Standard observer curves

Colorimeters. Also known as tristimulus colorimeters, these are instniments that do not measure spectral data but typically use four broad-band filters to approximate the jy, and the two peaks of the x color-matching functions of the standard observer curves of Figure 7. They may have lower accuracy and be less expensive, but they can serve adequately for most industrial color control functions. Examples of colorimeters are the BYK-Gardner Co. XL-835 the Hunter Lab D25 series the Minolta CA, CL, CS, CT, and CR series (the last of these is portable with an interface) and the portable X-Rite 918. 

Pruszkowska et al. [135] described a simple and direct method for the determination of cadmium in coastal water utilizing a platform graphite furnace and Zeeman background correction. The furnace conditions are summarised in Table 5.1. These workers obtained a detection limit of 0.013 pg/1 in 12 pi samples, or about 0.16 pg cadmium in the coastal seawater sample. The characteristic integrated amount was 0.35 pg cadmium per 0.0044 A s. A matrix modifier containing di-ammonium hydrogen phosphate and nitric acid was used. Concentrations of cadmium in coastal seawater were calculated directly from a calibration curve. Standards contained sodium chloride and the same matrix modifier as the samples. No interference from the matrix was observed. 

Fig. 4.16 shows a typical chromatogram for a standard 42pg L 1 phosphate solution and hypoxanthine peaks resulting from various phosphate samples after reaction with the enzyme. The calibration curve had a slope of 0.043 0.002 and an intercept of 0.124 0.033 with a correlation coefficient of 0.998. Linearity up to 30mg L 1 was observed. Relative standard deviation of triplicate runs was 10% or less. The detection limit, twice the signal of the blank, was determined to be 1.5mg L 1. 

Contrary to the increase in oxidation observed during standard RIE, films subjected to one of the passivation pretreatments (with the exception of the barrel reactor process) showed little further increase in oxide conversion beyond the initial pretreatment conversion. Only the films pretreated by high pressure oxygen plasma in the barrel reactor showed a further increase in oxide conversion on subsequent "standard" RIE (curve 5e). 

The external standard calibration method is a simple but less precise method and should only be used when the sample preparation is simple and small or no instrumental variations are observed. The method is not suitable for use with complicated matrices but is often used in pharmaceutical product analysis characterized by simple matrices and easy sample preparation. To construct a standard curve, standard solutions containing known concentrations of the analyte must be prepared and a fixed volume injected into the column. The resulting areas or heights of the peaks in the chromatogram are measured and plotted versus the amount injected. Unknown samples are then prepared, injected and analyzed in exactly the same manner, and their concentrations are determined from the calibration plot. The term external standard calibration implies that the standards are analyzed in chromatographic runs that are separate from those of the unknown sample. 

Once a reflectance curve is obtained spectrophotometrically, the measured information is mathematically transformed according to standard conventions, into the numbers used to describe the color of the sample. In practice, a standard observer and standard illuminant are selected based on test methods defined by the Commission International de I Eclairage (CIE). Calculation procedures involve the numerical integration of the product of the spectral power distribution S(l) of the light source and the reflectance factor R(l). Reflectance factors represent the percentage of light reflected by the sample at each wavelength. 

Figure 3 CIE 1931 Standard Colorimetric Observer curve. CIE, Commission Internationale de I Eclairage.

Fig. 3). For this reason, always radiometric units must be used in photosynthesis instead of photometric units. The radiometric analogue of the lumen (luminous flux) is the watt (radiant flux). At each wavelength both are coupled according to the standard observer curve at 555 nm 1 W of radiant flux corresponds to a luminous flux of 683 lumens by definition. As a comparison, at 650 nm, 1 W corresponds to 73 lumens only. 

Figure 11.4 Standard observer curves. Weighted contributions from each curve are used to recreate spectra. The red contribution (R) is represented by z, green (G) by y, and biue (B) by x. 

Figure 11.5 Weighting factors for each standard observer curve. 

Standard observer curves In the QE colorant space system, three spectral curves that describe the contribution of red, green, and blue l hf h) a perceived color. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

The GIE Standard Observer. The CIE standard observer is a set of curves giving the tristimulus responses of an imaginary observer representing an average population for three primary colors arbitrarily chosen for convenience. The 1931 CIE standard observer was deterrnined for 2° foveal vision, while the later 1964 CIE supplementary standard observer appHes to a 10° vision a subscript 10 is usually used for the latter. The curves for both are given in Eigure 7 and the differences between the two observers can be seen in Table 2. The standard observers were defined in such a way that of the three primary responses x(X),jy(X), and X), the value ofjy(X) corresponds to the spectral photopic luminous efficiency, ie, to the perceived overall lightness of an object. 

In tlie case of a random sample of observations on a continuous random variable assumed to have a so-called nonnal pdf, tlie graph of which is a bellshaped curve, tlie following statements give a more precise interpretation of the sample standard deviation S as a measure of spread or dispersion. 

The procedure comprises the addition of a constant amount of internal standard to a fixed volume of several synthetic mixtures which contain varying known amounts of the component to be determined. The resulting mixtures are chromatographed and a calibration curve is constructed of the percentage of component in the mixtures against the ratio of component peak area/standard peak area. The analysis of the unknown mixture is carried out by addition of the same amount of internal standard to the specified volume of the mixture from the observed ratio of peak areas the solute concentration is read off using the calibration curve. 

Plot the observed e.m.f. values against the concentrations of the standard solutions, using a semi-log graph paper which covers four cycles (i.e. spans four decades on the log scale) use the log axis for the concentrations, which should be in terms of fluoride ion concentration. A straight line plot (calibration curve) will be obtained. With increasing dilution of the solutions there tends to be a departure from the straight line with the electrode combination and measuring system referred to above, this becomes apparent when the fluoride ion concentration is reduced to ca 0.2 mg L-1. 

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

PbO and Pb304 are oxides of lead that are important primary products. Under certain circumstances, PbO is also observed in lead-acid batteries (cf. Sec. 4.4.5.1). Fig. 1 shows that these oxides are only stable in a neutral or alkaline environment. Their equilibrium potentials are represented by curves C-F and their standard values are compiled in Table 3. 

For the usual accurate analytical method, the mean f is assumed identical with the true value, and observed errors are attributed to an indefinitely large number of small causes operating at random. The standard deviation, s, depends upon these small causes and may assume any value mean and standard deviation are wholly independent, so that an infinite number of distribution curves is conceivable. As we have seen, x-ray emission spectrography considered as a random process differs sharply from such a usual case. Under ideal conditions, the individual counts must lie upon the unique Gaussian curve for which the standard deviation is the square root of the mean. This unique Gaussian is a fluctuation curve, not an error curve in the strictest sense there is no true value of N such as that presumably corresponding to a of Section 10.1—there is only a most probable value N. 

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