Absorbance or Optical Density

The quantity usually measured is the change in optical density or absorbance with the duration of the irradiation (dose) [eq. (17)]. 

The term on the left side is called optical density or absorbance, A. 

E being the optical density or absorbance, /q and I the intensities at a fixed wavelength (wave number v) of the exciting beam and the beam after passing through the sample, s is the decade molar extinction coefficient, c the polymer concentration, and d the layer thickness. E is determined directly by the... 

Several publications suggest that an optimum concentration of a pbotoinitiator should be such that the optical density or absorbance of the reacting composition should be 0.434 to yield an optical density at which maximum radiation is absorbed. This number has meaning, however, only for ideal systems, cured by monochromatic radiation. With commercial sources of radiation that consist of 20 to 40 emission lines this does not apply. Also, it is important to know that variations in the concentrations of a photoinitiator within various locations of the film that is being photocured, dramatically affect flic locations where the radiation is absorbed, such as at the surface, flnoughout the material, or at some other location. 

The relationship between the molar concentration of a chlorophyll solution and the optic density or absorbance at a particular wavelength (A) fits a linear response defined by the Beer-Lambert law ... 

This method is based on Beer s law, which says that A = e X I X c, where A is the optical density or absorbance, e (epsilon) is the mole extinction coefficient, I is the length of the cell and c is the concentration. 

Throughout this manual the term extinction (symbol E) is employed. The extinction is defined as log 1 1 where and 1 are incident and transmitted light intensities, respectively. Spectrophotometers are calibrated directly in extinction units (also called optical density or absorbency). If the absorptiometer employed reads percentage transmission, T, calculate E as logio(100/r). The use of transmission values in absorptiometry is to be discouraged as they are rarely if ever simply related to the concentration of substance being measured. 

One hundred milliliters of an aqueous solution of methylene blue contains 3.0 mg dye per liter and has an optical density (or molar absorbancy) of 0.60 at a certain wavelength. After the solution is equilibrated with 25 mg of a charcoal the supernatant has an optical density of 0.20. Estimate the specific surface area of the charcoal assuming that the molecular area of methylene blue is 197 A. ... 

The function on the y-axis of the spectrum in Figure 9.4 is the absorbance (as defined on p. 441). Absorbance is also called optical density or optical absorbance in older books these three terms each mean the same thing. We can see from the spectrum that more light is absorbed at 300 nm (in the near infrared) than at 500 nm... 

The logio /(>// is termed absorbance (A) or optical density (OD). Absorbance is currently the preferred term. It is the absorbance of a solution at a particular wavelength that is of use in the discipline of spectrophotometry. As is apparent from these equations, absorbance is a unitless value. 

Despite the fact that absorbance is more limited in sensitivity than fluorescence, the potential of an inexpensive miniaturized optical setup still provides an advantage in certain applications, such as when changes in optical density or color are sufficient for clinical diagnosis. 

The absorbance is a particularly useful quantity since it is directly proportional to the optical density or thickness of the sample. The reason for this is shown in Figure 2.4. 

Frequent practice is the use of the overnight cultures, like in the case of B. subtilis, or 6-h cultures, like in the case of E. coli. The optical density (OD) (absorbance) of the bacteria suspension is varied. It is recommended to be between 0.40 and 0.60, which represents 4xl0 cellsml. ... 

The absorbance, sometimes called the optical density or extinction, denoted A, DO, or A, is a dimensionless physical quantity that corresponds to a monochromatic radiation to the logarithm of the ratio of incident radiation intensity over a transmitted radiation intensity. Depending on the selected logarithm function type, both the Napierian ( and AJ and the decadic absorbance (A, A, , and DO) can be defined and related to transmittance ... 

Translucent or opaque liquids reflectance or diffuse transmittance High optical density (highly absorbing) tiny pathlengths in transmittance... 

Measurements of optical density, or relative absorbance, as a function of applied field strength and dye concentrations taken on cells prepared with a wide variety of mixtures of dyes and nematic liquids show that contrast can be optimized by keeping the dye concentration at or below 1%. This level appears to yield the greatest absorption change. The existence of a level indicates that nematic compounds can produce alignment of only a limited number of dye molecules. As the number of dissolved dye molecules increases, non-oriented molecules that contribute to absorption of the medium under an electric field reduce contrast. 

Spectral measurements can be done either ex situ, changing the doping level or redox state appropriately for every measurement, or in situ, the preferred method for greater accuracy. All such spectroelectrochemical data can be presented with Absorbance (Optical Density) or %-Transmission on the ordinate, and wavelength (nm, fim) or energy (eV, X (in pm) = ca. 1.24/eV), or sometimes, wavenumbers (cm ) on the abscissa. Occasionally, difference spectra, usually referenced to the pristine state of the CP, are also presented. 

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

The physicochemical properties of the reactants in an eiKyme-catalyzed reaction dictate the options for the assay of enzyme activity. Spectrophotometric assays exploit the abihty of a substrate or product to absorb hght. The reduced coenzymes NADH and NADPH, written as NAD(P)H, absorb hght at a wavelength of 340 run, whereas their oxidized forms NAD(P) do not (Figure 7—9). When NAD(P)+ is reduced, the absorbance at 340 run therefore increases in proportion to—and at a rate determined by—the quantity of NAD(P)H produced. Conversely, for a dehydrogenase that catalyzes the oxidation of NAD(P)H, a decrease in absorbance at 340 run will be observed. In each case, the rate of change in optical density at 340 nm will be proportionate to the quantity of enzyme present. 

Some simple rearrangement of Equation 3.1 leads to the concepts of transmission T = Io/1 and absorbance A = — log T, with the quantity s c l called the optical density. The choice of units here for the extinction coefficient (M-1 cm-1) is appropriate for measurement of the absorbance of a solution in the laboratory but not so appropriate for a distance Z of astronomical proportions. The two terms and c are contracted to form the absorption per centimetre, a, or, more conveniently (confusingly) in astronomy, per parsec. The intrinsic ability of a molecule or atom to absorb light is described by the extinction coefficient s, and this can be calculated directly from the wavefunction using quantum mechanics, although the calculation is hard. 

---