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
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.
2.2 Interaction of Optical Radiation with Matter
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 τλ, is given by the Beer–Lambert law:
(3)
where αλ 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 Aλ 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.