Find out more. Fortunately, since the GF/F surface has a clear mesh structure with fiber alignment, the first cause of variability could be removed by positioning the filter with the fibers aligned vertically. Result of transmittance measurement = TRANS, thick line; ODs by T-R and T methods = medium and thin lines, respectively. (Perkin Elmer, Varian Cary Shimatzu, etc.) The widest data dispersion is for pigment absorption (labeled PIG), due to the quadratic combination of the experimental error of the measurements of the original and oxidized sample. Part of the wave is reflected in the direction OR, and part refracted in the direction OT. not far from the combined errors of the two measurements. Reconsideration of the error source due to light scattered outside the integrating sphere port, with the aid of ad hoc measurements, led to the conclusion that it affects only the measured value of the ‘reference’ filter reflectance [i.e. A: Pure Appl. On the other hand, the detector of the double-beam spectrophotometer, viewing the sphere inside, measures the ratio of two diffuse light sources: the light reflected by the filter and that reflected by the Spectralon plate closing port B2. Particular attention has been devoted to the error contribution by the variability of the optical properties of the GF/F filters used. Nine 25-mm-diameter GF/Fs, wetted by immersion in distilled water for 1 h and accurately positioned as indicated above, were measured in the transmission and reflection modes. As a consequence, the T-R method, which performs an accurate correction for backscattering, yields ODs values larger than those measured by the T method towards the upper limit of the wavelength range, but the difference generally decreases with decreasing wavelength, and may invert sign for the lowest wavelength values. Therefore, the transmittance is 0%. For a completely opaque material, the light cannot pass through the material. This error represents the estimate of the overall error of the T-R measurement of the optical density of the filter-retained particle sample. The modifications of the original T-R procedure outlined in Section 2.2 suggested the performance of additional reliability tests of the method. In the experimental set-up used, the sphere port is an 8 × 20 mm opening, the light beam has a 2 × 8 mm cross-section and the 0.2-mm-thick filter is directly adjacent to the port. This is a colored sediment (clay containing iron oxides), with absorption as the prevailing contributor to ODtr(750) (~75%). By continuing to use this site you agree to our use of cookies. It follows that the standard deviation of (Tf + Rf) is presumably lower than the value [σ(Tf)2 + σ(Rf)2]1/2 that is computed if Tf and Rf are independent variables. For the first test, four 500 ml volumes taken from a 10 l volume of lake water were filtered through 25-mmdiameter GF/F filters under gentle (200 mmHg) depression. The stability of the ODsus versus ODs correlation obtained by the T-R method is probably due to the accurate correction for light scattering performed by this method. The pigment OD spectra yielded by the T and T-R methods are almost superimposed (the maximum difference is ~0.004 at 500 nm). The sample 6 discrepancy is not statistically compatible with the dispersion of the data of the other samples, and does not appear in the measurements performed in the reflection mode (Figure 2B). To find out more, see our, Browse more than 100 science journal titles, Read the very best research published in IOP journals, Read open access proceedings from science conferences worldwide, Journal of Optics A: Pure and Applied Optics, Exact solution of a two-type branching process: clone size distribution in cell divisionkinetics, Spectral and polarization effects in deterministically non-periodic multilayers containingoptically anisotropic and gyrotropic materials, Chiral tunneling in single-layer and bilayer graphene, Equivalence of time-domain inverse problems and boundary spectral problems, Use of the optical admittance function and its WKB approximation to simulate andevaluate transmittance spectra of graded-index colloidal films, On the choice of the phase constant of the Fresnel transmission coefficient of a slab. Figure 3 shows the estimate of the error caused by the use of equations (8) and (9), instead of equations (4) and (5), which is the error due to the fact that the transmittance and the reflectance of the ‘reference’ filter and of theparticle-retaining filter are not the same. When light passes through any surface or material, it can be transmitted, reflected, or absorbed. The second modification regards the sequence of measurements. The old IR-spectra can only be found in Transmittance mode. Experimental error (= 1σ) of the optical density of the particle suspension, ODsus, as obtained by the T-R method, due to all error sources (thick line) and due to the error of the measured ODs (thin line), for σ(ODs) = 0.002 (solid line) and σ(ODs) = 0.005 (dashed line). Full disclaimer here. Although the numerical constants of equation (10) were computed from measurements performed on the single phytoplankton species, S. obliquus, previous tests indicated that the empirical correlation obtained is insensitive to the particle size and thus has a quite general validity. The optical density (OD = log [1/T], where T is the transmittance) of the filter-retained particle sample measured by the T-R method, ODs(λ), was converted into the equivalent optical density of the particle suspension, ODsus(λ), by means of an empirical correlation (ODsus versus ODs), which is determined by comparing the scattering-free pigment optical densities of particles in suspension and retained on the filter. Figure 6 shows the plots versus wavelength of the suspension optical density ratios (10 ml sample/2 ml sample) measured by the T and T-R methods, as well as of the ratio of the directly measured optical densities of the suspension (labeled T, T-R and SUSP, respectively). Numerically, the T method underestimates detritus absorption by a factor 3.6 at 650 nm, 1.5 at 550 nm and 1.2 at 450 nm. Obviously, the addition of the particle deposit to the filter does not modify the angular distribution of the forward scattered light, since scattering by the glass-fibers is largely prevailing over that by the particles. Phototherapy and Radiation Protection, BTS256-EF Light-Color-Spectral and Flicker Meter. The tests yielded important variations. Two changes made to the experimental routine deserve to be mentioned. The transmittance is a ratio of intensity. – Solve equation (5) with as given by equation (4). As can be seen from the analysis of the terms of equation (4), the fact that any variation in the filter transmittance is balanced by the opposite variation of the filter reflectance reduces the contribution of the GF/F variability to the ODs error (see the next section). Ratio of measured suspension optical densities of 10 and 2 ml samples versus wavelength. To test this assumption, [σ(Tf)2 + σ(Rf)2]1/2 was computed using the Tf and Rf spectra derived from the optical density data of Figure 2A and B (e.g. The relation between transmittance (T) and absorbance (A) is given by the Beer-Lambert law (Beer’s law). – Place the particle-retaining filter on the sample holder as indicated above. – Transfer the bleached sample to the filtration apparatus and filter 25 ml of Milli-Q water under mild vacuum (Milli-Q water with 60 g l−1 Na2SO4 in the case of marine samples). In this practical case, the above error was found to increase to σ ~ 0.004 (Section 5). Efficient calculation of the reflectance and transmittance of finite inhomogeneous layers To cite this article: Raúl Castillo Pérez et al 2009 J. Opt. At the same wavelengths, the total absorption is underestimated by a factor 2.4, 1.7 and 1.15. The optical density spectrum of pigment, obtained by subtraction of the detritus OD spectrum from the original OD spectrum, is indicated by label P. The sample is characterized by very high detritus absorption (75% of total absorption at 440 nm).

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