Absorption Coefficient Of Pure Water Research Articles

Calibration of absorption coefficients of liquid solutions for measuring liquid film thickness by absorptiometry

Abstract Infrared absorptiometry is a widely used non-intrusive method for measuring the thickness of liquid films. The accuracy of that measurement depends crucially on having high-accuracy data of the absorption coefficient of the laser light used, which is, however, not easily available, especially for the wavelength range where the absorption is strong. Here we propose a method to calibrate the absorption coefficients in such cases. By measuring the light intensity reduction while scanning through a liquid film formed in a wedge, whose angle can be adjusted and determined a priori from interferometry, the absorption coefficient of the liquid can be accurately obtained without the need to create a flat liquid film with exact known thickness. The method is verified by calibrating the absorption coefficient of pure water at an infrared wavelength and the result agrees very well with the values found in the literature. As a demonstration of the application of the method, the absorption coefficients of soap solutions with different compositions were calibrated and used to measure the thicknesses of draining soap films. The results from the absorptiometry are in good agreement with the film thickness measured simultaneously from interferometry.

A new algorithm of retrieving a petroleum substances absorption coefficient in sea water based on a remote sensing image

Establishing the remote sensing algorithm of retrieving the absorption coefficient of seawater petroleum substances is an efficient way to improve the accuracy of retrieving a seawater petroleum concentration using a remote sensing technology. A remote sensing reflectance is a basic physical parameter in water color remote sensing. Apply it to directly retrieve the absorption coefficient of seawater petroleum substances is of potential advantage. The absorption coefficient of waters containing petroleum [ACWCP, ao(λ)], consists of the absorption coefficient of pure water [ACPW, aw(λ)], plankton [ACP, aph(λ)], colored scraps [ACCS, ad, g(λ)], and petroleum substance [ACPS, aoil(λ)]. Among those, ACCS consists of the absorption coefficient of nonalgal particle [ACNP, ad (λ)] and colored dissolved organic matter [ACCDOM, ag(λ)]. For waters containing petroleum, the retrieved ACCS using the existing method is a combination absorption coefficient of ACNP, ACCDOM and ACPA [CAC, ad,g,oil (λ)]. Therefore, the principle question is how to extract ACPS from CAC. Through the analysis of the three proportion tests conducted between the year of 2013 and 2015 and the corresponding remote sensing data, an algorithm of retrieving the absorption coefficient of petroleum substances is proposed based on remote sensing reflectance. First of all, ACPS and CAC are retrieved from the reflectance using the quasi-analytical algorithm (QAA), with some parameter modified. Secondly, given the fact that the backscatter coefficient [BC, bbp(555)] of total particles at 555 nm can be obtained completely from the reflectance, the relation between BC and ACNP in petroleum contaminated water can be established. As a result, ACNP can be calculated. Then, combining the remote sensing retrieving algorithm of ag(440), the method of achieving the spectral slope of the absorption coefficient can be established, from which ACCDOM, can be calculated. Finally, ACPS can be computed as the residual. The accuracy of ACPS based on this algorithm is 86% compared with the in situ measurements.

Measurement of the optical absorption coefficient for liquid based on optical microfiber

An inline measuring method of the optical absorption coefficient for liquid based on optical microfiber (OM) is studied. If an OM is immersed into absorptive liquid, the additional loss of the OM will increase. We have analyzed the relationship between the additional loss of the OMs and the absorption coefficients of the liquids, and found that the OM with determinate construction will be useful for measuring the absorption coefficients of the liquid whose refractive index is lower than that of the OM. Two OM samples are prepared to measure the absorption coefficient of pure water. A supercontinuum source is launched into the OM sample which is immersed into pure water, and absorption spectrum from 1400 to 1700nm has been achieved by monitoring the additional loss. The trend of the absorption spectrum is similar with that of the reported result, showing that the inline measurement of the optical absorption coefficient is a feasible method for those lower refractive index liquids.

