Absorbance, Reflectivity and Transmittance

The original intensity of the radiation is defined as Iq. A part of the intensity is absorbed, cuiother part is transmitted, still another part is scattered, and a part of the total intensity is reflected. The components, S and T, are processes which are independent of the wavelength (frequency) of the incident photons, whereas R and A are mostly wavelength dependent. 

The exact amount of energy extracted from Iq by each process is a complex set of variables depending upon the type and arrangement of 

That is, the same amount of energy absorbed in the scattering process is refimitted by the atom(s). Contrast this to absorption, where a part of the energy is changed into vibrational energy within the solid. We can summarize these properties as  

Consider an optically homogeneous thin film. optically homogeneous, we mean one that is thin enough so that no scattering can occur. If abeam of photons is incident to the surface at a given angle (but less than that where all of the beam is transmitted- the so-called Brewster angle), part of the beam will be reflected and part will be absmbed. Ihis is shown as follows  

The reflectance, R, is a consequence of the difference in refractive indices of the two media, (1) - air, and (2)- the semi- transparent thin film. The amount of absorption is a function of the nature of the solid. In this case. A, the absorbance, is defined as  

---

In another direction, words ending in -tion, such as absorption, reflection, radiation, transmission, and diffraction, are process terms. As such, they should not be used for the entity absorbed, radiated, or transmitted, nor for the corresponding measured values, such as absorptance, absorbance, reflectance, and transmittance. 

Reflectance data can be more deceiving with respect to the quantitative composition than transmittance data, as illustrated in Figure 6. Equivalent reflectance and transmittance spectra (Figure 6A) are converted into absorbance Ae and F(p) (Figure 6B and C). The F(p) representation... 

It was shown that the transition moment of the )3-carotene molecule is isotropically distributed in the plane of the SC film. As regards the normal direction ofthe film, the observed angular dependence ofboth reflectance and transmittance was in excellent agreement with the theoretical curves calculated assuming the isotropic absorbing medium. This means that the refractive index ofthe SC film is isotropic in the normal ofthe film. Since the refractive index can be directly related to the polarizability of a molecule using Clausius-Mosotti s law and a transition moment linearly depends on the polarizability, the present investigation reveals that the transition moment of the /3-carotene molecule is isotropically distributed both in the plane and in the normal of the SC film. The transition moment lies parallel to the molecular axis of /3-carotene, hence we can conclude that the /3-carotene molecules are randomly oriented in the SC film. 

Provided the film is non-absorbing, then, whilst both AR and AT are finite conservation of energy ensures that A ft = — AT18, as may be verified from eqns. (23) and (24). In addition, if l< , or, equivalently, nx < n2, then it is evident that AR < 0. In other words, even if the film does not absorb in the IR region being examined, some changes in reflectance and transmittance will occur on film formation. 

With non-absorbing films, A = 0, and for reflectance and transmittance the relation R + T = 1 is valid. It is therefore sufficient to know either R or T. Reflectance R is considered in the following treatment. 

Measure the thermal radiative properties of spacecraft thermal protection materials Covington and Sawko, Stewart, and White have measured the reflectance and transmittance for a variety of spacecraft thermal protection materials and calculated the appropriate blackbody-weighted emittance and absorbance for the predicted conditions. In addition, spectroscopy has been used to detect changes in spacecraft surface materials resulting from long-term exposure to the low earth orbit environment. 

In general, reflectance and transmittance pose no threats to the lifespan of the coating. Absorption is the problem. When energy from the sun is absorbed, it leads to chemical destruction (see Section 6.1.3). 

The performance of the beamsplitter is of vital importance to the successful operation of most FT-IR spectrometers and we consider the design and performance of beamsplitters in this section. Consider a beam of monochromatic radiation at wavenumber, V, entering a two-beam interferometer. Let the intensity of the radiation be 7(v) and let the reflectance and transmittance of the beamsplitter be and TV, respectively. For a beamsplitter that absorbs no radiation, Ry- -T = 1. As mentioned in Section 2.2, an ideal beamsplitter is one for which R and TV are equal to 0.5 across the entire spectrum that is measured. Usually, however, either R or TV is slightly greater than 0.5, which has the effect of reducing the beamsplitter efficiency. 

For ratioed spectra, it is of interest to ascertain the effect of the various noise sources on the ratioed spectrum (i.e., the transmittance or reflectance spectrum as the case may be), on the absorbance spectrum, and also to determine, as was done previously [1, 2, 5], the optimum value for the sample to have that will give the minimum error of the calculated value. 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

The International Union of Pure and Applied Chemistry recommends that the definition should now be based on the ratio of the radiant power of incident radiation (Pq) to the radiant power of transmitted radiation (P). Thus, A = log(Po/P) = log T. In solution, Pq would refer to the radiant power of light transmitted through the reference sample. T is referred to as the transmittance. If natural logarithms are used, the quantity, symbolized by P, is referred to as the Napierian absorbance. Thus, B = ln(Po/P). The definition assumes that light reflection and light scattering are negligible. If not, the appropriate term for log(Po/P) is attenuance. See Beer-Lambert Law Absorption Coefficient Absorption Spectroscopy... 

The following is a conversion table for absorbance and transmittance, assuming no reflection. Included for each pair is the percent error propagated into a measured concentration (using the Beer-Lambert law), assuming an uncertainty in transmittance of+0.005.1 The value of transmittance that will give the lowest percent error in concentration is 3.368. Where possible, analyses should be designed for the low uncertainty area. 

Modern NIR equipment is generally robust and precise and can be operated easily by unskilled personnel [51]. Commercial instruments which have been used for bioprocess analyses include the Nicolet 740 Fourier transform infrared spectrometer [52, 53] and NIRSystems, Inc. Biotech System [54, 55]. Off-line bioprocess analysis most often involves manually placing the sample in a cuvette with optical pathlengths of 0.5 mm to 2.0 mm, although automatic sampling and transport to the spectrometer by means of tubing pump has been used (Yano and Harata, 1994). A number of different spectral acquisition methods have been successfully applied, including reflectance [55], absorbance [56], and diffuse transmittance [51]. 

As long as 0 < p < 1 (deviations may occur in experiments), the conversion of reflectance into the Kubelka-Munk function can be performed, just as absorbance can be calculated from transmittance for 0 < t < 1 (and it usually is without any further consideration). However, the proportionally to the concentration may not hold true. As applies for the Lambert-Beer law, the Kubelka-Munk function is a limiting case for the range of weak absorption. For low reflectance (transmittance), the Kubelka-Munk function (absorbance) will not be proportional to the concentration of the absorbing species, and quantification is not possible. 

FIGURE 6 Comparison of the different appearance of band areas depending on representation. (A) reflectance or transmittance, (B) absorbance, and (C) Kubelka-Munk function. 

---