|Pixels: the more the better!
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 pixels size and thus increase the images
resolution. The digital images 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.