Simulated Large Hadron ColliderCMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons
Depending on the Planck energy
cutoff and other factors, the quantum vacuum energy contribution to the
effective cosmological constant is calculated to be between 50 and as
much as 120 orders of magnitude greater than observed, a state of affairs described by physicists as "the largest discrepancy between theory and experiment in all of science" and "the worst theoretical prediction in the history of physics".
History
The basic problem of a vacuum energy producing a gravitational effect was identified as early as 1916 by Walther Nernst.
He predicted that the value had to be either zero or very small. In 1926, Wilhelm Lenz concluded that "If one allows waves of the shortest observed wavelengths λ
≈ 2 × 10−11 cm, ... and if this radiation, converted to material density (u/c2 ≈ 106),
contributed to the curvature of the observable universe – one would
obtain a vacuum energy density of such a value that the radius of the
observable universe would not reach even to the Moon."
After the development of quantum field theory in the 1940s, the
first to address contributions of quantum fluctuations to the
cosmological constant was Yakov Zel'dovich in the 1960s.
In quantum mechanics, the vacuum itself should experience quantum
fluctuations. In general relativity, those quantum fluctuations
constitute energy that would add to the cosmological constant. However,
this calculated vacuum energy density is many orders of magnitude bigger
than the observed cosmological constant. Original estimates of the degree of mismatch were as high as 120 to 122 orders of magnitude; however, modern research suggests that, when Lorentz invariance is taken into account, the degree of mismatch is closer to 60 orders of magnitude.
With the development of inflationary cosmology
in the 1980s, the problem became much more important: as cosmic
inflation is driven by vacuum energy, differences in modeling vacuum
energy lead to huge differences in the resulting cosmologies. Were the
vacuum energy precisely zero, as was once believed, then the expansion of the universe would not accelerate as observed, according to the standard Λ-CDM model.
Cutoff dependence
The
calculated vacuum energy is a positive, rather than negative,
contribution to the cosmological constant because the existing vacuum
has negative quantum-mechanical pressure, while in general relativity, the gravitational effect of negative pressure is a kind of repulsion. (Pressure here is defined as the flux of quantum-mechanical momentum
across a surface.) Roughly, the vacuum energy is calculated by summing
over all known quantum-mechanical fields, taking into account
interactions and self-interactions between the ground states, and then
removing all interactions below a minimum "cutoff" wavelength to reflect
that existing theories break down and may fail to be applicable around
the cutoff scale. Because the energy is dependent on how fields interact
within the current vacuum state, the vacuum energy contribution would
have been different in the early universe; for example, the vacuum
energy would have been significantly different prior to electroweak symmetry breaking during the quark epoch.
Renormalization
The vacuum energy in quantum field theory can be set to any value by renormalization.
This view treats the cosmological constant as simply another
fundamental physical constant not predicted or explained by theory.
Such a renormalization constant must be chosen very accurately because
of the many-orders-of-magnitude discrepancy between theory and
observation, and many theorists consider this ad-hoc constant as
equivalent to ignoring the problem.
Estimated values
The vacuum energy density of the Universe based on 2015 measurements by the Planck collaboration is ρvac = 5.96×10−27 kg/m3 ≘ 5.3566×10−10 J/m3 = 3.35 GeV/m3 or about 2.5×10−47 GeV4 in geometrized units.
One assessment, made by Jérôme Martin of the Institut d'Astrophysique de Paris in 2012, placed the expected theoretical vacuum energy scale around 108 GeV4, for a difference of about 55 orders of magnitude.
