The Context - Artificial Human Sensors: Science and Applications - page 45

increase the individual's integrity and increase the awareness of identifying correct
rendering colours. In a society where colours are of importance, this also seems to
be a safety aspect in situations where colour perception is required.

2.2.3 The Colour of Water

If we look at earth from the space we explore, it is as a big water reservoir with
a huge water paradise. This may be true, but if we consider the actual available
water for human disposal, in areas as for example personal use, food production,
etc., we have to consider the fact that 97.4% of the water is salt water. Of the
existing fresh water, only 0.8% is available for human consumption. The rest of
the freshwater is bound at the poles, permanent snow and in the glaciers. The
fresh water will surely in the future, be an issue of importance in the world to
secure the living standard as the resources have to be equally shared between the
population. More than 1.2 billion people today have no access to safe drinking
water. The proposed and complementary drinking water test device, described
later in this topic, may be one step for pushing the safety of externally deciding
whether the water is drinkable or not.
Another essential fact is that water is not equally divided between the popu-
lation. Nine countries have 60% of the earth's fresh water resources within their
boundaries. The difference in the fresh water allocation is not only unevenly dis-
tributed, but also unequally perceived by individuals.
Water basically has a light blue colour, which however may shift into a colour
spectrum, from deeper blue as the depth of the observed water increases to green
when affected by other components in the water, e.g., minerals or algae. The
blue colour is caused by selective absorption and scattering of the light spectrum,
Fig. 2.7. Impurities dissolved or suspended in water may give water different
coloured appearances.
Large reservoirs of water such as oceans elucidate the water's inherent slightly
blue colour, not as was once believed a reflection of the blue colour from the sky.
The main reason why the ocean is blue is because water itself is blue-coloured. Op-
tical scattering from water molecules provides a second source of the blue colour,
but coloured light caused by scattering only becomes significant with extremely
pure water. According to the frequency spectra for pure liquid water, a short wa-
ter column has an obvious light shade of turquoise blue. Thicker layers (many
meters) appear as much darker blue colour or the colour of blue-green water set
against the stark backdrop of Victoria Glacier, as can be seen in Fig. 2.8. It is only
when collected in a large body that water's blue colour is less affected by external
sources, and becomes apparent.
Scattering from suspended particles also plays an important role in the colour
of lakes and oceans. A few tens of meters of water will absorb all light, so without
scattering, all bodies of water would appear black. Because most lakes and oceans
contain suspended living matter and mineral particles, i.e., coloured dissolved
organic matter, light from above is reflected upwards. Scattering from suspended