Transmittance measurement

The use of ultrasonic energy is different in on/off switches and in transmitters. Switches act on the attenuation of the acoustic signal in the gap between two crystals, while transmitters measure the time of flight of the ultrasonic pulse. 

A critical study has been carried out in order to evaluate the capabilities of Near Infrared spectroscopy for the analysis of commercial pesticide formulations using transmittance measurements. In this sense, it has been evaluated the determination of active ingredients in agrochemical formulations after extraction with an appropriate solvent. 

Such a simplification could be carried out in the example cited above for raspberries, where a transmittance measurement in the region of maximum absorption... 

Figure 9.2 Normalized transmittance measured by the Z-scan with and without the collecting aperture for CdTe QDs with the diameter of 4.1 nm excited at 803 nm (0.4 pj pulse ). The open aperture Z-scan corresponds to the nonlinear absorption and the closed aperture Z-scan to the nonlinear refractive index.

Since the measurement of reflectance and transmittance are defined by essentially the same equation, we will couch our discussion in terms of a transmittance measurement. The important difference lay, as we discussed previously, in the nature of the error superimposed on the measurement. Therefore, we begin by noting that transmittance (T) is defined by the equation 41-1 ... 

In the water tank level control system in the example above, the level transmitter measures the level within the tank. The level transmitter sends a signal representing the tank level to the level control device, where it is compared to a desired tank level. The level control device then computes how far to open the supply valve to correct any difference between actual and desired tank levels. 

Figure 6. Two transmittance measurements that are necessary to use absorption to determine the concentration of an analyte in solution.

This expression is interpreted as the relative uncertainty in the concentration, as related to the relative uncertainty of the transmittance measurements, ST/T. The graph below illustrates the effect of a 1% uncertainty in transmission measurements on the percent relative uncertainty in the concentration (Fig. 5.8). 

The single beam instrument measures directly the amount of energy transmitted by the sample. They give the most accurate transmittance measurements and is particularly helpful for quantitative analysis. It has simpler and more reliable systems than double beam. 

Optical Microscopy and Transmittance Measurement. A microscope equipped with polarizing optics and thermostatted stage was used in certain studies described below. Transmittance measurements were made by use of a power meter (Lexel model 504) positioned close to the cell, on the optic axis of the incident beam. The cell was mounted on the light scattering stage for these measurements, and the transmittance was determined for light with wavelength 647 nm. 

FIGURE 14.52 (a) Absorptance and (b) transmittance measured on days with varying degrees of cloudiness using aircraft colocated above and below the clouds where broadband (224 nm to 3.91 /urn), near-IR (680 nm to 3.30 /urn), and spectral band (10-nm width centered at 500 nm) measurements were made (adapted from Valero et at., 1997a). 

Another kind of effective or average optical constants involves mixtures of different particles such as atmospheric aerosols or soils. Effective optical constants for compacted samples of these mixtures might be inferred from reflectance and transmittance measurements as if the samples were homogeneous. But scattering or extinction calculations based on these optical constants would not necessarily be correct. 

Selected florinated norbomane copolymers were dissolved in propylene glycol methyl acetate to form 14 wt% solutions and then treated with two weight parts of triphenyl-sulfonium triflate as the acid generator. These solutions were then spin coated and light transmittance measured using a 100-nm-thick him at 157 nm. Testing results are summarized in Table 2. 

FIGURE 10 Diffuse reflectance, transflectance, and transmittance measurements. 

The fundamental elements of an ultraviolet-absorption analyzer include (a) a radiation source (b) suitable optical filters (c) a sample cell and (d) an output meter. A transmittance measurement is made by calculating the ratio of the reading of the output with die sample in the cell to die reading widi the cell empty (of ultraviolet-absorbing materials). The concentration can be calculated from the known absorptivity of the substances as previously demonstrated by the equations or it may be determined by comparison with known samples. 

Despite the fact that direct absorbance/transmittance measurements are well established in analytical chemistry owing to the simplicity of the instrumentation and their broad applicability and versatility towards a large number of analytes, most of the reported miniaturized optical devices are based on the measurement of variations of the real part of the refractive index, such as SPR sensors [84,109-111] or interferometric sensors [94,112]. 

Transmittancy measurements have principally been used in connection with the color problem in sugar products. The resulting data are used, because of the automatic cancellation of such effects as solvent correction, cell constant, etc.10... 

If the rare medium exhibits absorption, the penetrating wave becomes attenuated. In the case of transmittance measurements at weak absorption the law of Lambert-Beer is valid in a linear approximation also... 

Newer technology allows for opaque, translucent, transparent samples as well as multiple film thicknesses for both reflectance and transmittance measurements. The newer technology provides a spectral match (matches spectral curve of the sample at each wavelength across the visible spectrum 360-740 nm). The advantage of spectral matching is its ability to match both color and opacity from a single database. 

Both reflectance and transmittance measurements can be applied to the same database. For plastics and translucent liquids and for printing or coating on nonopaque substrates, this is a very important feature. In these applications it is not sufficient to match a standard in reflectance only a transmission match is equally important. Contemporary color-matching systems can combine both types of matches in one calculation using only one database. To use this capability, it is necessary that the spectrophotometer measure both the reflectance and total transmittance of the sample. 

Just as the reflectance and transmittance measurements can be used in one database, contrast measurements can also be applied to a single database (or even combined with reflectance and transmission measurements for the calibration of the database). In this case the two measurements that would be combined are the over white and over black measurements. Typically this technique is applied to coatings and printing inks on paper or screen inks on textiles (white and black cloth). 

For the majority of bioanalytical applications, measures of absorbance will be made at defined wavelengths, and the absorbance reading is sometimes referred to as the optical density (OD) reading. The absorbance is calculated from the transmittance measure using the logarithmic relationship described by the following equation ... 

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