Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 99

Knowledge of the modes of interaction of radiation with matter forms the basis for understanding biological effects, detection principles, and control principles. To have an effect on, or to be detected by, a medium or system, radiation must enter the material and be absorbed by it.

When electromagnetic radiation propagating through one medium meets an interface with another medium, some of the incident radiation may be refracted (cross the interface into the second medium) and the remainder will be reflected from the surface. The fraction of radiation reflected, called the reflectance, depends on the optical properties of the two media, the wavelength of the radiation, and the angle of incidence of the radiation. Reflectance from a specular surface (i.e. a surface that is smooth on the scale of the wavelength of the radiation) is generally lowest at normal incidence (perpendicular to the surface). Reflectance can be an important contributor to the shielding properties of a material.

As radiation moves through a medium, part of the energy may be absorbed by the medium. The fraction of radiation absorbed depends on the optical properties of the medium, the wavelength, and the distance traveled through the medium. The fraction of monochromatic radiation of wavelength λ transmitted through a thickness x of a medium, called the spectral transmittance τ_λ, is given by the Beer–Lambert law:

(3)

where α_λ is the absorption coefficient in that medium for radiation of wavelength λ. An opaque medium has a very high absorption coefficient, while a transparent medium has a negligible absorption coefficient. The absorbance A_λ of a filtering medium of defined thickness, also called its optical density (OD), is related to the transmittance by the expression:

(4)

An example of an absorption spectrum, that of pure water at 25°C (7), is illustrated in Figure 1.

In order for a medium to absorb optical radiation, the photon energy must match the energy difference between two allowed quantum states within the medium. UV and visible photon energies are high enough to excite transitions between electronic states in many materials. IR photons do not have sufficient energy to cause electronic transitions in most materials but may excite molecules into higher vibrational states. Energy transfer involving increased vibrational, rotational, or translational molecular motion is considered a thermal process.

Three possible outcomes of electronic excitation are photochemical change, fluorescence, and transfer of energy to thermal modes. In a photochemical reaction, an excited electronic state allows existing chemical bonds in the molecule to break or different bonds to be formed, either within the molecule or with other molecules. For example, as part of the visual process, upon absorbing a photon of visible light the retinal pigment rhodopsin isomerizes and then breaks down into two separate molecules. In fluorescence, a portion of the absorbed energy is dissipated by the relaxation of higher vibrational levels of the molecule in the excited electronic state. The electron then drops from the excited state to the ground state with the emission of a photon of wavelength longer than the absorbed photon. A molecule in an excited electronic state might alternatively relax to the ground electronic state by dissipating all of the absorbed energy through thermal processes.

FIGURE 1 Water absorption spectrum between 200 nm and 200 μm.

Source: Data from Ref. (7).

In solid‐state semiconductor materials, absorption of a photon can cause an electron to transition from a low‐energy state (the valence band), in which it is localized in a chemical bond, to a higher energy state (the conduction band), in which it is free to move throughout the material. The vacancy left in the valence band, called a “hole,” behaves as if it were a positive charge. In the presence of an electric field, electrons in the conduction band and holes in the valence band flow as electrical current, which can be measured. Semiconductors are widely used for the detection of optical radiation.

The thermal, photochemical, and/or electrical changes resulting from the absorption of optical radiation may lead to observable effects such as a biological response or a detector signal. The strength of a specific response of a system to radiation as a function of wavelength is called the action spectrum or the spectral response function.