A hybrid approach to estimate chromophoric dissolved organic matter in turbid estuaries from satellite measurements: A case study for Tampa Bay

Remote sensing of chromophoric dissolved organic matter (CDOM) from satellite measurements for estuaries has been problematic due to optical complexity of estuarine waters and uncertainties in satellite-derived remote sensing reflectance (Rrs, sr(-1)). Here we demonstrate a hybrid approach to combine empirical and semi-analytical algorithms to derive CDOM absorption coefficient at 443 nm (a(g)(443), m(-1)) in a turbid estuary (Tampa Bay) from MODIS Aqua (MODISA) and SeaWiFS measurements. The approach first used a validated empirical algorithm and a modified quasi-analytical algorithm (QAA) to derive chlorophyll-a concentration (Chla, mg m(-3)) and particulate backscattering coefficient at 443 nm (b(bp)(443), m(-1)), respectively, from which phytoplankton pigment and non-algal particulate absorption coefficient at 443 nm (a(ph)(443) and a(d)(443), m(-1)) were derived with pre-determined bio-optical relationships. Then, the modified QAA was used to estimate the total absorption coefficient at 443 nm (a(t)(443), m(-1)). Finally, a(g)(443) was estimated as (a(t)(443) - a(ph)(443) - a(d)(443) - a(w)(443)) where a(w)(443) is the absorption coefficient of pure water (a constant). Using data collected from 71 field stations and 33 near-concurrent satellite-field matchup data pairs covering a large dynamic range (0.3 - 8 m(-1)), the approach showed ~23% RMS uncertainties in retrieving a(g)(443) when in situ Rrs data (N = 71) were used. The same approach applied to satellite Rrs yielded much higher uncertainties of a(g)(443) (~85%) due to large errors in the satellite-retrieved Rrs(443). When the Rrs(443) was derived from the satellite-retrieved Rrs(550) and then used in the hybrid approach, uncertainties in the retrieved a(g)(443) reduced to ~30% (N = 33). Application of the approach to MODISA and SeaWiFS data led to a 15-year time series of monthly mean a(g)(443) distributions in Tampa Bay between 1998 and 2012. This time series showed significant seasonal and annual variations regulated mainly by river discharge. Testing of the approach over another turbid estuary (Chesapeake Bay, the largest estuary in the U.S.) demonstrated the potential (~25% uncertainties for a limited a(g)(443) range) of using this approach to establish long-term environmental data records (EDRs) of CDOM distributions in other estuaries with similar optical complexity.

Spectral variations in the near-infrared ocean reflectance

The optical properties of natural waters beyond the visible range, in the near-infrared (NIR, 700–900 nm), have received little attention because they are often assumed to be mostly determined by the large absorption coefficient of pure water, and because of methodological difficulties. It is now growingly admitted that the NIR represents a potential optical source of unambiguous information about suspended sediments in turbid waters, thence the need for better understanding the NIR optical behaviour of such waters. It has recently been proposed (Ruddick et al., Limnology and Oceanography. 51, 1167–1179, 2006) that the variability in the shape of the surface ocean reflectance spectrum in the NIR is negligible in turbid waters. In the present study, we show, based on both in situ and remote sensing data, that the shape of the ocean reflectance spectrum in the NIR does vary in turbid to extremely turbid waters. Space-borne ocean reflectance data were collected using 3 different sensors (SeaWiFS, MODIS/Aqua and MERIS) over the Amazon, Mackenzie and Rio de la Plata turbid river plumes during extremely clear atmospheric conditions so that reliable removal of gas and aerosol effects on reflectance could be achieved. In situ NIR reflectance data were collected in different European estuaries where extremely turbid waters were found. In both data sets, a flattening of the NIR reflectance spectrum with increasing turbidity was observed. The ratio of reflectances at 765 nm and 865 nm, for instance, varied from ca. 2 down to 1 in our in situ data set, while a constant value of 1.61 had been proposed based on theory in a previous study. Radiative transfer calculations were performed using a range of realistic values for the seawater inherent optical properties, to determine the possible causes of variations in the shape of the NIR reflectance spectrum. Based on these simulations, we found that the most significant one was the gradual increase in the contribution of suspended sediments to the color of surface waters, which often leads to the flattening of the reflectance spectrum. Changes in the scattering and absorption properties of particles also contributed to variations in the shape of the NIR surface ocean reflectance spectrum. The impact of such variations on the interpretation of ocean color data is discussed.

