When discussing electronic images, you must become familiar with the term pixel. The word pixel means picture element. A pixel is the smallest picture element that a peripheral device can display on screen. It is these individual elements that construct the image. The quality of an electronic image is determined by the pixels of the image-the more pixels per image, the greater the quality of the photograph.

A pixel is not an absolute unit, such as a millimeter. A pixel depends on other factors to determine its resolution. The image resolution depends on the number of pixels in a specific area. For example, a 1/2-inch square area containing 270,000 pixels has better resolution than a 1-inch area having the same number of pixels. However, resolution is not based solely on the number of pixels.

The resolution of electronic images depends on the size and the total number of individual pixels used to depict a single image. Each pixel can contain only one color or shade of gray. The smaller the pixels (more pixels per image), the closer the image can be viewed before the individual pixels are seen. Likewise, larger pixels may be objectionable. By increasing the number of pixels, you obtain finer detail.

When thinking in terms of resolution, you cannot compare pixels to the grain in film. Film resolution is not directly comparable to electronic-imaging resolution. Additionally, different types of imaging hardware use different types of measurements. For example, some equipment describes resolution in pixels and other equipment describes resolution in dots per inch (dpi). Pixels and dpi are not directly related or interchangeable.

Another problem in terms of resolution is there are no established conversion standards for images captured on different forms. An example where nonstandardization may present a problem is when comparing images from an analog camera to the products from a digital camera or the image output from an electronic printer or a film recorder.

ELECTRONIC-IMAGING WORKSTATION

Electronic imaging involves more than simply taking a photograph with an electronic camera. Like conventional photography, exposing the film is not enough. After the image is captured on film, it must be processed before the image can be viewed as a positive transparency or as a negative image that must later be printed to provide a useable positive image.

Images that are generated electronically must also be processed, but the methods to make the image visible and useable are completely different. On an electronic-imaging workstation, your "darkroom" is a computer system. To operate an electronic-imaging workstation, you must have both hardware and software. There are basically five components in an electronic-imaging system; they are the following: some type of input, a computer platform and software, display, storage, and hardcopy output (fig 3-3) .

Figure 3-3.-Five basic components to an electronic-imaging system.

One major problem you will encounter when setting up an electronic-imaging station is interconnecting the various components that make up the imaging system. The technology in each component area is developing at a rapid rate. With the ever-increasing number of hardware components and software packages available on the market, setting up the links between the devices can become frustrating. Before choosing new components for a system, you must look carefully at each piece of new equipment to be sure it is compatible with the existing system.

INPUT

Several ways to acquire photographs electronically are used. You can obtain these photographs from digital or still-video cameras, or you can scan and digitize existing film and paper images.

Before a computer with the appropriate software can be used to modify or enhance an image, the image must be converted to digital values. Images that are imported from a still-video camera are in analog waveform. An analog waveform is a value that varies continuously over time fig. 3-4 . For an analog waveform to become a digital signal, both the value and the time must be changed into noncontinuous, numeric values of ones and zeros fig 3-5. The process used to determine time is called sampling. Sampling is done at equal increments of time. Conversion of continuous values into distinct values is called QUANTITIZING. The combined process is called analog-to-digital conversion (A/D conversion) or DIGITIZING.

The A/D conversion process is an approximation. When the sampling rate is low, a very inaccurate representation of the signal results. When the sampling rate is high, virtually an exact copy of the original signal is attained. When color images are digitized, the red, green, and blue information is handled as three separate sets of data to produce three sets of digital information.

Figure 3-4-Analog waveform.

Figure 3-5.-Digital signal.

In this case, three A/D circuits are used and the encoding is done simultaneously.

When an image is digitized, a series of points are created. These points are called pixels. When the resolution of the display system is low, the individual pixels may be noticeable. This objectionable resolution is called pixelation.

Electronic Still Cameras

The advantage of using a digital or still-video camera is the image may be captured and inputed to the electronic-imaging workstation instantly. The features on these cameras are basically the same, and they are used in the same manner as conventional cameras. The features of conventional cameras and electronic cameras that are similar are as follows:

The lenses may be fixed or interchangeable, depending on the camera.

The lenses are identified by f/stops and focal length.

The focusing may be fixed, automatic, or manual, depending on the camera.

