Underwater visibility (often termed 'the vis' or 'the viz' by divers) is usually defined as the distance that a diver can see horizontally and is, mainly, a measure of the clarity of the water. Of course, actual 'visual detection' of a target must depend upon the contrast - in terms of brightness and/or colour - that the target has with the background against which it is viewed; even bright white cannot be seen against a bright white background. Therefore, underwater visibility is best judged against the background of the body of water itself ( equilibrium brightness Equilibrium brightness is the brightness of body of water when looking horizontally. It is termed 'equilibrium' because the rate at which light is scattered into the path of sight is equal to the rate at which it is scattered out. Consequently the background does not get brighter or darker as the observer moves towards or away from the direction of view. ).
In fact underwater visibility depends upon a number of factors including the optical characteristics of the target and water, the nature and intensity of the illumination and the physiology of the observer's eye-brain optical system.
We shall consider, briefly, each of the following aspects of underwater visibility:
- water clarity
- available sunlight
- tides in shallow water
- target and background brightness and colour
- depth of observer
- target direction
- seabed reflectance
- temperature and/or salinity gradients
We shall also consider the related topic:
- the measurement of underwater visibility
This is, of course, the most important factor affecting underwater visibility, and visible distance (see later) is sometimes taken as a measure of the clarity of the water. When a beam of light propagates through water it suffers
Attenuation in this case is the decrease in intensity of a beam of (image carrying) light due to the absorption of light by the water and its dissolved and suspended content, and the scattering of light out of the beam, also by the water molecules and the water's dissolved and suspended content.
(a decrease in intensity) which is the sum of the absorption and the scattering which takes place. Since a visual image is transmitted through the water by light then visibility is affected in exactly the same way.
Water clarity is affected by:
- organic particles - plankton, algae, faecal pellets of marine creatures and particles of dead animal and vegetable matter
- inorganic particles - suspended sediments of geological origin
- dissolved materials - mostly of organic origin, especially 'gelbstoff' (yellow substance), the stain imparted to water by decaying vegetation
- water molecules - these also absorb and scatter light so that even in the clearest water visibility is less than in, say, clear air
Of course visibility must depend greatly upon available light, and this will vary significantly with latitude and time of the year and the day (altitude of the sun) and cloud cover. When a beam of light from the sun strikes the sea surface then some of the light is refracted When light passes from one transparent medium (air, water, glass etc) to another then there is generally a change in direction at the interface. The magnitude of this change depends upon the difference in speed of light in the two media. The speed of light in water is about 3/4 of that in air (= 3x108 ms-1). Consequently, when a beam of light enters the sea from the air it is bent downwards. (bent downwards) into the sea and some is reflected away from the surface. At high altitudes of the sun most of the light is refracted into the body of water; at very low altitudes most of the light is reflected away from the surface. Of course this assumes a perfectly flat sea surface (specular reflection) whereas the sea surface is usually uneven (diffuse reflection) to an extent that depends mainly upon wind speed, i.e. the sea state.
Tides in shallow water
In shallow water, estuaries etc the state of the tide can have a marked effect upon sediment suspension and hence visibility. Maximum current flow - so, minimum visibility - occurs at mid-tides whilst the best visibility will occur at the stand at low water and high water, particularly the latter. This situation is further complicated by the fact that fine sediments, e.g. mud, stay in suspension for much longer than coarser sediments such as sand.
Target and background brightness and colour
As mentioned above visual detection of a target must depend upon the contrast - in terms of brightness and/or colour - that the target has with the background against which it is viewed. Generally, targets are best seen against a background of different brightness (e.g. black against white is excellent; black against black isn't) and/or complementry colour (red:cyan, green:magenta, blue:yellow etc).
Depth of observer
Since even pure water absorbs light it must follow that it gets darker with depth, and this limits visibility (in sea water, the dissolved salt has almost no effect upon its light absorption properties or colour). However there is another consequence of increased depth: due to the wavelength (colour) selective absorption of light, red is soon lost followed by orange, yellow and green. In pure water, the intensity of blue light is halved in about 39m, but red light is halved in less than 2m.
Table of half distances
It follow that in clear tropical waters the underwater scene becomes increasingly blue with depth whilst in higher latitudes, due to dissolved materials (see Ocean colour, below) the visual scene becomes increasingly green. Whatever the colour of the water, the underwater scene becomes increasingly monochromatic (one colour) with depth, and this must limit target-background contrast.
Since underwater visibility is very much about target-background contrast, the visibility of any given target will vary with the brightness of the background. Of course, brightness is greatest when looking upwards towards the illuminated sea surface and least when looking downwards into the darker depths. It is intermediate when looking horizontally - the usual definition of 'visibility'.
In shallow or very clear water the seabed may be visible when looking downwards so that the seabed rather than the body of water becomes the background, and this may range from dark mud to bright sand.
Temperature and/or salinity gradients
Just as light is refracted as it passes from air to water (or from water to air) due to the different refractive indices (basically different velocities of light) of these two distinctly different media, so will light be refracted within a body of water at the interface between layers of differing refractive index. Such layers comprise water of different temperature and/or salinity. Of course this gives rise to differences in density - and hence the layering (stratification).
Typically this phenomenon occurs in estuaries (fresh water river over denser seawater wedge) and in caves. The same effect has been reported by divers at shallow depths of the sea during heavy rainfall. At the interface the visibility becomes very blurry and confusing.
The measurement of underwater visibility
Clearly estimation of the viz by divers is subjective, not least because different divers reckon visible distance in different ways. Some use the distance at which a buddy diver can just be seen; others use their buddy’s bubbles. A useful estimate used by some divers is “the distance at which hand signals are no longer recognisable”. This very practical definition should, perhaps, be further specified as “..... against the background of the open water”.
This writer conducted a substantial number of underwater visibility experiments using a range of targets in both open waters and a laboratory ‘visibility tank’. The most successful definition of underwater visibility was found to be the hydrological range, the threshold distance of a black target viewed horizontally against the body of water. Strictly, the term threshold distance is the distance at which the target can just be seen on 50% of observations. The black target is defined as having zero brightness as this lends itself to mathematical treatment
(see Underwater visibility - mathematical version).
Choice of a ‘black target of zero brightness’ was not as straightforward as may be supposed; even a black painted surface may reflect some light - consider, for example, a well polished black car. The visibility target used was a black light trap , a black cone of diameter 30cm painted matt black and attached to a tape measure. The open end of the target was pointed towards the observer so that any light reflected by the cone’s inner surface was reflected inwards towards the cone’s peak.For practical use - and without getting wet - an estimation of the viz can be made by observing the Secchi depth, the depth of disappearance of a Secchi disc, a 30cm disc painted matt white. Fifty eight concurrent observations of Secchi depth and
Secchi disc in clear water
hydrological range (black cone) were obtained by this writer in Plymouth Sound (UK), SE Malaysia and Singapore. It was found that the vis (hydrological range, black cone) was about 80% (0.83, actually) of the Secchi depth.
Date of last revision: 25 September 2017