or film.

Before we look at digital printers, we need to clarify some confusing terminology about digital resolutions.

PPI or pixels-per-inch, refers to the spatial resolution we discussed in the Capture Module. An image scanned at 100-ppi has lower spatial resolution than an image scanned at 300-ppi.

DPI or dots-per-inch, refers to the size of the spots created by the output devices we will be discussing in this module. For example, a diagnostic image output at 2,400 dpi on a film recorder has a much higher resolution than an image printed at 300 dpi on a laser print.

LPI or lines-per-inch, refers to the lines on the screen used to create halftones in the graphic arts industry. For digital images eventually going onto the printing press, there are issues relating to ppi and lpi, but that is beyond the scope of this course.

Now let's look at the confusing variety of digital printers on the market today. Digital printers can be divided into two basic groups: halftone printers and continuous tone printers. Through a process called dithering, halftone printers group pixels to simulate continuous tone gray scale, or a continuous range of color. Remember, in Module 1 we saw how shades of gray can be simulated by black and white pixels.

For color printing, these printers increase color range by grouping dots of cyan magenta and yellow to simulate a third color. For example, alternating cyan and yellow dots look like green.

Since continuous-tone printers blend the colors as they are printed, they do not have to use dithering or halftoning. That's why thermal prints with a resolution of 300 dpi can be superior to a 600 dpi halftone print.

Before we take a closer look at halftone and continuous tone printing, we need to understand the role that page description languages and raster image processors play in digital printing. Through a process called dithering, halftone printers group pixels to simulate continuous tone gray scale, or a continuous range of color. Remember, in Module 1 we saw how shades of gray can be simulated by black and white pixels.

A page description language, like Postscript, is a programming language used by computers to describe the text, vector graphics and bit-mapped image content of the printed page.

The raster image processor is a hardware/software device which converts a page description file into a pixel image that can be printed.

Every printer is composed of two basic parts Every printer is composed of two basic parts. The front-end communicates with the host computer, performs image processing and controls the printing process. The print engine places dots of toner, ink, wax, or dye on the paper.

In Postscript printers, the front end is the raster image processor, commonly called a RIP.

In raster-based printers, the front-end is the controller which may also perform image processing functions, such as scaling and rotating.

Let's take a closer look at how halftoning or dithering works. Black & white halftone printers cannot vary the density of dots they print. They have only two choices: either fill a pixel with toner to make it black, or don't fill the pixel to leave it white.

To simulate shades of gray, these printers group pixels into a matrix, called a halftone cell. A cell containing many black pixels looks darker than a cell with few black pixels.

Color halftone printers use the same dithering technique, by grouping pixels of cyan, magenta, yellow, and black.

With digital halftone printers, there's a tradeoff between the spatial resolution and the number of gray scale levels which can be rendered.

Using a 2 x 2 matrix of pixels to create one halftone cell, the printer can render white, black and three shades of gray.

To increase the number of grays, the printer uses a 4 x 4 matrix, but this decreases spatial resolution.

Kodak: Digital Training: Output Module-Thermal Dye Diffusion Printers In thermal dye diffusion printers, red, green and blue pixel values control proportional values of cyan, magenta, and yellow dyes. Image data can be further enhanced for maximum sharpness and color control.

Thermal printers use heat and pressure to transfer dyes from a ribbon containing cyan, magenta and yellow dyes to a special thermal paper.

A linear thermal head, containing thousands of resistive heating elements, exposes the image pixel by pixel, line by line.

Each element can be pulse-code modulated to render 256 different levels of cyan, magenta or yellow dye which can combine to create any of 16.7 million colors.

By controlling the dye density of each pixel, dye diffusion produces 24-bit continuous tone images.

Another type of continuous tone printer is the film recorder used for making presentation slides, photo retouching, high-end printing and medical imaging applications. Film recorders send a laser beam through an optical system to produce an image, dot-by-dot, on photosensitive film or resin-coated paper, which is then processed photographically.

With recent developments in digital image capture, processing and storage, more and more images are joining other digital data transmission across computer networks, telephone lines and via satellite. For desktop publishing projects, digital images are being shared by teams working in the same building, via local area networks such as Ethernet, AppleTalk and Token Ring.

Stock images are being transferred between photographers, agencies and printers across town and across the country by linking computers via telephone lines.

News photos are being transmitted around the world via telecommunications networks that involve telephone cable, broadband coaxial and fiber optic cable.

Diagnostic images are being transmitted to specialists around the world via satellite communication links.

As transmission lines are upgraded to fiber optics, digital images are increasingly transmitted as on/off pulses of laser light at hundreds of millions of pulses per second.

The global telecommunications network is gradually being switched over to a totally digital format called the Integrated Services Digital Network. On this network voice, data and images will all be sent and received digitally.