Editing Nephelometer (section)

==Turbidity units==
[[File:Nephelometer acadia.jpg|right|thumb|A nephelometer installation at [[Acadia National Park]]]]
[[File:Turbidimeters.JPG|thumb|Turbidimeters used at a water purification plant to measure turbidity (in NTU) of raw water and clear water after filtration]]

*Because optical properties depend on suspended particle size, a stable synthetic material called "[[Formazin]]" with uniform particle size is often used as a standard for calibration and reproducibility.<ref>Formazin was first used for standardizing turbidity measurements in 1926.</ref> The unit is called Formazin Turbidity Unit (FTU).
*Nephelometric Turbidity Units (NTU) specified by [[United States Environmental Protection Agency]] is a special case of FTU, where a white light source and certain geometrical properties of the measurement apparatus are specified. (Sometimes the alternate form "nephelos turbidity units" is used<ref>[http://www.freepatentsonline.com/5034211.html ''Reducing turbidity in chromic acid solutions''], Zeller, III, Robert L.; Morgan, Russell J.; Rabbe, Gilbert D.; Fiscus, Donna R.; Wilkes, Jr., Richard L.;
United States Patent 5034211, Filing date 1990-10-29, Publication date 1991-07-23</ref><ref>[https://web.archive.org/web/20070926195339/http://www.floridaconservation.org/codebook/2006/Deptartment_of_Agriculture_and_Consumer_Services.pdf ''Florida Department of Agriculture and Consumer Services 2006 Codebook Chapter 5L-1: The Comprehensive Shellfish Control code''] </ref>)
*Formazin Nephelometric Units (FNU), prescribed for 9 measurements of turbidity in water treatment by [[ISO 7027]], another special case of FTU with near infrared light (NIR) and 90° scatter.
*Formazin Attenuation Units (FAU) specified by [[ISO 7027]] for water treatment standards for turbidity measurements at 0°, also a special case of FTU.
*Formazin Backscatter Units (FBU), not part of a standard, is the unit of [[optical backscatter detector]]s (OBS), measured at c. 180°, also a special case of FTU.
*European Brewery Convention (EBC) turbidity units
*Concentration Units (C.U.)
*Optical Density (O.D.)
*Jackson "Candle" Turbidity Units (JTU; an early measure)
*Helms Units
*American Society of Brewing Chemists (ASBC-FTU) turbidity units
*Brantner Haze Scale (BHS) and Brantner Haze Units (BHU) for purposefully hazy beer
*Parts Per Million of standard substance, such as PPM/DE (Kieselguhr)
*"Trübungseinheit/Formazin" (TE/F) a German standard, now replaced by the FNU unit.
*diatomaceous earth ("ppm SiO<sub>2</sub>") an older standard, now obsolete

A more popular term for this instrument in water quality testing is a '''turbidimeter'''. However, there can be differences between models of turbidimeters, depending upon the arrangement ([[geometry]]) of the source beam and the detector. A nephelometric turbidimeter always monitors light reflected off the particles and not [[attenuation]] due to cloudiness. In the [[United States]] [[environmental monitoring]] the [[turbidity]] standard unit is called [[Nephelometric turbidity units|Nephelometric Turbidity Units]] (NTU), while the international standard unit is called Formazin Nephelometric Unit (FNU). The most generally applicable unit is Formazin Turbidity Unit (FTU), although different measurement methods can give quite different values as reported in FTU (see below).

Gas-phase nephelometers are also used to study the [[atmosphere]]. These can provide information on [[visibility]] and atmospheric [[albedo]].