BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 7

From a biological point of view, wavelengths below 180 nm (vacuum UV) are of
little practical significance since they are readily absorbed in air; hence, the spectral
region below ~ 180 nm is also referred to as “the vacuum ultraviolet.” UV-C
wavelengths are more photochemically active, because these wavelengths correspond to
the most energetic photons, are strongly absorbed in certain amino acids and therefore
by most proteins; whereas, UV-B wavelengths are somewhat less photochemically
active, but more penetrating in most tissues. 2-6 UV-A wavelengths are far less
photobiologically active, but are still more penetrating than UV-B wavelengths and
often play an interactive role when exposure occurs following UV-B exposure. 2-6
Although useful, it is very important to keep in mind that these photobiological
spectral bands are merely "short-hand" notations, and they can be used to make general
(but not absolute) statements about the relative spectral effectiveness of different parts
of the UV spectrum in producing effects. The dividing lines, while not arbitrary, are
certainly not fine dividing lines between wavelengths that may or may not elicit a given
biological effect. One should always provide a wavelength band or spectral emission
curve for the UV source being used and not rely totally on these spectral terms. There
are also many authors who use 320 nm rather than the CIE defined dividing line of
315 nm to divide UV-A from UV-B. Some authors also may divide the UV-A band into
two regions: UV-A1 and UV-A2, with a division made at about 340 nm. The exposure
limits often overlap bands or do not even make use of them. For this reason, providing
action spectra and lamp spectra in research reports are critical. 7
2. Optical phenomena
Polarization
An electromagnetic wave is a transverse wave. The fluctuating electric and
magnetic fields are perpendicular to each other and to the direction of propagation
(orthogonal). When all of the waves in a beam of light have the electric vector in only
one plane, e.g., the wave has only Y displacements, we say the wave is linearly
polarized.
Reflection
When light arrives at an interface, a certain fraction of the incident radiant
energy is reflected. This reflection may be very orderly as from a mirror or a sheet of
glass (specular reflection), or the light rays may be reflected in random directions and
produce a diffuse reflection.
Absorption
When energy is not transmitted or reflected from a medium, it must be absorbed.
In simple media where generally only one scattering of a photon takes place, the
absorption follows Beer's Law, or “the exponential law of absorption.” Beer's Law is
expressed as:
ĭ/ĭ 0 = e -Įx (4)
where ĭ is the radiant power (radiant flux) exiting the medium, ĭ 0 is the initial radiant
power, Į is the absorption coefficient, and x is the thickness of the medium. The