Photometric error

The minimum uncertainty (ca. 3%) of photometric error ranges from approximately 20 to 60% transmittance or an absorbance range of 0.2-0.7, a 5% relative error in concentration has a photometric range of 0.1-1.0. 

Fig. 5.8. Relative photometric error in concentration as a function of %T. Optimum range of transmittance is shown as 20-60%T for approximately 3% error for a 1% error in T.

Based on minimizing the photometric error, what range of absorbances is optimal for absorbance spectroscopy What is the relative dynamic range of absorbance measurements ... 

Optics Issues relative to hygroscopic optics and the need to pay attention to mirror mounts relative to vibration and/or thermal effects have already been addressed. Zinc selenide is an important alternative material, especially when antireflection (AR) coated. If potassium bromide absolutely has to be used for its lower transmission range (down to 400 cm ) then a protective coating such as Parylene must be used. Most process analyzers use protective windows between the spectrometer and the sample interface. If used, back reflections from the window surfaces into the interferometer must be avoided because these will cause photometric errors. Such reflection can be eliminated by wedging or tilting the optical windows, relative to the beam path. 

Relative photometric errors. The fixed current error for the vidicon and the variable error for the image dissector have been discussed under the headings of independent and square-root errors (32J. Using eq 8c from the photometric errors paper (32), and noting that the relative absorbance error, RS, is equal to the relative concentration error, RSq, it can be shown that the relative concentration errors for the vidicon, RSq image dissector, RSp jp, are given by ... 

The precise measurement of eluent ion absorption is also extremely important in indirect photometric detection. It is known from classical spectrophotometry [31] that the photometric error is only small in an absorbance range between 0.2 and 0.8. There-... 

There are several recorded determinations of the absorption curves of the aromatic amino-acids. Most of these were obtained with photographic methods of spectrophotometry which have been superceded by more accurate photoelectric methods. It will be shown that in the spectrophotometric analysis of tyrosine and tryptophan in proteins, the photometric error is magnified in the final estimate of tyrosine and tryptophan contents. This fact is inevitably bound up with the form of the equations of mixture analysis. It is therefore important that the absorption constants be measured as accurately as possible. 

Minimal emission. In an a.c. system, emission from the burner will not produce a photometric error. However, high emission will contribute to the flicker of the output, because the noise current from the photomultiplier detector increases as a function of the total light faUing on it. A very bright flame will therefore tend to produce a fluctuating output. 

Figure 64 Sampling problems that can result in photometric errors.

The following are words of caution about the preparation of evaporated films from solvent-based solutions If KBr or NaCl windows are used, ensure that any solvents or solutions used are water free. When the film is cast, allow the solvent to evaporate slowly (to prevent moisture condensation) and finish evaporation under a heat lamp to remove any residual solvent. If a solid is cast from solution, make sure that crystallization has not occurred. Note that for some crystalline solids a preferred orientation can occur (such as along the crystal lattice lines of the salt window), causing intensity variations as a function of position of the evaporated film. Generally, in the preparation of any film—cast, smear, or capillary—ensure that the film is regular and free from voids, air bubbles, or pinholes. Failure to do this will result in photometric errors in the recording of the spectrum. 

Successful application of NIRS greatly depends on the robustness, specificity, selectivity and transferability of the calibration. In particular, it is highly desirable to be able to support a plant analyser from a laboratory instrument. Sampling-based optical artefacts and the optical characteristics of each instrument, including mechanical and photometric errors, impair transferability of data between instruments. Collaborative NIRS projects with transfer between instruments of calibration files, equations and spectra have been described [245] and interlaboratory collaborative studies of NIRS calibration methodology (for moisture analysis) have been carried out [246]. Blanco et al. [247] have compared various calibration methods in NIR diffuse reflectance spectroscopy. An absolute procedure for instrument calibration and standardisation has been presented [248]. A cloning procedure and... 

At z=1.02, R-band measurements correspond to the rest frame U-band and H-band measurements correspond to the rest frame I-band. This choice of bands provides a long baseline for measuring colors, is sensitive to the 4000A break as a discriminant for galaxy type, and is less sensitive to photometric errors. Figure 2 indicates the rest frame colors of the objects within an AbeU core radius of 3C208.1 on the assumption that they are aU at z=1.02. 

Figure 1. Radial CO 2.36 m Profiles. Each point is center of a 1.5 arcsec bin. The formal 1(7 zero point uncertainty is 0.01 mag. The error bars show the photometric errors while the dashed profiles show the effect of a 1

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