Effects of Light Absorption and Scattering in Water Samples on OBS ...

<strong>Effects</strong> <strong>of</strong> <strong>Light</strong> <strong>Absorption</strong> <strong>and</strong> <strong>Scatter<strong>in</strong>g</strong> <strong>in</strong><strong>Water</strong> <strong>Samples</strong> on OBS ® Measurements<strong>Light</strong> transmission through a water sample is determ<strong>in</strong>ed by physical properties such as particle size, shape,suspended solids concentration (SSC), <strong>and</strong> composition, <strong>and</strong> chemical properties such as the presence <strong>of</strong> near<strong>in</strong>frared(NIR) absorb<strong>in</strong>g dissolved matter. There is enormous variation <strong>in</strong> these properties <strong>in</strong> the environment,result<strong>in</strong>g <strong>in</strong> a nearly <strong>in</strong>f<strong>in</strong>ite number <strong>of</strong> unique optical characteristics for natural <strong>and</strong> man-<strong>in</strong>fluenced water,however, consistent light transmission through a water sample is essential for precise measurements. Suspendedsediment concentration <strong>and</strong> particle size span a 1000-fold range while NIR reflectivity varies by a factor <strong>of</strong> about10. The absorption <strong>of</strong> light by dissolved matter can affect light-scatter<strong>in</strong>g measurements by 10% to 50% <strong>in</strong> run<strong>of</strong>ffrom m<strong>in</strong>e tail<strong>in</strong>gs. Lack <strong>of</strong> underst<strong>and</strong><strong>in</strong>g <strong>of</strong> these properties can lead to mis<strong>in</strong>terpretation <strong>of</strong> SSC valuesdeterm<strong>in</strong>ed with an optical sensor. This application note reviews their effects <strong>and</strong> addresses answers to questionsabout size, color, <strong>and</strong> disaggregation effects.OverviewKnow<strong>in</strong>g some basics about light transmission through water samples will enhanceyour underst<strong>and</strong><strong>in</strong>g <strong>of</strong> how OBS ® sensors operate <strong>and</strong> help you select the bestsystem/<strong>in</strong>strument for an application. It will also help you recognize technical<strong>and</strong> operational limitations that can reduce the value <strong>of</strong> your data <strong>and</strong> challengethe success <strong>of</strong> your monitor<strong>in</strong>g program. <strong>Light</strong> transfer through a water sample isaffected <strong>in</strong> complex ways by water molecules, material dissolved <strong>in</strong> the water, <strong>and</strong>scatter<strong>in</strong>g by suspended particles. All light-transfer processes depend on wavelengthas <strong>in</strong>dicated by the symbol <strong>in</strong> the follow<strong>in</strong>g discussion.<strong>Absorption</strong> Coefficient [a()]<strong>Light</strong> is an ensemble <strong>of</strong> photonsthat are absorbed <strong>and</strong> scattered bywater, suspended particles, <strong>and</strong>dissolved matter as they travelthrough a sample. The absorptioncoefficient, a(), is a measure <strong>of</strong>the conversion <strong>of</strong> radiant energyto heat <strong>and</strong> chemical energy. It isnumerically equal to the fraction<strong>of</strong> energy absorbed from a lightbeam per unit <strong>of</strong> distance traveled<strong>in</strong> an absorb<strong>in</strong>g medium.As Figure 1 shows, the absorptioncoefficient <strong>of</strong> pure, particle-free,water ranges from 0.03 to 0.06 cm -1<strong>in</strong> the 760 to 1000 nm b<strong>and</strong>. OBSsensors operate <strong>in</strong> the near <strong>in</strong>fraredspectral b<strong>and</strong> (780 <strong>and</strong> 860 nm)to limit their response to direct<strong>and</strong> reflected sunlight, the NIRcontent <strong>of</strong> which is stronglyabsorbed by water.Figure 1. Graph <strong>of</strong> a shows that the absorption coefficient <strong>of</strong> pure, particlefree,water ranges from 0.03 to 0.06 cm -1 <strong>in</strong> the 760 to 1000 nm b<strong>and</strong>.Copyright © 2005, 2008 Campbell Scientifi c, Inc. 1815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342 App. Note: 2Q-Q