The range of shutter speeds is similar.

The flash may be built-in or have a dedicated hot shoe.

Each electronic camera has an image sensor. The image sensor, called the "charge-coupled device" (CCD), is the main component of an electronic camera. The CCD is rated in size, pixels, and ISO. The larger the CCD, the more pixels it can record, thus the higher the resolution. However, the resolution quality and the exposure range of an electronic camera is not as great as what can be achieved with film.

Electronic cameras use one of three devices to store images. These three devices are a 2-inch video floppy disk, a hard drive or Random-Access Memory (RAM), and an integrated circuit (IC) card or chip.

In the 1980s, the still-video camera was introduced in the Navy. The still-video camera uses a 2-inch video floppy disk capable of recording 50 or 25 images. The number of images that can be recorded on a floppy disk depends on whether the image is recorded in the field" or "frame" mode.

The FIELD MODE uses one track per image on the floppy disk and allows 50 images to be recorded on one disk. The field mode provides poorer resolution because there are less pixels per picture. The FRAME MODE uses two tracks per image and allows 25 images to be stored on one floppy disk. The frame mode provides higher quality because more pixels per image are recorded.

Still-video cameras use an analog signal to record the images. This variable waveform represents density and colors that are created electronically by the intensity and the color of light striking an image sensor within the camera. This analog signal is the same type of signal used to record most motion-video images. It is also the same type of signal used in conventional television. Many still-video cameras have a playback capability and may be connected directly to a television monitor to view the images.

An analog signal records the lowest resolution of the electronic cameras, thus using the least amount of memory per image. Most still-video cameras have a limited resolution of approximately 380,000 pixels.

A still-video camera used by the Navy is the Sony Pro Mavica MVC-5000. This camera has three CCD chips that are used as the image pickup device and the high-band format to improve resolution.

An important factor to remember is that a still-video camera is an analog system, not a digital system. The format and configuration of a still-video floppy disk is different than that of a computer system, which is digital. By using the appropriate hardware and software, you must convert an image captured on a still-video camera from analog to digital format before it can be modified or printed in a digital-imaging system.

Still-Digital Cameras

A still-digital camera is superior to a still-video camera. As the name implies, a still-digital camera

records an image in digital format. This digital format uses the binary system of "Os and Is." The combination of these digits represents densities and colors created electronically by the intensity and the color of light striking an image sensor within the camera.

A digital image has a much higher resolution than an analog image. This higher resolution provides more pixels per image, but it also requires much more memory per image. Digital cameras use an IC card or chip and RAM or a hard drive. The RAM is built permanently into the camera and must be downloaded to another storage device. This storage device is an internal or external hard drive. This hard drive is similar to the hard drive found in personal computers. Kodak's Digital Camera System (DCS) uses a hard drive to store images.

The Kodak DCS still-digital camera combines a Nikon F-3 body and a standard lens with a digital-image back to capture high-resolution color or black-and-white images. The Nikon body operates similar to a camera with conventional film. The major difference between the Nikon being used with film compared to the DCS back is that the image area of the DCS is only one half of the size of a 35mm-film frame. This change in image area affects the effective focal length. For example, a conventional 35mm lens becomes a 70mm lens with the DCS. The Nikon F-3 functions, aperture settings, shutter speeds, and light metering operate the same as with film. Three major components that make up the Kodak DCS are as follows: an electronic back, a camera winder, and a digital storage unit.

Kodak's DCS models have a digital-image back that contains a 1,280 by 1,024 pixel CCD imager. This means the CCD is capable of recording about 1.3 million pixels. The color back equates to film speeds of 200,400, and 800. The monochrome back equates to film speeds of 400, 800, 1600, and 3200.

With a winder (spooler), you can shoot up to 2.5 images per second. The system has a standard 6 megabyte (Mb) buffer that can store six images in one burst. Thus it is possible to shoot faster than the images are stored.

The camera body of the DCS 100 is tethered to a Digital Storage Unit (DSU) that contains a hard drive. The 200Mb hard drive can store 158 uncompressed images or about 600 compressed images. The DSU also has a key pad for system control and a 4-inch monochrome monitor so you can view the images immediately.

