Scanning training help and issues with pixels, vectors, scans, resolution, ppi, lpi dpi,

ppi :pixels per inch. Describes the digital pixel resolution of an image. The term dpi is often used to describe the digital resolution but this is inappropiately used. Scanning devices are sometimes advertised with their scanning resolution expressed in dots per inch. Strictly speaking, this is an incorrect use of the term dpi, because input devices like scanners produce pixels only output devices produce dots. However, its fallen into common practice now only to add to the confusion. Monitor resolution is specified in ppi. Macintosh monitors have a resolution of 72dpi, whereas PC monitors are usually 96ppi. On of the reasons for the Mac default resolution is that it helps graphic designers get a better feel for the weight of their forms when laying out a page. The print industry traditionally has worked on 72 points to the inch. Tip: 72 points of PostScript is a slightly measurement to the traditional 72 points to the inch ,(PostScript works in pixals not inches) this can cause rounding problems in conversion over large areas.

Resolution

What defines

Low Resolution

Digital photographs are constructed with pixels and as such are resolution dependent. You can scale up a pixel image, but as you do so the finite information can only be streched so far before the underlying pixel structure becomes apparent (it bitmaps and looks blurred or blocky). Adobe Illustrator is a vector based programme. Objects drawn in a vector based programme are defined mathmatically, so if you draw a rectangle, the proportions of the rectangle edges, the relative placement on the page and the colour it is filled with can all be expressed by using mathmatical expressions. The rectangle can be output at any resolution, whether it is at 72ppi on the screen or at 300ppi on an A1 blowup print. It will always be rendered a crisply defined image.

Object orientated applications such as QuarkXpress and In-Design observe the resolution setting even though you can resize the image in the application, therefore the image needs preparing in Photoshop as completely as possible before importing into QuarkXpress. Ideally, you should never have to resize, rotate, or crop an image in any other progamme.

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ppi	ppi :pixels per inch. Describes the digital pixel resolution of an image. The term dpi is often used to describe the digital resolution but this is inappropiately used. Scanning devices are sometimes advertised with their scanning resolution expressed in dots per inch. Strictly speaking, this is an incorrect use of the term dpi, because input devices like scanners produce pixels only output devices produce dots. However, its fallen into common practice now only to add to the confusion. Monitor resolution is specified in ppi. Macintosh monitors have a resolution of 72dpi, whereas PC monitors are usually 96ppi. On of the reasons for the Mac default resolution is that it helps graphic designers get a better feel for the weight of their forms when laying out a page. The print industry traditionally has worked on 72 points to the inch. Tip: 72 points of PostScript is a slightly measurement to the traditional 72 points to the inch ,(PostScript works in pixals not inches) this can cause rounding problems in conversion over large areas.
dpi	dots per inch: Refers to the resolution of a printing device. An output device such as an imagestter (ie, an Agfa excalibur X-45) is able to produce tiny 100% black dots at a specified resolution. The X-45 is capable of a resolution of 2400dpi and say the printer wished to use a screen ruling of 150Lpi (lines per inch). You would then divide the dpi of 2400 by the Lpi of 150 then you get the figure of 16. this will makeup a halftone dot of 16 x 16 which has 256 cells within it (16 x 16=256). This is known as the number of levels of grey in a geyscale image. At 1200dpi with a line screen ruling of 133 you will get 81 levels of grey. 1200 divided by 133 = 9, 9 x 9 = 81. 256 levels of grey is shown in the levels command in Photoshop.
Lpi	lines per inch: The number of halftone lines or 'cells' in an inch, also described as screen ruling. The origin of the term (like many others) go way back before digital desktop printing. Logically the term should be pixels per inch, ppi but the mixed term of dpi & Lpi has no clear definative answer. It is an example of how two separate disciplines of traditional repro and those who developed the digital technology apply different meanings to these terms. Originally, halftones were made by placing an etched glass plate over an image and exposing it to produce dots, Lpi refers to the frequency of the horizontal and vertical lines.

Digital Workflow Issues
Resolution What defines Low Resolution	Two of the first questions everyone wants answered are, 1. How do you define resolution? 2. How big a file do you need?. Digital images are made up of picture elements called pixels and every digital image contains a finite number of these blocks of tonal information. In the digital world resolution refers to the number of pixels, the more pixels in an image the finer its resolving capacity. the digital resolution therefore refers to the fineness of the output, the number of pixels per inch used to construct the final print. So if we have a digital image of 3000 pixels, the same image can be output either at 10 inches at a resolution of 300 ppi or for example 12 inches at 250ppi. So there will be a point at which this image will go low resolution at a certain enlargement. This can be expressed in the following formula: pixel size = physical dimension x resolution (ppi). In other words there is a reciprocal relationship between pixel size, the physical dimension and resolution.
Pixels versus Vectors	Digital photographs are constructed with pixels and as such are resolution dependent. You can scale up a pixel image, but as you do so the finite information can only be streched so far before the underlying pixel structure becomes apparent (it bitmaps and looks blurred or blocky). Adobe Illustrator is a vector based programme. Objects drawn in a vector based programme are defined mathmatically, so if you draw a rectangle, the proportions of the rectangle edges, the relative placement on the page and the colour it is filled with can all be expressed by using mathmatical expressions. The rectangle can be output at any resolution, whether it is at 72ppi on the screen or at 300ppi on an A1 blowup print. It will always be rendered a crisply defined image.
TIP	Object orientated applications such as QuarkXpress and In-Design observe the resolution setting even though you can resize the image in the application, therefore the image needs preparing in Photoshop as completely as possible before importing into QuarkXpress. Ideally, you should never have to resize, rotate, or crop an image in any other progamme.