Visual content	Issues for visual content	Image production	Image acquisition	Image manipulation	Image creation

Web images have one critical unit of measurement - the pixel. 'Pixels' represent the individual units of resolution on your computer screen. The pixel dimensions of an image specify the size it will appear on screen in relation to the pixel resolution of the screen.

Digital images that are to be printed have an image resolution (usually expressed in ppi (pixels per inch) or occasionally spi (samples per inch) or dpi (dots per inch), and a size measurement (eg 10 x 15 cm, or 4 x 6 inches).

This translates into a 'pixel' measurement, which is the basic unit for describing the size of any digital image.

For instance, a 4" x 6" image scanned in at 100% size and 300ppi will be 1200 x 1800 pixels.

The same image scanned in at 75ppi will be 300 x 450 pixels.

These images will appear very different sizes if viewed on an average computer screen (size 600 x 800 pixels), although they would print at the same size.

Scanner software will generally prompt you to enter a size (as a percentage of original document size) and a resolution (in ppi) for scanning your image. These relate to PRINTED size and resolution only, the critical factor for WEB use is the final PIXEL measurement.

Image resolution (pixels per inch) x size (inches) will therefore allow you to determine the pixel measurement of your image, even though these two elements do not in themselves have a bearing on how the image will appear on screen. Screen appearance can be changed only by changing the pixel dimensions - 'resampling' the image.

Although resolution is not a factor in determining screen size of an image, it is sometimes considered useful to use a resolution of 75ppi when scanning images for web use, as it approximates the resolution of the average computer screen. For instance, a 4" x 6" image scanned at 75ppi will appear approximately 4" x 6" on screen, depending, of course, on the size of the monitor and the resolution that is selected for the display.

Some scanning software will allow you to set preferences to set your image size in pixels, which makes things simpler when scanning for on-screen use.

Summary:

• Digital images are measured in 'pixels'.
• The document size multiplied by the resolution equals the pixel size.
• Images must be 'resampled' to change the pixel dimensions.
• Image resolution of 75ppi will make document size and screen display size similar.
• When possible, set scanning software to use pixels as the size unit when scanning for the web.