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The Essence of

Scanning images


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Following on some elements discussed on other pages, just a few basic points regarding the use of your scanner. Some aspects regarding digital cameras may also be relevant, plus some observations on printers.

Scanning -

Most scanners, at least the ''average'' priced ones most Typical flatbed scannerpeople use at home, use a simple process of passing a light source across the image and at the same time digitally scanning each ''line'' of picture as it passes.

Portions of each line are ''read'' as pixels by a CCD (charge coupled device array) usually, and the number of available sensing sections on this determines the resolution available across the image. The motion of the scan head equally affect resolution and in this case, a stepper motor has to rapidly ''stop and start'' the required number of times every inch of travel.

The color value of each dot area (or pixel as it will become) is determined by reading throughprime color filters after being directed through a mirror array - and the analogue levels are then translated into digital, and thus binary values - and if ''full color'' has been selected then this will usually be 24 bit (16 million colors). If however ''grayscale'' had been selected, then shades of black/white will be saved - usually as 8 bit for good gradation, or 4 bit for reduced tonal values. ''Line art'' need only be theoretically 1 bit ... each pixel having a value of simply black or white.Schematic to show essential parts of a scanner.  The scan unit moves from left to right and then returns.

It is rare to find the real or ''optical'' performance of these to be a lot better than 300 to 600 DPI ........ beyond that most make use of ''software interpolation''. However, this is hardly a problem if you mostly scan smallish prints, and as mentioned elsewhere, it is really not necessary very often to scan at much in excess of 200 DPI anyways. Even at this resolution, file sizes get larger quickly!!

The one situation where very high resolution is useful, is if you need to scan a negative (say 35mm ... 1" x 1 1/2") - then of course you are going to want a considerable degree of enlargement. Say we wanted to increase this to a size that will ''physically'' print at 6" x 3", then that is a 6 times increase. If we require for example a minimum resolution of 100 DPI on the finished, enlarged image, then we will have to scan at 600 DPI and when we enlarge, it will become effectively 100 DPI.

Note however - working with film original material requires a dedicated unit using transmitted light instead of reflected.

Considerations -

Always try if you can to decide in advance what you require from your image scan. Is it only for web use??? If so then resolution can be kept lower. In many cases 75 DPI will suffice but it should be noted that JPG compression of this low resolution will cause losses very quickly. I favor often scanning at 200 and pre-cropping so I only get the picture, and not the whole potential size of the scanner bed. I can also, on my scanners, choose to enlarge or reduce at this stage too - remembering of course what this does to the final resolution.

200 DPI scans will compress down further and often get close in final file size to a less compressed 75 DPI ..... and so in many cases look better. The more pixel density there is, the less obvious the effects of compression.

Remember ...... many is the time the resolution of your original material is not all that high ... to scan that at a higher res than needed is wasteful ... on file size and scanning time.

''Best descreening'' is something many scanners offer ....... what is it? This is a method used to help minimize those annoying interference patterns you sometimes get from scans of magazine pictures for example. They will probably be made up of a ''dot matrix'' (half tone) and if this is at a similar ''pitch'' or frequency to the scanning rate, then you will see a pattern, akin to ''Moiré'' patterns. Often the scanner still cannot cope and then it is best to experiment with different scan resolutions ... one will usually be obviously much better than the rest.

Scanner light sources - these are usually very slim fluorescent tubes with a high color temperature (''cold'), to approximate daylight. They have a finite life and I would always suggest switching the unit off between sessions to extend the life.

Anyone considering a scanner and not having had one before, I can recommend getting if you can, a SCSI version. If you already have a SCSI card in your PC then that is ideal, but, many come with a small dedicated SCSI card too. Data transfer is faster with this and, unlike with Parallel port models (was more usual) you will not have to have the potentials problems sometimes encountered from a shared printer port.

Now though with USB 2, serial connection is much more acceptable, giving 1.5/12/480 Mbps. Many scannersand printers are available now with this feature.

The information above is only really an outline, to try and give those less familiar with scanners some idea of what they are about.

Printers after Scanning

If you need to print after scanning, these considerations are important .... covering some of the aspects discussed above but also needing further consideration. For those wanting more information - it is available on a separate page

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