Visibility is a term which generally predicts the ability of some human or instrument to detect an object in the given environment, and may be expressed as a measure of the distance at which an object or light can be discerned. The theoretical black body visibility of pure water based on the values for the optical properties of water for light of 550 nm has been estimated at 74 m. For the case of a relatively large object, sufficiently illuminated by daylight, the horizontal visibility of the object is a function of the photopic beam attenuation coefficient (spectral sensitivity of the eye). This function has been reported as 4.6 divided by the photopic beam attenuation coefficient. Factors affecting visibility include: particles in the water (
turbidity), salinity gradients (
haloclines), temperature gradients (
thermoclines) and dissolved organic matter. Reduction of contrast with distance in a horizontal plane at a specific wavelength has been found to depend directly on the beam attenuation coefficient for that wavelength. The inherent contrast of a black target is -1, so the visibility of a black target in the horizontal direction depends on a single parameter, which is not the case for any other colour or direction, making horizontal visibility of a black target the simplest case, and for this reason it has been proposed as a standard for underwater visibility, as it can be measured with reasonably simple instrumentation. The photopic beam attenuation coefficient, on which diver visibility depends, is the attenuation of natural light as perceived by the human eye, but in practice it is simpler and more usual to measure the attenuation coefficient for one or more wavelength bands. It has been shown that the function 4.8 divided by the photopic beam attenuation coefficient, as derived by Davies-Colley, gives a value for visibility with an average error of less than 10% for a large range of typical coastal and inland water conditions and viewing conditions, and the beam attenuation coefficients for a single wavelength band at about 530 nm peak is a suitable proxy for the full visible spectrum for many practical purposes with some small adjustments.
Measurement of visibility The standard measurement for underwater visibility is the distance at which a
Secchi disc can be seen. The range of underwater vision is usually limited by
turbidity. In very clear water visibility may extend as far as about 80m, and a record Secchi depth of 79 m has been reported from a coastal
polynya of the Eastern
Weddell Sea, Antarctica. In other sea waters, Secchi depths in the 50 to 70 m range have occasionally been recorded, including a 1985 record of 53 m in the Eastern and up to 62 m in the tropical Pacific Ocean. This level of visibility is seldom found in surface freshwater.
Crater Lake,
Oregon, is often cited for clarity, but the maximum recorded Secchi depth using a 2 m disc is 44 m. The lakes of the
McMurdo Dry Valleys of Antarctica and
Silfra in Iceland have also been reported as exceptionally clear. Visibility can be measured in an arbitrary direction, and of various colour targets, but horizontal visibility of a black target reduces the variables and meets the requirements for a straight-forward and robust parameter for underwater visibility, which can be used to make operational decisions for mine hunters and explosive ordnance disposal teams. An instrument for measuring underwater visibility basically measures light transmission through the water between the target and the observer, to calculate the loss, and is called a
transmissometer. By measuring the amount of light which is transmitted from a light source of known strength and wavelength distribution, through a known distance of water to a calibrated light meter, the clarity of water can be objectively quantified. A wavelength of 532 nm (green) aligns well with the peak of the human visual perception spectrum, but other wavelengths may be used. Transmissometers are more sensitive at low particulate concentration and are better suited for measuring relatively clear water.
Measurement of turbidity Nephelometers are used for measuring suspended particles in turbid waters where they have a more linear response than transmissometers. Turbidity, or cloudiness, of water is a relative measure. It is an apparent optical property which varies depending on the properties of the suspended particles, illumination, and instrument characteristics. Turbidity is measured in nephelometer units referenced to a turbidity standard or in
Formazin Turbidity Units. Nephelometers measure the light scattered by suspended particles and respond mainly to the first-order effects of particle size and concentration. Depending on manufacturer, nephelometers measure scattered light in the range between about 90° to 165° off the axis of the beam, and usually use infra-red light with a wavelength of around 660 nm because this wavelength is rapidly absorbed by water, so there is very little contamination of the source due to ambient daylight except near to the surface.
Low visibility Low visibility refers to a diving environment where the diving medium is
turbid and objects cannot be seen clearly at short range even with artificial illumination. The term is not usually used to refer to a simple lack of illumination when the medium is clear. Zero visibility is used to describe conditions when the diver can effectively see nothing outside the mask of helmet, and a light must be put against the viewport to see if it is switched on, and it is not possible for a person with normal vision to read normal instruments. (some mask-integrated head-up displays may be legible) Low visibility is defined by NOAA for operational purposes as: "When visual contact with the dive buddy can no longer be maintained."
DAN-Southern Africa suggest that limited visibility is when a "buddy cannot be discerned at a distance greater than 3 metres." == See also ==