Proposed solutions
Some physicists propose an anthropic solution, and argue that we live in one region of a vast multiverse that has different regions with different vacuum energies. These anthropic arguments
posit that only regions of small vacuum energy such as the one in which
we live are reasonably capable of supporting intelligent life. Such
arguments have existed in some form since at least 1981. Around 1987, Steven Weinberg
estimated that the maximum allowable vacuum energy for
gravitationally-bound structures to form is problematically large, even
given the observational data available in 1987, and concluded the
anthropic explanation appears to fail; however, more recent estimates by
Weinberg and others, based on other considerations, find the bound to
be closer to the actual observed level of dark energy.Anthropic arguments gradually gained credibility among many physicists
after the discovery of dark energy and the development of the
theoretical string theory landscape,
but are still derided by a substantial skeptical portion of the
scientific community as being problematic to verify. Proponents of
anthropic solutions are themselves divided on multiple technical
questions surrounding how to calculate the proportion of regions of the
universe with various dark energy constants.
Other proposals involve modifying gravity to diverge from general
relativity. These proposals face the hurdle that the results of
observations and experiments so far have tended to be extremely
consistent with general relativity and the ΛCDM model, and inconsistent
with thus-far proposed modifications. In addition, some of the proposals
are arguably incomplete, because they solve the "new" cosmological
constant problem by proposing that the actual cosmological constant is
exactly zero rather than a tiny number, but fail to solve the "old"
cosmological constant problem of why quantum fluctuations seem to fail
to produce substantial vacuum energy in the first place. Nevertheless,
many physicists argue that, due in part to a lack of better
alternatives, proposals to modify gravity should be considered "one of
the most promising routes to tackling" the cosmological constant
problem.
Bill Unruh
and collaborators have argued that when the energy density of the
quantum vacuum is modeled more accurately as a fluctuating quantum
field, the cosmological constant problem does not arise. Going in a different direction, George F. R. Ellis and others have suggested that in unimodular gravity, the troublesome contributions simply do not gravitate.
Another argument, due to Stanley Brodsky and Robert Shrock, is that in light front quantization, the quantum field theory vacuum becomes essentially trivial. In the absence of vacuum expectation values, there is no contribution from QED, weak interactions, and QCD to the cosmological constant. It is thus predicted to be zero in a flat space-time.From light front quantization insight, the origin of the cosmological constant problem is traced back to unphysical non-causal terms in the standard calculation, which lead to an erroneously large value of the cosmological constant.
In 2018, a mechanism for cancelling Λ out has been proposed through the use of a symmetry breaking
potential in a Lagrangian formalism in which matter shows a
non-vanishing pressure. The model assumes that standard matter provides a
pressure which counterbalances the action due to the cosmological
constant. Luongo and Muccino have shown that this mechanism permits to
take vacuum energy as quantum field theory predicts, but removing the huge magnitude through a counterbalance term due to baryons and cold dark matter only.
In 1999, Andrew Cohen, David B. Kaplan and Ann Nelson proposed that correlations between the UV and IR cutoffs in effectivequantum field theory are enough to reduce the theoretical cosmological constant down to the measured cosmological constant due to the CKN bound. In 2021, Nikita Blinov and Patrick Draper confirmed through the holographic principle
that the CKN bound predicts the measured cosmological constant, all
while maintaining the predictions of effective field theory in less
extreme conditions.
Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionizeatoms, it can cause chemical reactions and causes many substances to glow or fluoresce.
Many practical applications, including chemical and biological effects,
derive from the way that UV radiation can interact with organic
molecules. These interactions can involve absorption or adjusting energy states in molecules, but do not necessarily involve heating.
Short-wave ultraviolet light damages DNA and sterilizes surfaces with which it comes into contact. For humans, suntan and sunburn are familiar effects of exposure of the skin to UV light, along with an increased risk of skin cancer.
The amount of UV light produced by the Sun means that the Earth would
not be able to sustain life on dry land if most of that light were not
filtered out by the atmosphere.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes
air so strongly that it is absorbed before it reaches the ground. However, ultraviolet light (specifically, UVB) is also responsible for the formation of vitamin D in most land vertebrates, including humans. The UV spectrum, thus, has effects both beneficial and harmful to life.
