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 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."