|Fundamental HIGH-END and FLATBED scanners differences|
suggested minimum prior reading - INTRO TO COLOUR, INTERPOLATION
The Drum Scanner
Most high-end scanners today are used for bitmap image output linking in with or bridging to the digital desktop environment, and now replacing them is the smaller machines referred to as Desktop Drum Scanners. Without the film recording units, desktop drum scanners can be more compact, however their method of image capture is much the same as on the high-end machines
A very fundamental difference between Drum Scanners and Flatbed machines is in the ability of the operator to study any/all areas of the original on a drum scanner, directly from the original and make adjustments to the set up and colour controls as necessary. The operator is working in real time with the light transmitted from the original and can fine tune focus etc. etc.. The operator manually moves the viewing/scanning head and drum to do this and responds to a digital display or bargraph type information displayed on a monitor. No colour information, other than pre-scan correction settings, is recorded at this stage. It is all prepared directly from the original.
Once set up for output the machine spins the drum at high speed, moves to align the head with the original and then, via the mechanically accurate threaded shaft, scans slowly across the original as the drum spins. Variation to output size can be assisted with variable sideways carriage movement speed, different lenses for various reproduction ranges and different drum sizes (circumference). As the image is scanned the information is fed into the colour computer - described below aided by the second diagram.
The light reflecting off or transmitting through the original passes through an adjustable lens system, and when setting up, is transferred to either the viewer or towards the colour computer. To get to the colour computer the light from the lens is directed towards a prism (see Bitmaps and CMYK process - Filtering and Separations) that spreads the spectrum of visible light accurately on devices called Photo Multipliers.
Analogue to Digital
For digital output and control of the thousands of components the analogue values are converted to any one of a set number of digital values. Most components work with the common 8bit (256) counting system where as today the colour depth range for the colour computers can be very high indeed (48bit+) because of the availability of 32bit and 64bit microprocessors.
Every Halftone Dot is different
For what is called high resolution on a film output recorder, up to 6 laser beams (standard resolution) doubled to 12 could make up the shape of just one halftone dot. Each beam of light could be separately switched on and off. The result is a halftone dot that is not truly round, square or elliptical but also contains the smallest picture detail directly from the original (viewing the dot under a microscope). Naturally this has to enhance the result even after the printing press has "splodged!" the dots a little.
Output Lab - Image Output & Grey levels" pages
discuss the very different methods used for the
rasterising of digital "Halftone Cells".
Mega squillions !!!
For today's digital systems the Stochastic Screening technology has offered some improvement in the areas of apparent detail enhancement, but so far has proved very expensive and difficult to implement needing a super controlled environment (don't bother using the 15 year old well worn press - and where are the platemaking skills of old?).
to Bitmap - the Flatbed killer
The Flatbed Scanner
Compared to a drum scanner, a flatbed scanner has very few components.The operator can scan a preview (which is converted data - analog light to digital), and with the more sophisticated scanning preview software, make adjustments to the result - for an improved result! Confused, ...hmmm. I think I know the point I am making so I will try to enlighten you. It is important to catch on and understand - why only flatbeds costing Au$15,000 to Au$30,000 can start to come close to drum scanners or - when to use your el cheapo flatbed and when to use a prepress scanning service.
To begin with look at the purposely simple diagram below. A copy is placed on the glass top of the flatbed scanner, the light source illuminates it and the reflected or transmitted image traverses the path to a lens via a redirecting mirror(s) and an RGB filter system. The copy glass is a minimum of 81/4 inches wide for a Letter sized scanner and the lens reduces this to cover an array of Charge Coupled Devices that, depending on the strength of light falling on each one, vary at their output a constant input current.
Lens and Lens Housing
The focal length of the lens does not alter to suit the operators requests for enlargement or reduction, in fact nothing changes from copy to CCD array (a number of very expensive flatbed scanners do use two or three lens, each covering different ranges of reproduction only).
So even when we produce a preview scan to check cropping and, with more sophisticated software, make colour adjustments with eyedropper tools etc. we are working on INTERPOLATED bitmap information that has already gone through the 'guesswork' system - as apposed to a Drum scanner where the operator works directly with light and the original. Having read the page on interpolation you should now understand that the pre-scan image is not exactly like the copy and neither will the final scan because clever as most of it is, interpolation is guesswork.
We would have to consult the engineers that design each machine to know for sure, but hypothetically a scan at 100% reproduction (same size output) at an image resolution matching the number of CCDs per inch exactly WITH an array of CCDs that matches the width of the copy holder would be the best image obtainable from a flatbed scanner. The reasoning here is that no interpolation should be used. But how many copies are reproduced at same size using a full width array, plus each image file would be considerably larger than needed for postscript output thus overloading the computer and imaging system.
Another serious drawback with CCDs is their weak response to variations of low light levels, unlike the fairly linear response of Photomultipliers. Advances have been made in this area in the last year or so but will remain a weakness for some time. What this means in simple terms is that flatbeds reproduce shadow detail, subtle changes in image contrast in dark colours, very poorly.
The Colour Depth page will describe just what "colour depth" refers to, an important issue that needs to be addressed when obtaining a flatbed scanner.
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