<strong>Light</strong> <strong>Absorption</strong> & <strong>Scatter<strong>in</strong>g</strong> <strong>in</strong> <strong>Water</strong> <strong>Samples</strong><strong>Scatter<strong>in</strong>g</strong> Coefficient [b()]Written by John Down<strong>in</strong>g<strong>Light</strong> scatter<strong>in</strong>g changes the direction <strong>of</strong> photon transport, “dispers<strong>in</strong>g” them asthey penetrate a sample, without chang<strong>in</strong>g their wavelength. The scatter<strong>in</strong>gcoefficient, b(), is equal to the fraction <strong>of</strong> energy dispersed from a light beam perunit <strong>of</strong> distance traveled <strong>in</strong> a scatter<strong>in</strong>g medium, <strong>in</strong> cm -1 . For example, water withb() = 1 cm -1 will scatter 63% <strong>of</strong> the energy out <strong>of</strong> a light beam over a distance <strong>of</strong>1 cm whereas another sample with b() = 0.1 cm -1 will scatter the same proportion<strong>of</strong> energy <strong>in</strong> 10 cm. Both absorption <strong>and</strong> scatter<strong>in</strong>g reduce the light energy <strong>in</strong> abeam as it travels through a sample <strong>and</strong> larger values <strong>in</strong>dicate stronger effects.The scatter<strong>in</strong>g coefficient <strong>of</strong> pure water is less than 0.003 cm -1 so light scatter<strong>in</strong>g has animmeasurably small <strong>in</strong>fluence on an OBS sensor compared to absorption. Pure water is scarce<strong>and</strong> particles are ubiquitous <strong>in</strong> the environment <strong>and</strong> light scatter<strong>in</strong>g from a m<strong>in</strong>iscule amount(< 100 gl -1 ) will cause consistent response from an OBS sensor.Attenuation Coefficient [c()]The attenuation coefficient, c(), is a measure <strong>of</strong> the light loss from the comb<strong>in</strong>edeffects <strong>of</strong> scatter<strong>in</strong>g <strong>and</strong> absorption over a unit length <strong>of</strong> travel <strong>in</strong> an attenuat<strong>in</strong>gmedium. The unit for c() is cm -1 . An easy way to remember the relationshipamong these properties is to recall that a() + b() = c(). It is also important toremember that a(), b(), <strong>and</strong> c() are <strong>in</strong>herent optical properties (IOP) that do notdepend on how a sample is illum<strong>in</strong>ated. They are the same dur<strong>in</strong>g the day <strong>and</strong>night <strong>and</strong> when a sample is measured with turbidity meter A or sediment meter B.<strong>Absorption</strong> <strong>and</strong> scatter<strong>in</strong>g processes can be illustrated by us<strong>in</strong>g an overhead projection <strong>of</strong> petridishes conta<strong>in</strong><strong>in</strong>g <strong>in</strong>ky water <strong>and</strong> skim milk. The projected images <strong>of</strong> both dishes will be graybecause their contents attenuate light, however, the milky sample does so ma<strong>in</strong>ly by scatter<strong>in</strong>gwhereas the <strong>in</strong>ky one ma<strong>in</strong>ly absorbs light.<strong>Light</strong> <strong>Scatter<strong>in</strong>g</strong>We elaborate a bit on light scatter<strong>in</strong>g because <strong>of</strong> its central importance <strong>in</strong> underst<strong>and</strong><strong>in</strong>ghow OBS' respond <strong>in</strong> the environment. The angular distribution <strong>of</strong> light<strong>in</strong>tensity scattered from a beam by a water sample is called the volume scatter<strong>in</strong>gfunction, VSF. The angle between this beam <strong>and</strong> scattered light rays is the scatter<strong>in</strong>gangle. Forward-scattered radiation occupies the hemisphere surround<strong>in</strong>g the<strong>in</strong>cident beam <strong>and</strong> oriented away from the source <strong>and</strong> back scattered radiation fillsthe opposite hemisphere. Figure 2 shows VSFs computed from Mie theory for airbubbles, m<strong>in</strong>eral gra<strong>in</strong>s, <strong>and</strong> biological material as well as the forward- (0° to 90°)<strong>and</strong> back-scatter<strong>in</strong>g (> 90°) VSF regions.2 Copyright © 2005, 2008 Campbell Scientifi c, Inc.App. Note: 2Q-Q 815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342