The lower wavelength limit of the visible spectrum is conventionally taken as 400 nm, so ultraviolet rays are not visible to humans, although people can sometimes perceive light at shorter wavelengths than this. Insects, birds, and some mammals can see near-UV (NUV), i.e., slightly shorter wavelengths than what humans can see.
Visibility
Ultraviolet rays are usually invisible to most humans. The lens of the human eye blocks most radiation in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. Humans also lack color receptor adaptations for ultraviolet rays. Nevertheless, the photoreceptors of the retina are sensitive to near-UV, and people lacking a lens (a condition known as aphakia) perceive near-UV as whitish-blue or whitish-violet. Under some conditions, children and young adults can see ultraviolet down to wavelengths around 310 nm. Near-UV radiation is visible to insects, some mammals, and some birds.
Birds have a fourth color receptor for ultraviolet rays; this, coupled
with eye structures that transmit more UV gives smaller birds "true" UV
vision.
History and discovery
"Ultraviolet" means "beyond violet" (from Latinultra, "beyond"), violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency (thus a shorter wavelength) than violet light.
UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them "(de-)oxidizing rays" (German: de-oxidierende Strahlen) to emphasize chemical reactivity and to distinguish them from "heat rays",
discovered the previous year at the other end of the visible spectrum.
The simpler term "chemical rays" was adopted soon afterwards, and
remained popular throughout the 19th century, although some said that
this radiation was entirely different from light (notably John William Draper, who named them "tithonic rays"). The terms "chemical rays" and "heat rays" were eventually dropped in favor of ultraviolet and infraredradiation, respectively.
In 1878, the sterilizing effect of short-wavelength light by killing
bacteria was discovered. By 1903, the most effective wavelengths were
known to be around 250 nm. In 1960, the effect of ultraviolet radiation
on DNA was established.
The discovery of the ultraviolet radiation with wavelengths below
200 nm, named "vacuum ultraviolet" because it is strongly absorbed by
the oxygen in air, was made in 1893 by German physicist Victor Schumann.
Subtypes
The electromagnetic spectrum
of ultraviolet radiation (UVR), defined most broadly as
10–400 nanometers, can be subdivided into a number of ranges recommended
by the ISO standard ISO 21348:
Entirely ionizing radiation by some definitions; completely absorbed by the atmosphere.
Vacuum ultraviolet
V-UV
10–200
6.20–12.4, 0.993–1.987
Strongly absorbed by atmospheric oxygen, though 150–200 nm wavelengths can propagate through nitrogen.
Several solid-state and vacuum devices have been explored for use in
different parts of the UV spectrum. Many approaches seek to adapt
visible light-sensing devices, but these can suffer from unwanted
response to visible light and various instabilities. Ultraviolet can be
detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive UV photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation. Silicon detectors are used across the spectrum.
Vacuum UV, or VUV, wavelengths (shorter than 200 nm) are strongly absorbed by molecular oxygen in the air, though the longer wavelengths around 150–200 nm can propagate through nitrogen.
Scientific instruments can, therefore, use this spectral range by
operating in an oxygen-free atmosphere (commonly pure nitrogen), without
the need for costly vacuum chambers. Significant examples include
193-nm photolithography equipment (for semiconductor manufacturing) and circular dichroism spectrometers.
Technology for VUV instrumentation was largely driven by solar
astronomy for many decades. While optics can be used to remove unwanted
visible light that contaminates the VUV, in general; detectors can be
limited by their response to non-VUV radiation, and the development of solar-blind devices
has been an important area of research. Wide-gap solid-state devices or
vacuum devices with high-cutoff photocathodes can be attractive
compared to silicon diodes.
