The full-colour picture of the Moon used in The Independent last Thursday surprised some people, because we're used to seeing a black and white (or grey and white) object in the sky, not something like Jupiter. Such false-colour images, though, are important nowadays for all sorts of astronomy, and also mapping the Earth and even in medical systems.

But by adding the full spectrum of colour to what appears at first to be just shades of grey and white - or even invisible - false-colour images add a whole new layer to our understanding of the world.

A typical false-colour image is produced by photographing an object through three separate filters, each "tuned" to a different range of light frequencies - or even, in the case of radio and X-ray astronomy, to radio frequencies. The "view" of the object from each filter is recorded separately on special photographic film, or frequently nowadays on an electronic equivalent, called a "charge-coupled device" (CCD) array

If you are viewing a distant galaxy, for example, you may want to know how much free hydrogen and how much free helium it contains, and at what sort of temperatures. Chemists already know the frequencies at which hydrogen and helium emit light: with that data you can set up your filters so that one will only allow through frequencies from hydrogen, another from helium, and another from other trace elements you are interested in.

To build the final image, each of the filtered images is printed only in one of the three "primary colours" - red, blue and yellow - and then combined together to form a full-colour picture. This will show where there is just hydrogen or just helium; but where the colours overlap, you know both elements are present.

For the false-colour picture of the Moon, the "blue" filters were tuned to the wavelengths emitted by the metal titanium; the more there was, the bluer the area looked. The less there was, the redder it looked.

The power of this technique isn't limited to space. Satellites can see minute variations in the height of the sea and magnify them using false- colour techniques, so a difference of a few centimetres turns a strip of ocean purple - indicating a powerful El Nino. We can observe the growth of cities. And by applying false-colour methods to medical techniques such as positron emission tomography (PET) scans, we can see exactly where the active regions of the brain are when we are thinking about something.