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Scanning
Fundamental HIGH-END and FLATBED scanners differences
 
suggested minimum prior reading - INTRO TO COLOUR, INTERPOLATION

Understanding Flatbed Limitations
Now, we are not talking about producing a nice pamphlet for the local pizza shop here, we are looking at the theory and technology required for serious good to high quality reproduction. The creation of an image and indeed the preparation on a high-end scanner is very different to image creation and preparation on a flatbed scanner. By understanding the workings of High End Drum scanners we can appreciate and perhaps better work with our Flatbed scanners.

The Drum Scanner
Drum scannerThe term "High-End scanner" refers to the very expensive machines used in prepress companies that traditionally output images direct to film. These film images would have the halftone dots applied to them at output and could go straight into the manual preparation stages of Film Combining and Platemaking. High-end scanners have an abundance of controls to affect the result, but probably the most important issues we should understand about these machines is the way in which the output is set up and adjusted before the actual scanning takes place, plus the method of creating actual halftone dots is very different to the way they are created via a RIP and image setter from a bitmap.

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

rotating drum & moving scanning head The image on the left describes:
1. A clear plastic rotatable drum
2. An original fixed to the drum
3. Light source 1 for reflective originals
4. Light source 2 for transparent originals (in drum)
5. The operators viewer
6. The lens system
7. Fine precision threaded track (mechanical, exact)

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.

High End Scanner - 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
The Photo Multipliers are very expensive and not unlike the glass encased Radio Valves used in the early days of broadcast transmission and reception. They are still used to this day because of their very fast reactance times (in electronic terms of speed) and clean signal amplification. At this point the light is converted into electrical values, but the values are analogue - constant variation between high and low values. Many popular older scanners kept the values in analogue form well into the workings of the colour computer and those machines were easy to operate because of the virtually infinite variation of values and feedback the operator had at his/her fingertips on the many controls. A quick visual scan of all the knobs showed how the machine was set up

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
The High End scanners with film recording units then sent the electrical information to sophisticated laser devices. Up to 6 laser beams could be activated and directed towards an output unit basically made up of a large drum that held a sheet of high contrast film securely by vacuum, and an exposing head that moved in unison with the scanning head. As the big drum rotated also at high speed the carefully focused laser beam light is interrupted, or allowed continuation, by simple (relatively) crystals that respond to an electrical signal. A current through a crystal could make the crystal go dark (opaque) and so switch the light directed towards the film on and off. With the drums spinning so fast the switching on and off of the laser is very fast indeed. Look at the small image below and consider the length of exposure (black) for any one of the fine scanned lines making up the dot, remembering that one complete halftone cell (any value) is perhaps only 1/150th of an inch in size for #150 halftone screen output.

high-end dot constructionFor 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.

Rasterised Haltone dots (imagesetter output)The "Image Output Lab - Image Output & Grey levels" pages discuss the very different methods used for the rasterising of digital "Halftone Cells".

Mega squillions !!!
The fully digital era will never match this sort of enhancement for quality work until we have super computers with mega squillions! of memory (a lot more than currently possible) plus unbelievable CPU / component speed. i.e. Currently the industry standard bitmap resolution for reasonable quality 4 colour process work (CMYK @ 150 halftone dots per inch) is 300ppi (pixels per inch). A 4 inch X 4 inch bitmap would have 1,440,000 pixels per channel. Relatively a resolution of 1200ppi would be required (12 pixels per halftone dot, not 2 - horizontally) plus the software/RIP technology to handle the output that match's the detail of the drum film recorders. How many pixels would a 1200ppi image contain for 4 channels (CMYK)?.

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?).

Drum to Bitmap - the Flatbed killer
As mentioned in the third paragraph the modern drum scanner outputs scanned information into digital bitmap form as do the flatbeds. Using the described method of image capture and control, pixel data can be constructed very accurately while image enhancement, sharpening and colour correction etc., is applied to data received before any pixels are created. The opposite is true for flatbed scanners where all enhancement is applied to already digitised pixel data.

 
The Flatbed Scanner
Flatbed scannerCompared 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.

Flatbed Scanner - diagram

The Filters
RGB FiltersThis is a sample RGB filter holder that is moved back and forth in turn separating the reflected image into 3 channels, section by section until the required image is constructed. Also used amongst scanner manufacturers are various kinds of filter wheels that perform the same task as the device shown.

The Lens and Lens Housing
Lens HousingA light tight cover encapsulates the CCD Array and holds the factory focused lens system in place. The sample I used from an old scanner also shows the circuit board the lens and CCD array housing is mounted on. NONE of these components are user accessible - meaning if little fingers start fiddling with them, the carriage system or mirrors etc., serious focus, alignment or colour shift problems will probably result.

A CCD Array
Linear CCD ArrayA Linear array refers to one single line of CCDs. Some modern scanners, like digital cameras and movie cameras use multi line arrays. With this sample angled to the light correctly I could barely see the tiny circuitry supporting the CCDs when viewed with a 20X magnifier. They are very complex, delicate and expensive devices so do not thump the scanner (or computer for that matter) when "doing the block"! Hit your own head instead - it is probably less complex and costs less - oops!

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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