Extreme UV (EUV or sometimes XUV) is characterized by a
transition in the physics of interaction with matter. Wavelengths longer
than about 30 nm interact mainly with the outer valence electrons
of atoms, while wavelengths shorter than that interact mainly with
inner-shell electrons and nuclei. The long end of the EUV spectrum is
set by a prominent He+ spectral line at 30.4 nm. EUV is strongly absorbed by most known materials, but synthesizing multilayer optics that reflect up to about 50% of EUV radiation at normal incidence is possible. This technology was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, and it has been used to make telescopes for solar imaging. See also the Extreme Ultraviolet Explorersatellite.
Some sources use the distinction of "hard UV" and "soft UV". For instance, in the case of astrophysics, the boundary may be at the Lyman limit (wavelength 91.2 nm), with "hard UV" being more energetic; the same terms may also be used in other fields, such as cosmetology, optoelectronic,
etc. The numerical values of the boundary between hard/soft, even
within similar scientific fields, do not necessarily coincide; for
example, one applied-physics publication used a boundary of 190 nm
between hard and soft UV regions.
Solar ultraviolet
Very hot objects emit UV radiation (see black-body radiation). The Sun
emits ultraviolet radiation at all wavelengths, including the extreme
ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars (such as O- and B-type) emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m2 in vacuum.
The atmosphere blocks about 77% of the Sun's UV, when the Sun is
highest in the sky (at zenith), with absorption increasing at shorter UV
wavelengths. At ground level with the sun at zenith, sunlight is 44%
visible light, 3% ultraviolet, and the remainder infrared.
Of the ultraviolet radiation that reaches the Earth's surface, more
than 95% is the longer wavelengths of UVA, with the small remainder UVB.
Almost no UVC reaches the Earth's surface.
The fraction of UVA and UVB which remains in UV radiation after passing
through the atmosphere is heavily dependent on cloud cover and
atmospheric conditions. On "partly cloudy" days, patches of blue sky
showing between clouds are also sources of (scattered) UVA and UVB,
which are produced by Rayleigh scattering
in the same way as the visible blue light from those parts of the sky.
UVB also plays a major role in plant development, as it affects most of
the plant hormones.
During total overcast, the amount of absorption due to clouds is
heavily dependent on the thickness of the clouds and latitude, with no
clear measurements correlating specific thickness and absorption of UVA
and UVB.
The shorter bands of UVC, as well as even more-energetic UV
radiation produced by the Sun, are absorbed by oxygen and generate the
ozone in the ozone layer when single oxygen atoms produced by UV photolysis
of dioxygen react with more dioxygen. The ozone layer is especially
important in blocking most UVB and the remaining part of UVC not already
blocked by ordinary oxygen in air.
Blockers, absorbers, and windows
Ultraviolet absorbers are molecules used in organic materials (polymers, paints, etc.) to absorb UV radiation to reduce the UV degradation
(photo-oxidation) of a material. The absorbers can themselves degrade
over time, so monitoring of absorber levels in weathered materials is
necessary.
For clothing, the ultraviolet protection factor (UPF) represents the ratio of sunburn-causing UV without and with the protection of the fabric, similar to sun protection factor (SPF) ratings for sunscreen. Standard summer fabrics have UPFs around 6, which means that about 20% of UV will pass through.
Suspended nanoparticles in stained-glass prevent UV rays from causing chemical reactions that change image colors. A set of stained-glass color-reference chips is planned to be used to calibrate the color cameras for the 2019 ESA Mars rover mission, since they will remain unfaded by the high level of UV present at the surface of Mars.
Common soda–lime glass, such as window glass, is partially transparent to UVA, but is opaque to shorter wavelengths, passing about 90% of the light above 350 nm, but blocking over 90% of the light below 300 nm. A study found that car windows allow 3-4% of ambient UV to pass through, especially if the UV was greater than 380 nm. Other types of car windows can reduce transmission of UV that is greater than 335 nm. Fused quartz, depending on quality, can be transparent even to vacuum UV wavelengths. Crystalline quartz and some crystals such as CaF2 and MgF2 transmit well down to 150 nm or 160 nm wavelengths.
