Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 98

All electromagnetic radiation, including optical radiation, has the same essential physical nature. Electromagnetic radiation is a form of energy that propagates through space as mutually perpendicular oscillating electric and magnetic fields. (See Figure 4 from Ionizing Radiation.) The wave‐like nature of electromagnetic radiation is exhibited in optical phenomena such as refraction (the behavior of light crossing the interface between two media) and diffraction (the bending of light around obstacles). The distance between two successive peaks in a wave is the wavelength, designated by λ. The number of oscillations passing a point in a given time is the frequency, commonly designated by ν and measured in hertz (Hz), which is the same as a cycle per second (s⁻¹). The speed of propagation of a wave, c, is related to the wavelength and the frequency and is given by

(1)

Electromagnetic radiation propagates through vacuum at a speed of 2.998 × 10⁸ m s⁻¹ (meters per second). The frequency of an electromagnetic wave is invariant with the medium through which it propagates, but the speed and hence also the wavelength of the radiation changes depending on the refractive index of the medium. The speed of light in air is nearly the same as in vacuum.

Electromagnetic radiation is emitted or absorbed by matter in discrete amounts or “quanta” of energy. A quantum of electromagnetic energy, termed a photon, behaves like a particle that cannot be divided or combined. The energy E of a photon is related to its frequency by the equation

(2)

where h, Planck's constant, has the value 6.63 × 10⁻³⁴ J s (joule‐seconds) or 4.14 × 10⁻¹⁵ eVs (electron volt‐seconds). A beam of electromagnetic radiation can be understood with equal validity as a train of waves characterized by their frequency or as a stream of photons characterized by their quantum energy.

Broadband radiation consists of photons with some distribution of quantum energies. Such a distribution is termed a spectrum, or spectral density function. The spectral range can be defined in terms of photon energy or frequency or wavelength. For optical radiation, wavelength is generally used as the spectral unit, where λ is understood to be the wavelength of the radiation in vacuum or air, regardless of the medium in which the radiation is actually propagating.

Photon energies of more than about 12 eV can cause ionization in biological matter. This corresponds to a wavelength of about 100 nanometers (nms). Nonionizing radiation by definition has wavelengths longer than 100 nm. The optical radiation spectrum has been divided by the International Commission on Illumination (Commission Internationale de l'Eclairage – CIE) into biologically significant bands as follows (4):

UV‐C (“germicidal”)	100–280 nm
UV‐B (“erythemal”)	280–315 nm
UV‐A (“black light”)	315–400 nm
Visible	400–780 nm
IR‐A	780–1400 nm
IR‐B	1.4–3 μm
IR‐C	3–1000 μm

The divisions between bands are somewhat arbitrary. In older terminology, the UV‐B and UV‐C bands were sometimes referred to collectively as “actinic radiation.” Some photobiologists divide UV‐A from UV‐B at 320 nm and UV‐B from UV‐C at 290 nm (5). The UV‐A band may be further split into UV‐A1 (340–400 nm) and UV‐A2 (315–340 nm) based on the ability of the shorter UV‐A wavelengths to cause direct photochemical damage to nucleic acids (6). The limits of the visible region are not consistently defined due to individual variability in the ability to perceive violet light in the 380–400 nm range and red light in the 700–780 nm range.

UV radiation of wavelength shorter than about 190 nm is rarely encountered in occupational settings because it is strongly absorbed by air. This spectral range is referred to as the “vacuum UV” because historically it could be studied only under high vacuum. Exposure to vacuum UV can occur outside the earth's atmosphere or in proximity to very intense UV‐C sources such as some high‐power xenon arc lamps and certain excimer lamps.