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Scanning

With the availability of low cost desktop scanners many clients now undertake their own scanning for PowerPoint presentations, publication and posters, etc. the Media Resources Centre has compiled the following advice to help clients with their scanning:

Before starting

Before scanning an image it is useful to answer a few general questions:

  1. How will the final image be used? Will it be for a poster, a presentation for slide or data projection, or publication in print or used on a web page?
  2. Which presentation program, e.g. PowerPoint, will the image be imported into?
  3. What printer or other output device will be used?

It is only with these questions answered that it is possible to produce scans at the most suitable resolution, file size and format.

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Quality originals

However good the scanner, image processing and editing software is, the quality of the final scanned image is greatly influenced by the quality of the original. Poorly exposed, unsharp slides or prints will never look great, despite the range of retouching tools that might be applied to them.

Half-toned images (i.e. those printed through a dot screen e.g. in books or journals, etc.) can cause problems in scanning. The dot screen can create a moiré pattern and, although some scanner programs include software to try to eliminate this problem, they are not always satisfactory. Moiré patterns can also occur with the intensification screens used with some x-ray films. 

Tip

If you have to scan a dot screen image, try setting the original at a slight angle, e.g. 5 degrees, to the direction of the scanner head.

It should be remembered that the law relating to Copyright applies to scanning images just as it does to photocopying or photographic copying. If you are unsure of the current law as it relates to copyright, please ask.

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Image types

When setting up, it is important to consider the type of image being scanned. Originals can generally be described as one of three types: Line Art, Greyscale (i.e. Black & White) or Colour. Making the right choice at this stage can make a huge difference to the quality of the image, the size of the file and whether the output device will be able to handle the file.

Line Art

Line art is defined as simple black and white images with no colour, shading or grey tones. This is 1-bit scanning and the scanner only captures either black or white.

Line illustrations should be saved as a 1-bit Bitmap (bmp) or a PICT file (which offers a 1Bit resolution option). This will keep the file size small.

For a line scan to be successful the lines need to be reasonably thick and a good solid black. Any faint lines, such as pencil, may disappear, therefore the contrast levels in the scanner's editing software may need to be adjusted accordingly.

Greyscale

Greyscale scanning is used for conventional black and white photographs, drawings and radiographs, or any black and white original where there are grey tones between black and white.

If your scanner gives the option, scan at 256 shades of grey and save the scans as 8-bits/per pixel tif, jpg or PICT files.

Colour

Scanning from photographs and slides can create particular problems in relation to file sizes.

For example, a colour image scanned in RGB colour mode to produce an output 27cm x 18cm at 72dpi (i.e. screen resolution) will create a 1.12MB file; three times the file size of a greyscale image. With this in mind, it is very important to work to the following guidelines on resolution and file size, in order to produce manageable files.

If the choice is offered, files should be saved as RGB (or CMYK colour mode if the scanned image is to be used for litho printing) and in a format that will be accepted by the presentation program, such as jpg or tif.

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Resolution

The sharpness and visual quality of the scanned image, (whether the image appears pixelated), is largely determined by the resolution at which the original is scanned. This is usually measured in dots per inch (dpi).

The higher the dpi, the greater the resolution. However quality is also related to the resolution of the printer or output device, which is why it is essential to know how the final image will be used. The original scan should be made at a resolution that matches, and doesn't exceed, the resolution of the output device.

Tip

It is worth spending time considering the use of the scanned image before you start and working out the correct resolution. Don’t scan at a high resolution just to be on the safe side.

Scanning at a resolution greater than the output device is capable of resolving will not yield any increase in quality, but will simply lead to longer scan times and larger file sizes which take much longer to process and can cause output devices to fail.

Listed below are the printers used by the Media Resources Centre to help in selecting an appropriate resolution:

Output Device Scanning Resolution Required
Digital Black and White & Colour Printers 200 - 400dpi
Large Format Ink-Jet Poster Printer 300dpi
Data Projector 72dpi

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Image Size

Digital imaging combines what have traditionally been two different forms of output, with the ability to view the image on a computer screen or as a print. These two forms of presentation express image size in different units.

Screen images are expressed in pixel dimensions (e.g. 800 x 600 pixels), and printed images expressed in centimetres at a specified resolution (e.g. 12cm x 8cm at 300dpi).

As well as setting the resolution, some scanners require the dimensions to be set for the final image size, i.e. selecting the final image size in cm, mm, inches or pixels. In some cases the final image size is achieved by scaling software within the scanner program.

The dimensions you set for the final scanned image should depend on the presentation program into which the image will be imported, e.g. PowerPoint, and how it will be used within that program.

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Importing into PowerPoint for data projection

The scanned image size depends on the number of images to be put onto one slide. Use the following optimum image size guide for PowerPoint:

Optimum scanning image sizes for PowerPoint

 

Avoid making the final scanned image larger than the size it will appear in the presentation and then resizing it smaller in PowerPoint. The image file size will not be reduced when the image is resized and large image files can cause the presentation to run slowly.

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Importing into PowerPoint for a poster

Posters produced in PowerPoint should be designed on an A4 page, to be enlarged on the printer to the final size required, therefore the initial scans should be sized to fit the design of your poster within the A4 page layout (i.e. at 300dpi without any re-sizing on the page).

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File formats

The file format in which the image should be saved depends again on the presentation program into which it will be imported. Check in the 'Import' or 'Insert' menus of the program that you use, to see which formats are supported. The most common file formats used are tif or jpg.

Tip

All jpg files are compressed files. Data is averaged (digitally compressed) to reduce the size of the file. The degree to which the file size is reduced is dependent on the amount of compression that takes place. In a compressed image detail is lost, i.e. the greater the compression; the greater the loss of detail.

Therefore it is always better to calculate the correct size for an image rather than simply scan at the highest resolution and use high compression to produce a usable file size.

When saving a tif file you are asked to choose either Mac or Windows byte order. If you know the computer on which the file will be used, select the appropriate platform. If you are not sure choose Windows, as a Mac will read both formats whereas a PC will only read a Windows file.

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Constructing final presentations

When creating your posters and presentations, please bear in mind that large image files increase the chance that they may fail to run smoothly or print properly.

Before a poster can be made from a PowerPoint presentation, the file has to go through a Raster Image Processor (RIP). This complex process takes the data from the file and reconstructs it in a form acceptable to the output device.

The other major factor causing RIP failure is simply file size. If the file is too big, the RIP can be timed-out or simply run out of memory during the processing stage.

The problem of RIP failure is inconsistent and hard to predict. The best advice is to construct presentations carefully and produce optimum files by scanning at the right resolution and size.