Wood's glass is a deep violet-blue barium-sodium silicate glass with about 9% nickel oxide developed during World War I
to block visible light for covert communications. It allows both
infrared daylight and ultraviolet night-time communications by being
transparent between 320 nm and 400 nm and also the longer infrared and
just-barely-visible red wavelengths. Its maximum UV transmission is at
365 nm, one of the wavelengths of mercury lamps.
Artificial sources
"Black lights"
Two
black light fluorescent tubes, showing use. The longer tube is a
F15T8/BLB 18 inch, 15 watt tube, shown in the bottom image in a standard
plug-in fluorescent fixture. The shorter is an F8T5/BLB 12 inch, 8 watt
tube, used in a portable battery-powered black light sold as a pet
urine detector.
A black light lamp emits long-wave UV‑A radiation and little visible light. Fluorescent black light lamps work similarly to other fluorescent lamps, but use a phosphor on the inner tube surface which emits UV‑A radiation instead of visible light. Some lamps use a deep-bluish-purple Wood's glass optical filter that blocks almost all visible light with wavelengths longer than 400 nanometers.
The purple glow given off by these tubes is not the ultraviolet itself,
but visible purple light from mercury's 404 nm spectral line which
escapes being filtered out by the coating. Other black lights use plain
glass instead of the more expensive Wood's glass, so they appear
light-blue to the eye when operating.
Incandescent black lights are also produced, using a filter
coating on the envelope of an incandescent bulb that absorbs visible
light (see section below). These are cheaper but very inefficient, emitting only a small fraction of a percent of their power as UV. Mercury-vapor black lights in ratings up to 1 kW with UV-emitting phosphor and an envelope of Wood's glass are used for theatrical and concert displays.
Black lights are used in applications in which extraneous visible light must be minimized; mainly to observe fluorescence,
the colored glow that many substances give off when exposed to UV
light. UV‑A / UV‑B emitting bulbs are also sold for other special
purposes, such as tanning lamps and reptile-husbandry.
Short-wave ultraviolet lamps
9 watt germicidal UV bulb, in compact fluorescent (CF) form factor
Commercial germicidal lamp in butcher shop
Shortwave UV lamps are made using a fluorescent lamp tube with no phosphor coating, composed of fused quartz or vycor,
since ordinary glass absorbs UV‑C. These lamps emit ultraviolet light
with two peaks in the UV‑C band at 253.7 nm and 185 nm due to the mercury
within the lamp, as well as some visible light. From 85% to 90% of the
UV produced by these lamps is at 253.7 nm, whereas only 5–10% is at
185 nm.
The fused quartz tube passes the 253.7 nm radiation but blocks the
185 nm wavelength. Such tubes have two or three times the UV‑C power of a
regular fluorescent lamp tube. These low-pressure lamps have a typical
efficiency of approximately 30–40%, meaning that for every 100 watts of
electricity consumed by the lamp, they will produce approximately
30–40 watts of total UV output. They also emit bluish-white visible
light, due to mercury's other spectral lines. These "germicidal" lamps
are used extensively for disinfection of surfaces in laboratories and
food-processing industries, and for disinfecting water supplies.
Incandescent lamps
'Black light' incandescent lamps are also made from an incandescent light bulb with a filter coating which absorbs most visible light. Halogen lamps with fused quartz
envelopes are used as inexpensive UV light sources in the near UV
range, from 400 to 300 nm, in some scientific instruments. Due to its black-body spectrum a filament light bulb is a very inefficient ultraviolet source, emitting only a fraction of a percent of its energy as UV.
Specialized UV gas-discharge lamps containing different gases produce UV radiation at particular spectral lines for scientific purposes. Argon and deuterium arc lamps are often used as stable sources, either windowless or with various windows such as magnesium fluoride. These are often the emitting sources in UV spectroscopy equipment for chemical analysis.
