iXBT Labs - Computer Hardware in Detail






Scanners and Photographer

The active development of digital technologies has taken place for the last decade. The majority of printing houses has changed over only optical methods of preparation of photos for printing to electronic methods. All these changes have entered a new word "scanner" into a lexicon of many photographers.

However from all computer devices used by the photographer scanner is the oldest invention. The systems for scanning image are an integral part of such devices as a telephoto, telefax, television camera and they exist for already more than hundred years.

In 1855 the Italian physicist Kazelli has invented the instrument to transfer image called "pantetelegraph". In this instrument the needle scanned the image drawn by current-conducting ink. With the invention of a photocell the telephoto in which the thin light beam moved on a surface of the photo supported on a drum was made. Light reflects from the surface of image and gets on the cathode of a photocell causing an emission current proportional to reflectivity.

In the beginning of the century the German physicist Korn made the telephoto which doesn't differ from modern drum-type scanners. (In the figure on the right you can see the circuit of Korn's telegraph and the portrait of the inventor scanned and transmitted on the distance more than 1000 km on November 6, 1906). Mechanical scanning of image on two coordinates takes place in it and each dot is separately lit. Light passing through it is received by one selenium photodetector and therefore there is no error connected with unidentity of pick-off. It is the oldest and the most qualitative but also the most expensive way. It has no basic limiting on number of dots the image will be composed of. However scanners of this type demand fixing the image on a drum and therefore do not suit scanning not bending glass plates.

The development of semiconducting technologies has allowed to unite some photodetectors in one bar and to do with relocating only on one coordinate. It has lead to the birth of tablet, rolled, projection and hand-held scanners. Their optical circuit is absolutely identical and can be represented as an objective focusing the line of image on the photodetectors bar. The difference is in the way of relocating a photo, photodetectors bar and an objective. An objective and a bar of photocells are usually hard connected and transfered about the photo. Resolution of similar devices is limited by a number of pick-off in the bar and if the width of a photo is less than a working surface of the scanner, only part of photocells is used. In some projection scanners and studio digital cameras there is relocating bar of photodetectors about the image formed by moveless objective. Projection scanners allow to focus the object on all width of the pick-off bar and thus without dependence from the size of the image to receive the greatest possible resolution.

In the extreme case the mechanical relocating of a photodetector and light source is absent and there is an electronic scanning of the whole image. The television cameras and a majority of digital cameras can be reffered to similar devices. The devices at price up to $1000 usually have a matrix keeping no more than a million pick-off. There are also hybrid devices in which the matrix makes microrelocatings thus first all even points and then all uneven ones are read out.

Colour scanners don't differ from half-tone and image input is made through colour filters in three doses when at each passage before a photodetector the blue, green or red filter is installed or have three pick-off before which the appropriate filters are posed.

What scanner to select is defined by what you are going to use it for.

For the photographer a construction of the scanner is not so important as its characteristics which are defined by resolution capability (number of dots per unit of image square), number of shades which can be represented numerically (24, 30, 36 bits on a dot depending on the used analog-to-digital converter) and the range of densities depending on a dynamic range of the system illuminator-photodetector. It is necessary to note that the variation of intensity on three orders can be represented by 16 shades and therefore it is possible to make the scanner which has only 4 bits for a dot and the intensity is three.

If a scanner for you is auxiliaries in daily work of a photographer and it is used for creation of photoarchive and check review of negatives in real colours then the device for scanning transparent and opaque stuffs about a card suits you by size. Cost of similar equipment with permission of 400 dots per inch is today less than $200.

It is necessary to mark that the hand-held scanners allow to solve the majority of problems connected with digitisation and to scan the image on transparent basic by some resourcefulness and availability of blueprinting table.

The tablet scanners are the most universal. They can be used not only for scanning photos but also together with the printer for copying documents or together with the appropriate programs for recognition of the text and input it in ASCII format in the computer. Today optical resolution of scanners cost about $300 are 600 dots per inch. The majority of them can be completed by console for scanning transparent stuffs.

To evaluate real possibilities of equipment it is necessary to remember that at polygraph printing and printing on the printer halftones are made by variation of width line or dots the raster consists of. The printing quality is defined by number of raster lines per inch (lpi). For determination of required resolution in dots per inch at scanning you should clarify what index lpi the printer has and multiply it by 1,5. At printing newspapers the parameter lpi is 85, for laser and colour ink-jet printers this parameter is varied from 56 up to 133. At high-quality printing the number of lines per inch is usually 133 thus you can without loss of quality print out a photo by size 10x6 cm in a magazine from scanned on the tablet scanner 35 mm frame. If you have possibility to make a print by size about all working surface of the scanner, you can print in a printing house your photo of width 24 cm on a page with the side 70 cm. It is necessary to mark that the posters are usually typed with the raster in 35 lines per inch and therefore your photo can be rendered on a four-meter panel poster. For the photographer the operation with prints has also the advantage that magnifying an exposition it is possible to have normal prints even from very dense negatives while the dynamic range of scanners is usually much less. The exception is drum-type scanners in which photo-electronic multiplier is used and the brightness of a source can vary. However scanning on them is the same skill as printing of photos.

The scanners for films 35 mm are largely designed for the reporters and allow them to get and pass stuff to the editorial office rather fast and not spending time on printing. The devices with permission in 2000 dots per inch cost about one thousand dollars. If you have the necessity to input slides of the different sizes or their fragments into the computer, then probably the best decision for you will be the digital camera.

However the scanners interfere and radically change not only procedure of the photographer operation with snapshots but also become the separate instrument of creativity.

The circle has become isolated. On a new turn of a spiral we have returned to phonogram in its electronic creation. We can create rather expressive compositions just by setting some subjects on the glass of the tablet scanner. The depth of the sharp image for scanners is insignificant but nevertheless it is not zero therefore we receive an original view from the glass underground. And this method unlike phonograms allows to make not only abstract - shadow composition but also to receive quite realistic snapshots of flat subjects - coins, sections of stones, leaves...

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