Absorption

During absorption, the intensity of an incident electromagnetic wave is attenuated in passing through a medium. The absorbance of a medium is defined as the ratio of absorbed and incident intensities. Absorption is due to a partial conversion of light energy into heat motion or certain vibrations of molecules of the absorbing material. A perfectly transparent medium permits the passage of light without any absorption, i.e. the total radiant energy entering into and emerging from such a medium is the same. Among biological tissues, cornea and lens can be considered as being highly transparent for visible light. In contrast, media in which incident radiation is reduced practically to zero are called opaque.

The terms "transparent" and "opaque" are relative, since they certainly are wavelength-dependent. Cornea and lens, for instance, mainly consist of water which shows a strong absorption at wavelengths in the infrared spectrum. Hence, these tissues appear opaque in this spectral region. Actually, no medium is known to be either transparent or opaque to all wavelengths of the electromagnetic spectrum.

A substance is said to show general absorption if it reduces the intensity of all wavelengths in the considered spectrum by a similar fraction. In the case of visible light, such substances will thus appear grey to our eye. Selective absorption, on the other hand, is the absorption of certain wavelengths in preference to others. The existence of colors actually originates from selective absorption. Usually, body colors and surface colors are distinguished. Body color is generated by light which penetrates a certain distance into the substance. By backscattering, it is then deviated and escapes backwards from the surface but only after being partially absorbed at selected wavelengths. In contrast, surface color originates from reflection at the surface itself. It mainly depends on the reflectances which are related to the wavelength of incident radiation by (2.12).

The ability of a medium to absorb electromagnetic radiation depends on a number of factors, mainly the electronic constitution of its atoms and molecules, the wavelength of radiation, the thickness of the absorbing layer, and internal parameters such as the temperature or concentration of absorbing agents. Two laws are frequently applied which describe the effect of either thickness or concentration on absorption, respectively. They are commonly called Lambert's law and Beer's law, and are expressed by

where z denotes the optical axis, I(z) is the intensity at a distance z, I0 is the incident intensity, a is the absorption coefficient of the medium, c is the concentration of absorbing agents, and k ' depends on internal parameters other than concentration. Since both laws describe the same behavior of absorption, they are also known as the Lambert-Beer law. From (2.13), we obtain z = 0-ln . (2.15)

The inverse of the absorption coefficient a is also referred to as the absorption length L, i.e.

The absorption length measures the distance z in which the intensity I(z) has dropped to 1/e of its incident value I0.

In biological tissues, absorption is mainly caused by either water molecules or macromolecules such as proteins and pigments. Whereas absorption in the IR region of the spectrum can be primarily attributed to water molecules, proteins as well as pigments mainly absorb in the UV and visible range of the spectrum. Proteins, in particular, have an absorption peak at approximately 280 nm according to Boulnois (1986). The discussion of the absorption spectrum of water - the main constituent of most tissues - will be deferred to Sect. 3.2 when addressing thermal interactions.

In Fig. 2.4, absorption spectra of two elementary biological absorbers are shown. They belong to melanin and hemoglobin (HbO2), respectively. Melanin is the basic pigment of skin and is by far the most important epidermal chromophore. Its absorption coefficient monotonically increases across the visible spectrum toward the UV. Hemoglobin is predominant in vascular-ized tissue. It has relative absorption peaks around 280 nm, 420 nm, 540 nm, and 580 nm, and then exhibits a cut-off at approximately 600 nm. A general feature of most biomolecules is their complex band structure between 400 nm and 600 nm. Since neither macromolecules nor water strongly absorb in the near IR, a "therapeutic window" is delineated between roughly 600 nm and 1200 nm. In this spectral range, radiation penetrates biological tissues at a lower loss, thus enabling treatment of deeper tissue structures.

The absorption spectra of three typical tissues are presented in Fig. 2.5. They are obtained from the skin, aortic wall, and cornea, respectively. Among these, skin is the highest absorber, whereas the cornea is almost perfectly transparent1 in the visible region of the spectrum. Because of the uniqueness of the absorption spectra, each of them can be regarded as a fingerprint of the corresponding tissue. Of course, slight deviations from these spectra can occur due to the inhomogeneity of most tissues.

1 Actually, it is amazing how nature was able to create tissue with such transparency. The latter is due to the extremely regular structure of collagen fibrils inside the cornea and its high water content.

Spectral Scan 540
Fig. 2.4. Absorption spectra of melanin in skin and hemoglobin (HbO2) in blood. Relative absorption peaks of hemoglobin are at 280 nm, 420 nm, 540 nm, and 580 nm. Data according to Boulnois (1986)
Haemoglobin Absorbance 540

Fig. 2.5. Absorption spectra of skin, aortic wall, and cornea. In the visible range, the absorption of skin is 20-30 times higher than the absorption of corneal tissue. The absorption spectrum of aortic wall exhibits similar peaks as hemoglobin. Data according to Parrish and Anderson (1983), Keijzer et al. (1989), and Eichler and Seiler (1991)

Fig. 2.5. Absorption spectra of skin, aortic wall, and cornea. In the visible range, the absorption of skin is 20-30 times higher than the absorption of corneal tissue. The absorption spectrum of aortic wall exhibits similar peaks as hemoglobin. Data according to Parrish and Anderson (1983), Keijzer et al. (1989), and Eichler and Seiler (1991)

When comparing Figs. 2.4 and 2.5, we find that the absorption spectra of the aortic wall and hemoglobin are quite similar. This observation can be explained by the fact that hemoglobin - as already previously stated - is predominant in vascularized tissue. Thus, it becomes evident that the same absorption peaks must be present in both spectra. Since the green and yellow wavelengths of krypton ion lasers at 531 nm and 568 nm, respectively, almost perfectly match the absorption peaks of hemoglobin, these lasers can be used for the coagulation of blood and blood vessels. For certain clinical applications, dye lasers may be an alternative choice, since their tunability can be advantageously used to match particular absorption bands of specific proteins and pigments.

Not only the absorption of biological tissue, though, is important for medical laser surgery. In certain laser applications, e.g. sclerostomies, special dyes and inks are frequently applied prior to laser exposure. By this means, the original absorption coefficient of the specific tissue is increased, leading to a higher efficiency of the laser treatment. Moreover, adjacent tissue is less damaged due to the enhanced absorption. For further details on sclerostomy, the reader is referred to Sect. 4.1.

In Table 2.2, the effects of some selected dyes on the damage threshold are demonstrated in the case of scleral tissue. The dyes were applied to the sclera by means of electrophoresis, i.e. an electric current was used to direct the dye into the tissue. Afterwards, the samples were exposed to picosecond pulses from a Nd:YLF laser to achieve optical breakdown (see Sect. 3.4). The absolute and relative threshold values of pulse energy are listed for each dye. Obviously, the threshold for the occurrence of optical breakdown can be decreased by a factor of two when choosing the correct dye. Other dyes evoked only a slight decrease in threshold or no effect at all. In general, the application of dyes should be handled very carefully, since some of them might induce toxic side effects.

Table 2.2. Effect of selected dyes and inks on damage threshold of scleral tissue. Damage was induced by a Nd:YLF laser (pulse duration: 30ps, focal spot size: 30 ^m). Unpublished data

Dye

Damage threshold (||J)

Relative threshold

None

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