Written by John Down<strong>in</strong>g<strong>Light</strong> <strong>Absorption</strong> & <strong>Scatter<strong>in</strong>g</strong> <strong>in</strong> <strong>Water</strong> <strong>Samples</strong>Figure 2. Graph shows VSFs computed from Mie theory for airbubbles, m<strong>in</strong>eral gra<strong>in</strong>s, <strong>and</strong> biological material as well as the forward-(0° to 90°) <strong>and</strong> back-scatter<strong>in</strong>g (> 90°) VSF regions.The VSF for bubbles is strongly peaked <strong>in</strong> the forward direction relative to the othermaterials <strong>and</strong> biological material back scatters 10 to 50% less light than the otherparticles. Like the material properties <strong>of</strong> water samples, the variety <strong>of</strong> VSFs is huge.Their shape depends, among other th<strong>in</strong>gs, on particle size <strong>and</strong> refractive <strong>in</strong>dex. Thesize factor, x = D -1 , where D is particle diameter, measures the important relativeeffects <strong>of</strong> the operat<strong>in</strong>g spectrum <strong>and</strong> the size <strong>of</strong> the scatter<strong>in</strong>g particles on the response<strong>of</strong> a meter to a particular sample. For example, the VSF for 100 µm sphericalparticles when illum<strong>in</strong>ated by red light (650 nm) would be similar to one causedby illum<strong>in</strong>at<strong>in</strong>g 131 µm particles with 850 nm light, other th<strong>in</strong>gs be<strong>in</strong>g equal.Most suspended particles <strong>in</strong> streams, lakes, <strong>and</strong> the ocean are larger than the wavelength<strong>of</strong> a meters’ illum<strong>in</strong>ation system, <strong>and</strong> consequently they scatter about halfthe <strong>in</strong>cident light energy <strong>in</strong>to a 10-degree forward-directed cone <strong>and</strong> less than 2.5%<strong>of</strong> it <strong>in</strong> the backward direction. Because <strong>of</strong> the many comb<strong>in</strong>ations <strong>of</strong> particle size,shape, <strong>and</strong> color, similar turbidity read<strong>in</strong>gs can be obta<strong>in</strong>ed from samples conta<strong>in</strong><strong>in</strong>gphysically dist<strong>in</strong>ct particles. <strong>Effects</strong> <strong>of</strong> sediment properties <strong>and</strong> sample-h<strong>and</strong>l<strong>in</strong>gprocedures on the values <strong>in</strong>dicated by our meters are discussed the "Particle Size<strong>Effects</strong>", "NIR Reflectivity & Particle Color", <strong>and</strong> "<strong>Absorption</strong> <strong>Effects</strong>" application notes.The scatter<strong>in</strong>g coefficient <strong>of</strong> pure water is less than 0.003 cm -1 so light scatter<strong>in</strong>g has animmeasurably small <strong>in</strong>fluence on an OBS sensor compared to absorption. Pure water isscarce <strong>and</strong> particles are ubiquitous <strong>in</strong> the environment <strong>and</strong> light scatter<strong>in</strong>g from a m<strong>in</strong>isculeamount (< 100 gl -1 ) will cause consistent response from an OBS sensor.Copyright © 2005, 2008 Campbell Scientifi c, Inc. 3815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342 App. Note: 2Q-Q

<strong>Light</strong> <strong>Absorption</strong> & <strong>Scatter<strong>in</strong>g</strong> <strong>in</strong> <strong>Water</strong> <strong>Samples</strong>ReferencesWritten by John Down<strong>in</strong>gSmith, R.C. <strong>and</strong> K.S. Baker. 1981. Optical Properties <strong>of</strong> the Clearest Natural <strong>Water</strong>s(200-800 nm). Applied Optics, Vol. 20 (177).Petzold, T.J. 1972. Volume <strong>Scatter<strong>in</strong>g</strong> Functions for Selected Ocean <strong>Water</strong>s. SIO 72-28,Scripps Institution <strong>of</strong> Oceanography, La Jolla, California, 79 pages.Bohren, C.F. <strong>and</strong> D.R. Huffman. 1983. <strong>Absorption</strong> <strong>and</strong> <strong>Scatter<strong>in</strong>g</strong> <strong>of</strong> <strong>Light</strong> by SmallParticles. John Wiley & Sons. 530 pages..4 Copyright © 2005, 2008 Campbell Scientifi c, Inc.App. Note: 2Q-Q 815 W. 1800 N., Logan, UT 84321-1784 (435) 753-2342