The Kodak DCS 200 is a modified Nikon 8008 equipped with a Kodak DCS 200 camera back fig. 3-6. The DCS 200 is capable of 1.54 million pixels of resolution. It also has an internal hard drive that can store up to 50 images. A small "hitchhiker" 40Mb hard drive is also available. This 40Mb hard drive plugs directly into a small computer system interface (SCSI) port on the camera for additional image storage. The SCSI is pronounced SCUH-zee.

The advantages of the Kodak DCS 200 over the Kodak DCS are higher resolution and portability. It is more portable than the DCS because it has a built-in hard drive. The disadvantages of the DCS 200 are it stores less images than the DCS 100, and it has no built-in compression or transmission capabilities.

One of the biggest advantages of capturing an image digitally is the way the images can be processed. A digital image may be re-recorded without loss of image quality and the color and sharpness can be enhanced. This digital signal is identical to the signal used in a computer; thus, by using the proper interface, the signal from a digital-still camera can be imported directly into a computer.

Figure 3-6.-Kodak DCS 200.

Scanners

Film transparencies, negatives, and prints are sources of images that can be produced and edited electronically. Scanners can create digitized images with extremely high resolution. Scanners are also capable of providing resolution equal to the original negative or print.

Scanners come in three categories: rotary drum, flatbed, and film. Rotary-drum scanners provide the highest quality for converting images from film or prints, but they are very expensive. Rotary-drum scanners are capable of producing resolution ranging from 1,000 to 5,000 dpi. When a rotary-drum scanner is used, the film or photograph is placed on the surface of a drum that rotates while the original is scanned by a single beam of light. The beam of light and the speed of the drum can be adjusted to control the amount of resolution desired.

Scanners that use charge-coupled devices (CCDs) provide excellent quality. They are used in many Navy imaging facilities. Scanners operate similar to a photocopy machine. A CCD chip with a row of light receptors scans a photograph or negative and changes the colors or shades of gray (analog signal) into digital values.

Full-color scanners have three rows of CCDs: one for red, one for green, and one for blue. This tricolor array permits full-color scanning with a single pass of the scanning head. The number of elements in the CCD array determines the resolution of the images being scanned. For example, an 8.5-inch linear array with 2,540 elements has approximately 300 elements per inch. This array can produce a digitized image with a resolution of 300 dots per inch (dpi). Most standard desktop scanners operate in the 300- to 400-dpi range. When an image is scanned on a scanner that produces 1,000 to 5,000 dpi, a higher resolution results, but the scan time and file size also increase. Generally, the resolution required for a scanned image is limited to the output of the imaging system.

A flatbed scanner is used for scanning photographs and artwork. Some flatbed scanners are also capable of scanning transparencies and color negatives. The resolution of flatbed scanners range from 200 to 1,200 dpi. Unlike a rotary-drum scanner, a flatbed scanner scans an entire line at one time with a linear CCD array.

Film scanners are used to scan negatives and transparencies. Many of these scanners come with software packages that allow you to crop the image and make color corrections before the image is scanned into a digital file. By making these adjustments before scanning an image, you can save time and file size. Scanners are produced by a number of manufacturers. Scanners used in Navy imaging facilities are produced by Nikon and Kodak (fig. 3-7).

Once an image is converted to digital format, the data is passed from the scanner to the computer through an interface. Because of the enormous amount of data involved in electronic imaging, information is passed through a SCSI.

A SCSI is a special kind of parallel interface that allows for faster data transmission. The SCSI interface permits a number of peripheral devices to be connected to the computer through a single SCSI port. This is accomplished by chaining the devices together with a SCSI cable. The last device in the chain must have a special adapter, known as a terminator. Some devices have built-in terminators.

Each device in the SCSI chain is identified by a unique identification number. When the scanner is connected, you must verify that none of the peripherals have the same identification number. These identification numbers may be changed by using either dipswitch settings or software.

The scanner uses two programs to operate. One program is a paint program to manipulate the image once it is in the computer. The second program is a driver that acts as a translator between the scanner and the paint program. Once the image is digitized,

Figure 3-7.-Kodak 7720 thermal-dye transfer printer.

limitless modifications and enhancements can be made to the image.

Because a scanner scans at such high resolution, the end file is quite large. Thus a considerable amount of storage space is required. It is common for a 24-bit color image to require 20 to 25 megabytes of storage.