Intralipid: towards a diffusive reference standard for optical tissue phantoms

Measurements of optical properties carried out at visible and NIR wavelengths on many samples of Intralipid 20% showed a high stability and surprisingly small batch-to-batch variations. Measurements have been carried out in a short time interval using samples from nine different batches with expiry dates spreading over ten years. For the specific reduced scattering coefficient, the values we have obtained, averaged over the nine batches, are 25.9, 21.2, and 18.4 mm−1 at λ = 632.8, 751, and 833 nm, respectively, and the corresponding maximum deviations from the average were 2.2%, 1.1%, and 1.4%. For the absorption coefficient, we obtained values slightly smaller with respect to the absorption coefficient of pure water at 751 and 833 nm, and slightly larger at 632.8 nm. These results suggest that Intralipid 20% can be the first step towards a diffusive reference standard for tissue-simulating phantoms.

Contribution of Raman scattering to water-leaving radiance: a reexamination

We have reexamined the contribution of Raman scattering to the water-leaving radiance in case 1 waters by carrying out radiative transfer simulations that combine the latest reported measurements of the absorption coefficient of pure water with direct measurements of the spectral variation of the Raman-scattering coefficient. The resulting contribution of Raman scattering is then compared with experimental measurements of the water-leaving radiance, and the fractional contribution of radiance produced by Raman scattering to the total radiance measured at a given wavelength is determined. The results show that (1) the contribution of Raman scattering to the water-leaving radiance in an ocean of pure seawater is as much as 50-100% larger than earlier predictions, and (2) the Raman contribution does not decay as rapidly with increasing concentrations of chlorophyllouslike pigments C as predicted earlier. In fact, the Raman fraction for C <or= 1 mg/m(3) is approximately >8% at wavelengths of interest in ocean color remote sensing and therefore cannot be ignored in ocean color modeling.

Temperature‐dependent absorption of water in the red and near‐infrared portions of the spectrum

We looked at the influence of temperature and salinity on the absorption coefficient of water with emphasis on the red and near‐IR portions of the spectrum. The absorption coefficient of pure water was found to have a strong dependence on temperature and little dependence on salinity near the harmonics of the O−H bond‐stretching frequency. We found an increase in the absorption coefficient of 0.009 m−1 °C−1 at 750 nm and 0.0015 m−1 °C−1 at 600 nm.

Optical absorption coefficients of water

THE absorption spectrum of water in the visible region has been widely investigated1–9 because of the basic, technological and environmental importance of liquid water. Despite these efforts, there are significant disagreements between experimental results: discrepancies of factors of ∼2 in the absorption coefficients are not uncommon between various data. Possible reasons for the disagreements among the various studies are: (1) a lack of a reliable, sensitive technique for measuring small absorpton coefficients in liquids; (2) the presence of a significant amount of light scattering by particles (note that the amount of Rayleigh scattering by pure water is quite small and predictable in the visible8; and (3) measurements are often not done for pure distilled water stored in a noncontaminating vessel. We present here the first accurate measurement of the absorption coefficient of pure water at 21 °C in the 450–700-nm region. We have utilised a recently developed opto–acoustic (OA) technique10, using pulsed dye lasers and immersed piezoelectric transducers. This technique is ideally suitable for measuring weak linear or nonlinear absorptions in nonfluorescing neat liquids or solutions11,12. Our present absorption spectra for water, having typical accuracies of ±5%, should be useful not only for a basic understanding of the properties of water, but also for practical applications like underwater light propagation or intracavity dye laser measurement of dissolved materials in aqueous solutions.

Absorption coefficient of pure water at 488 and 5415 nm by adiabatic laser calorimetry*

The absorption coefficient of pure water was found to be 0.00017 cm−1 at 488 nm and 0.00029 cm−1 at 541.5 nm using adiabatic laser calorimetric techniques. These values are in good agreement with the generally accepted long-path transmission spectra of Clarke and James.