Spectrophotometer, measuring transmittance

Willey R R 1976 Fourier transform infrared spectrophotometer for transmittance and diffuse reflectance measurements Appl. Spectrosc. 30 593-601... 

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. 

A spectrophotometer measures the sample transmittance, T = I Ir, which is the ratio of the light transmitted through the sample and that transmitted through a reference. 

See color measurement and control for a discussion on applying color quality standards (color evaluation, comparison, matching, and specification). However, a case of special interest is assoicated with the tendency for many initial colorless or white, transparent, or translucent polymers to develop undesirable yellowish color, with aging in particular. A standard method (ASTM D1925), based on the use of a recording spectrophotometer for transmittance and reflectance measurement, permits the determination of a yellowness index (YI). 

Spectrophotometers measure the ratio of the intensity of the incident and transmitted radiation, which is known as the transmittance, T. 

Transmission and reflection spectra of the boron films on sapphire were measured over a wavelength range 200-2500 pm using an MPS-50 (Shimazu) spectrophotometer. In the transmission and reflection measurements, an uncoated sapphire and an evaporated aluminum film were used as the references, respectively. Absorption coefficient, refractive index, and extinction coefficient of the boron films were calculated from the measured transmittance and reflectance at normal incidence. 

The scales of spectrophotometers are often calibrated to read directly in absorbances, and frequently also in percentage transmittance. It may be mentioned that for colorimetric measurements I0 is usually understood as the intensity of the light transmitted by the pure solvent, or the intensity of the light entering the solution /, is the intensity of the light emerging from the solution, or transmitted by the solution. It will be noted that ... 

When using a spectrophotometer for a colorimetric analysis, both the 0% and 100% transmittance (oo and 0 absorbance) readings must be set. Once the instrument has warmed up, with the light beam blocked and with nothing in the sample compartment, the readout is set to 0% transmittance (oo abs.). Again, this measurement is done to set / in the absorbance equation shown earlier. A blank, a solution containing all the components used in the analysis except the analyte being measured, is placed in a cuvette, placed in the sample... 

The transmittance values for the calibration and experiment solutions can be measured either with the spectrophotometer or with the pyroelectric probe. In the first case, this reading can obviously be done at any time after the calorimeter is filled with the solution. In the latter case, the value is computed by using equation 13.21. 

Optical spectrophotometers work in different modes to measure optical density, absorbance, or transmittance. 

The optical density can be easily related to other well-known optical magnitudes that are also directly measurable by spectrophotometers, such as the transmittance, T = I//o, and the absorbance, A = I - I//q ... 

The phase transition was traced by monitoring the transmittance of a 500 nm light beam on a Spectronic 20 spectrophotometer (Baush Lomb). The concentration of the aqueous polymer solution was 5 wt%, and the temperature was raised from 15 to 70°C in 2° increments every 10 min. To observe their pH/temperature dependence, the phase transitions of polymers in citric-phosphate buffer solution versus temperature at two pH values (4.0 and 7.4) were measured. 

Iodine in water also may be determined by the Leucocrystal violet colorimetric method. An aqueous sample is treated with mercuric chloride followed by Leucocrystal violet reagent [4,4 ,4 —methylidynetris(N,N-dimethylani-hne)] in the pH range 3.5 to 4.0. A violet color is produced. The absorbance or transmittance is measured at 592 nm by a spectrophotometer or filter photometer. Iodine concentration is calculated from a standard calibration curve. 

A Hardy-type spectrophotometer may be used for determining luminous reflectance, transmittance, and color of polymers (ASTM-791). The transmittance of plastic films is measured by ASTM-D1746. 

Photometric accuracy is determined by comparing the difference between the measured absorbance of the reference standard materials and the established standard value. Many solid and liquid standards are commonly used to verify the photometric accuracy of a spectrophotometer. An optically neutral material with little wavelength dependency for its transmittance/absorbance is desirable because it eliminates the spectral bandwidth dependency of measurements. The advantages and disadvantages of various commonly used photometric accuracy standards are summarized in Table 10.6. Even for a relatively stable reference standard, the intrinsic optical properties may change over time. Recertification at regular intervals is required to ensure that the certified values of the standards are meaningful and accurate for the intended use. 

Some spectrophotometers allow the measurement of absorbance over a dynamic range of 4 to 6 decades. However, elevated values of absorbance are less reliable because they correspond to very weak transmitted intensities (///0 = 10-6 for A = 6). For most instruments, there are three independent causes of error that can affect transmittance (Fig. 11.23) ... 

For absorbance values <0.3, one should try to concentrate the extract (e.g., by evaporation), make a new extract using more plant material and less solvent, or extract the pigments in a separatory funnel into a small volume of a hydrophobic solvent in the epiphase. Various spectrophotometers are constructed to measure absorbance (extinction) values only up to 1.0 (i.e., a transmittance of 10%). In such cases, an absorbance >0.85 is notsuitable, and the extract solution should be diluted to obtain valid Chi b and carotenoid values. In both cases, care must be taken to ensure that the final volume of the extract solution is carefully recorded and considered in the calculation of total Chls and carotenoids. 

The films contained between 0.1 and 1% residual monomer according to gas chromatographic analysis. The transmittance and fluorescence spectra were measured with a Spectronic 505 spectrophotometer by Bausch and Lomb. To obtain correct fluorescence spectra with respect to energy, the above mentioned apparatus was calibrated using a wolfram-band lamp and an aperture driven by a wavelength-dependent correction... 

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