Nowadays desktop scanners are perfectly able to carry out many tasks which were previously only possible using expensive professional drum scanners. This booklet confines itself mainly to the realm of the desktop scanner of which there are currently five main categories:
hand-held
camera
sheetfed
flatbed
slide
Desktop drum scanners are also now available, but unfortunately their current price-tag will be prohibitive for most in-house work. In addition to the types of desktop scanner listed above, various multi-functional devices which are a combination of scanners / printers / copiers / fax machines and telephones are becoming more common.
The arena is expanded still further with the advent of Photo CD technology which is able to bring together the worlds of photography and digital imaging.
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{3}Hand-held scanners
Hand driven type
Hand-held scanners are the most common type of scanner in the marketplace and are relatively inexpensive. The hand driven type rely on a steady hand to pull the scanning head accurately across the material being scanned, although they are usually equipped with a sensing device to compensate for slight speed variations. With the capacity to distinguish anything between 16 and 256 greyscales and from 256 to 16.7 million colours, they have output resolutions ranging from 100 dpi to 400 dpi.
Hand-held scanners are often used for importing small images into the computer environment and are normally offered with entry-level OCR and image-editing software.
{4}Window type
Window type hand-held scanners try to overcome the need for manual precision by positioning a picture frame device over the object to be scanned. A linear sensor is driven across the image within this frame and the resultant image data is sent to the computer.
A common criticism of both types of hand-held scanner is that the scanning head is too limited in width to cope with large images. A partial solution to this problem is to use stitching functions to 'stitch' two scans into one image.
{5}Cameras
Video cameras
Still video cameras, conventional video cameras and camcorders can also be used as scanners. They offer the flexibility of mobile and remote analogue recording onto magnetic disk or tape, without the need for a computer to be present.
Video cameras which can connect with computers have the advantage that they are able to work with three-dimensional objects, but are less suitable for scanning two-dimensional images such as photographs and transparencies because of the limited resolution available. The video camera is connected to the computer by a video grabber (digitiser) board which converts the video image into a single frame. This frame can be 'grabbed' at the press of the appropriate button.
{6}The process of converting analogue video signals (PAL, SECAM, NTSC) to a digital frame or bitmap means that the final image quality may not be as good as that produced by a purely digital device. Thus, although the resultant scanned images are adequate for most TV, video or computer display use, they are not usually up to the standard required for quality printed material. Video cameras can be mounted as overhead scanners to scan images of whatever is placed in the subject area below the lens.
Still video technology has a history stretching back to 1984 when the first still video images were demonstrated at the Los Angeles Olympic Games.
However, due to its relative expense, it was not until recently that this type of camera found its functional niche in the production of high volume small ads., videos, presentations and as a cataloguing, image storage and database tool.
{7}Canon's ION RC 560 has a CCD sensor (See How scanners work) of about 475,000 pixels which means that it can be used to capture images of up to 3" x 4" for use in typical page layouts. Above this size, the resolution becomes too low for quality printed reproduction.
Digital Cameras
Available in both monochrome and colour versions, digital cameras use a matrix array CCD (see How scanners work) to achieve resolutions typically as high as 2,048 x 2,048 pixels.
Although most desktop scanners can achieve higher resolutions with A4-sized subjects, digital cameras offer more flexibility in mobility and range, especially if they are equipped with bayonet-mounted lens attachment systems.
{8}Digital cameras normally take the form of a modified professional camera with the back containing electronics, a CCD, hard disk storage and power supply instead of the normal film compartment. However, high resolution desktop overhead versions are available with resolutions comparable to those of flatbed scanners.
The beauty of digital cameras lies in their ability to integrate professional standard photography with high quality digital processing. This does not come cheap - with price tags from ú10,000 to over ú30,000, digital camera technology is, at the time of writing, beyond the budget of most photographers.
Digital cameras are aimed mainly at the Photo Journalism market where speed is of the essence. Photographers can use a digital camera to scan or take photographic images, then compress and store them on a portable hard disk unit for transmission directly to the news room. Other current applications include sports photography, catalogue production, forensic work and military surveillance.
{9}Sheetfed scanners
Sheetfed scanners work in the same way as fax machines. The document to be scanned is fed through a system of rollers similar to a sheet feeder on a printer while the sensor and light sources remain fixed.
Sheetfed scanners are usually relatively inexpensive monochrome devices. They can have anything from 16 to 256 greyscales available and usually scan at 200-300 dpi. They are at their best with similar shaped sheet work in an OCR, fax or archiving application. The design of their paper-handling mechanism obviously renders them unsuitable for scanning photographic or film originals.
{10}Flatbed scanners
Flatbed scanners are available in both colour and greyscale versions, and have become the standard configuration for DTP, OCR, archiving and many other types of work. Flatbed scanners work very much like photocopiers. You place the original face down on a piece of glass, typically A4 in size, then a travelling light source passes under the glass, scanning the image. (The next section How scanners work provides more detail).
Flatbed scanners are currently available with optical resolutions between 300 and 600 dpi. The output resolutions from the scanners may be even higher because many scanners incorporate on-board intelligence which allows them to interpolate to higher resolutions before they pass the data to the host computer.
{11}With flatbed and sheetfed greyscale scanners, illumination is likely to be a full spectrum fluorescent light source. Colour scanners use either three strobed coloured fluorescent lights (red, green, blue), a system of filters to change the colour of a single light source or a prism that splits the single light source into its red, green and blue components.
Single-pass scanners are usually quicker as they only travel up and down the bed once. Single light source scanners that rely on filters have to travel up and down the bed three times for a full-colour red, green and blue image.
{12}Slide scanners
Slide scanners which are purpose-built to scan 35 mm slides are now becoming more readily available in the marketplace. They need a high optical dpi resolution (1,000 to 4,000 dpi) since the resulting images usually need to be enlarged for reproduction in publications.
Specialist slide scanners are relatively expensive, currently costing up to ú10,000. There are a few transparency scanners available which can handle images up to 4" x 5" although these are usually more expensive still.
{13}Photo CD
Photo CD is a standard developed by Kodak and Philips for storing photographic images on compact disc (CD). It is included in this section because it is a technology which offers wide scope for storing and retrieving scanned images.
Nowadays when you take your holiday snaps to be processed, you have the option of getting your 35 mm film processed into digital format onto a Photo CD. You can then display your photos on your television screen using a special Photo CD player which also can be used as a conventional audio CD player. Photo CD film processing usually costs considerably more and takes longer than standard film processing.
{14}The Photo CD format stores five versions of each image, bundled together in what is termed an 'Image Pac'. There are low and medium resolution 'thumbnail' images, a 'base' resolution suitable for TV display and two higher resolution versions called 'base x 4' and 'base x16' images.
These images can also be read into a computer equipped with appropriate Photo CD hardware and software. Each disk can store around 100 images, and 'multi-session' technology means that these 100 images need not all be written at once. The base resolution of Photo CD is 768 x 512 pixels in 24-bit full colour which is about a 1.1 MB of data - adequate for display on most computer systems.
{15}The resolution of the x16 image is 3,072 x 2,048 pixels, about 18 MB. With a virtually lossless compression technique (see the File formats and storage section) about 100 Image Pacs are saved on a 600 MB Photo CD.
A higher resolution format, called Pro Photo CD, is being launched at the time of writing. This will include a new base x 64 image (72 MB file) which will allow medium format (6 cm x 6 cm and 5" x 4") negatives and transparencies to be stored in addition to the standard
35 mm format. The advent of Pro Photo CD has extended the application of Photo CD even further into the pre-press market.
