A given image can be represented using different numbers of pixels. In this example, the image at the top contains 230 x 176 pixels, the one in the middle 80 x 61 pixels, and the one at the bottom only 32 x 24 pixels. While the object in the last image is still recognisable, some details, such as the lashes, have disappeared.

If we increase the number of pixels composing an image, we obviously reduce each pixel’s size and thus increase the image’s resolution. The digital image’s resolution is usually expressed in pixels per inch (1 inch = 2.54 cm) or ppi. The resolutions of the pictures shown above are 72, 25 and 10 pixels per inch, respectively. As a rule, a good-quality image displayed on a computer screen has a resolution of about 75 ppi or roughly 3 pixels per millimetre. Beyond this limit, that is to say, if the pixels are smaller, the eye no longer sees the individual pixels and the image appears to be continuous. Indeed, photographs are also composed of a very large number of very small dots corresponding to the sensitive grains on the photographic film.

A grey-scale digital image could be produced by juxtaposing a very large number of small radiometers, each recording the intensity of the electromagnetic radiation received by each pixel (in the visible part of the spectrum). This is actually what most of the instruments that yield digital images (digital cameras, scanners, etc.) do.ese are ‘active systems’. The latter usually operate in the microwave or radar wave range, working with wavelengths of from 1cm to 1m.