A raster image refers to a rectangular bitmap, or in other words a picture held in digital form. The image is made up of pixels, and may be thought of as having dimensions defined by a number of rows and columns. Each pixel is assigned a numeric value representing a colour. Raster images are defined by the image resolution (measured as pixels per centimetre or per inch - often also called DPI of dots per inch). A low resolution such as 72 dpi (TV picture quality) will give a poor image on the computer screen, whereas a high resolution (e.g. 400dpi) will give a very sharp image. The downside is that doubling the resolution will quadruple the file size. The colour depth refers to the amount of data held for each pixel. At the simplest 1 byte (8 bits) is held allowing the display of 256 shades of gray. By using a colour table to allocate colours to each of the numbers 0 - 255, a full colour image (but restricted to 256 shades) can be held while still only using 1 byte per pixel. This is called an Indexed Colour image. Holding a normal Red - Green - Blue image requires 1 byte per colour, and is thus termed a 24 bit image. This allows 256 by 256 by 256 colour combinations, but uses three times the disk space of Indexed Colour. Because images tend to consume large amounts of disk space, various compression methods have been developed to reduce the file sizes. Compressed image formats include LZW Tiff, GIF, and JPEG. GIF and JPEG images are generally used for Web applications as these tend to result in the smallest files. Some compression methods (e.g. LZW) do not result in any loss of data, while others can cause some irreversible alteration or loss of quality (e.g. JPEG).

By default our images are supplied as 256 Indexed Colour TIFF images either with or without LZW compression. Some software can not handle LZW compression so please be sure to check your requirements and specify the appropriate option when placing an order. The resolution of our Raster Images is normally either 100 pixels per cm (254 dpi) or 300 dpi. We can also supply BMP format (common in Windows 3.x and above systems), GIF, JPEG and many others.