The excimer lamp,
a UV source developed in the early 2000s, is seeing increasing use in
scientific fields. It has the advantages of high-intensity, high
efficiency, and operation at a variety of wavelength bands into the
vacuum ultraviolet.
Ultraviolet LEDs
Light-emitting diodes
(LEDs) can be manufactured to emit radiation in the ultraviolet range.
In 2019, following significant advances over the preceding five years,
UV‑A LEDs of 365 nm and longer wavelength were available, with
efficiencies of 50% at 1.0 W output. Currently, the most common types of
UV LEDs are in 395 nm and 365 nm wavelengths, both of which are in the
UV‑A spectrum. The rated wavelength is the peak wavelength that the LEDs
put out, but light at both higher and lower wavelengths are present.
The cheaper and more common 395 nm UV LEDs are much closer to the
visible spectrum, and give off a purple color. Other UV LEDs deeper
into the spectrum do not emit as much visible light LEDs are used for applications such as UV curing
applications, charging glow-in-the-dark objects such as paintings or
toys, and lights for detecting counterfeit money and bodily fluids. UV
LEDs are also used in digital print applications and inert UV curing
environments. Power densities approaching 3 W/cm2 (30 kW/m2)
are now possible, and this, coupled with recent developments by
photo-initiator and resin formulators, makes the expansion of LED cured
UV materials likely.
UV‑C LEDs are developing rapidly, but may require testing to
verify effective disinfection. Citations for large-area disinfection are
for non-LED UV sources known as germicidal lamps. Also, they are used as line sources to replace deuterium lamps in liquid chromatography instruments.
Gas lasers, laser diodes, and solid-state lasers can be manufactured to emit ultraviolet rays, and lasers are available that cover the entire UV range. The nitrogen gas laser
uses electronic excitation of nitrogen molecules to emit a beam that is
mostly UV. The strongest ultraviolet lines are at 337.1 nm and 357.6 nm
in wavelength. Another type of high-power gas lasers are excimer lasers. They are widely used lasers emitting in ultraviolet and vacuum ultraviolet wavelength ranges. Presently, UV argon-fluoride excimer lasers operating at 193 nm are routinely used in integrated circuit production by photolithography. The current wavelength limit of production of coherent UV is about 126 nm, characteristic of the Ar2* excimer laser.
Direct UV-emitting laser diodes are available at 375 nm. UV diode-pumped solid state lasers have been demonstrated using cerium-doped lithium strontium aluminum fluoride crystals (Ce:LiSAF), a process developed in the 1990s at Lawrence Livermore National Laboratory. Wavelengths shorter than 325 nm are commercially generated in diode-pumped solid-state lasers. Ultraviolet lasers can also be made by applying frequency conversion to lower-frequency lasers.
The vacuum ultraviolet (V‑UV) band (100–200 nm) can be generated by non-linear 4 wave mixing
in gases by sum or difference frequency mixing of 2 or more longer
wavelength lasers. The generation is generally done in gasses (e.g.
krypton, hydrogen which are two-photon resonant near 193 nm)
or metal vapors (e.g. magnesium). By making one of the lasers tunable,
the V‑UV can be tuned. If one of the lasers is resonant with a
transition in the gas or vapor then the V‑UV production is intensified.
However, resonances also generate wavelength dispersion, and thus the
phase matching can limit the tunable range of the 4 wave mixing.
Difference frequency mixing (i.e., f1 + f2 − f3) has an advantage over sum frequency mixing because the phase matching can provide greater tuning.
In particular, difference frequency mixing two photons of an ArF
(193 nm) excimer laser with a tunable visible or near IR laser in
hydrogen or krypton provides resonantly enhanced tunable V‑UV covering
from 100 nm to 200 nm. Practically, the lack of suitable gas / vapor cell window materials above the lithium fluoride
cut-off wavelength limit the tuning range to longer than about 110 nm.
