Scor­pio News

ruler is someway down the picture. Use a pencil or run your eye along the edge of the ruler. At each millimetre transition make your mind up if the area you’re looking at is black or white. Ok, so some points will be difficult, about half a millimetre will be white and half black, either way, make your mind up, it’s either white or black, ‘don’t knows’ are not allowed. So as you ‘scanned’ along the line, you end up with a pattern of blacks and whites. Move the ruler down a shade and do it again. This is what the scanner sees. Black and white dots, and its output is electrical, in binary form representing the blacks and whites,

Now I’ve heard a lot of twaddle about scanning resolution. If you tried it yourself using a ruler, then that was at a pitch of 1mm, That’s about 25 dots to the inch. Scanning resolutions are at worst 75 dots to the inch up to some real hairy scanners which work at 2,400 dots to the inch or more. Typical for imaging work is about 200 to 400 dots to the inch, scanned along the line, and the paper or scanner advances down the paper at the same rate, 200 to 400 lines to the inch. So for each inch square, there are some 40,000 to 160,000 dots. 300 dots to the inch is commonly used as being a trade off between resolution and speed. It’s good enough to reproduce the finest detail the human eye is capable of seeing unaided and the amount of information produced is not so large as to be unmanageable. Of course, the higher the resolution used, the more detail scanned, but what’s the point, you don’t normally read books or drawings or documents with a microscope. So 300 dpi is adequate, yet even so, the information scanned at this rate is represented by over 8,000,000 dots to the A4 page. Pretty big.

The Display system.

The data from the scanner is temporarily stored in the computer’s memory, and there are three choices from here: throw the data away and start again because it was the wrong document or something; have a look at it and then decide to save it or throw it away; or save it without having had a look at it.

It’s usual to have a look before deciding what to do next, and that involves the display system. There’s not a lot to say about the display system, except you get what you pay for. The simplest and cheapest is to use the computer’s original display screen. This is usually the wrong shape (or at least for portrait displays) and it’s resolution is low, typically 50 to 75 dpi. Don’t worry about the loss of resolution in the picture you see, the image in the memory is fine and the resolution is unchanged, it’s just some software reduced the resolution of the displayed image because the computer’s display couldn’t cope with the full resolution. Next type of display is the portrait shaped (approximately) A4 size screen. These usually display the whole of the image, but usually the resolution is reduced. These displays are typically resolve at about 100 dpi. This is entirely adequate for looking at documents and drawings to check the scan quality, but text can be a bit tiring to read. It’s a trade off again, cost versus speed in this case. There are some A4 size displays which work at the