Tunable V‑UV wavelengths down to 75 nm was achieved using window-free
configurations.
Plasma and synchrotron sources of extreme UV
Lasers have been used to indirectly generate non-coherent extreme UV (E‑UV) radiation at 13.5 nm for extreme ultraviolet lithography.
The E‑UV is not emitted by the laser, but rather by electron
transitions in an extremely hot tin or xenon plasma, which is excited by
an excimer laser. This technique does not require a synchrotron, yet can produce UV at the edge of the X‑ray spectrum. Synchrotron light sources can also produce all wavelengths of UV, including those at the boundary of the UV and X‑ray spectra at 10 nm.
The impact of ultraviolet radiation on human health has implications for the risks and benefits of sun exposure and is also implicated in issues such as fluorescent lamps and health. Getting too much sun exposure can be harmful, but in moderation, sun exposure is beneficial.
Beneficial effects
UV light (specifically, UV‑B) causes the body to produce vitamin D,
which is essential for life. Humans need some UV radiation to maintain
adequate vitamin D levels. According to the World Health Organization:
There is no doubt that a little sunlight is good for you!
But 5–15 minutes of casual sun exposure of hands, face and arms two to
three times a week during the summer months is sufficient to keep your
vitamin D levels high.
Vitamin D can also be obtained from food and supplementation. Excess sun exposure produces harmful effects, however.
Vitamin D promotes the creation of serotonin. The production of serotonin is in direct proportion to the degree of bright sunlight the body receives. Serotonin is thought to provide sensations of happiness, well-being and serenity to human beings.
Skin conditions
UV rays also treat certain skin conditions. Modern phototherapy has been used to successfully treat psoriasis, eczema, jaundice, vitiligo, atopic dermatitis, and localized scleroderma. In addition, UV light, in particular UV‑B radiation, has been shown to induce cell cycle arrest in keratinocytes, the most common type of skin cell. As such, sunlight therapy can be a candidate for treatment of conditions such as psoriasis and exfoliative cheilitis, conditions in which skin cells divide more rapidly than usual or necessary.
Harmful effects
In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system and retina. The risk is elevated at high altitudes and people living in high latitude areas where snow covers the ground right into early summer and sun positions even at zenith are low, are particularly at risk. Skin, the circadian system, and the immune system can also be affected.
The differential effects of various wavelengths of light on the
human cornea and skin are sometimes called the "erythemal action
spectrum". The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to cause photokeratitis
and skin redness (with lighter skinned individuals being more
sensitive) at wavelengths starting near the beginning of the UVB band at
315 nm, and rapidly increasing to 300 nm. The skin and eyes are most
sensitive to damage by UV at 265–275 nm, which is in the lower UV‑C
band. At still shorter wavelengths of UV, damage continues to happen,
but the overt effects are not as great with so little penetrating the
atmosphere. The WHO-standard ultraviolet index
is a widely publicized measurement of total strength of UV wavelengths
that cause sunburn on human skin, by weighting UV exposure for action
spectrum effects at a given time and location. This standard shows that
most sunburn happens due to UV at wavelengths near the boundary of the
UV‑A and UV‑B bands.
Skin damage
Overexposure to UV‑B radiation not only can cause sunburn but also some forms of skin cancer.
However, the degree of redness and eye irritation (which are largely
not caused by UV‑A) do not predict the long-term effects of UV, although
they do mirror the direct damage of DNA by ultraviolet.
All bands of UV radiation damage collagen fibers and accelerate aging of the skin. Both UV‑A and UV‑B destroy vitamin A in skin, which may cause further damage.
UVB radiation can cause direct DNA damage. This cancer connection is one reason for concern about ozone depletion and the ozone hole.
The most deadly form of skin cancer, malignant melanoma,
is mostly caused by DNA damage independent from UV‑A radiation. This
can be seen from the absence of a direct UV signature mutation in 92% of
all melanoma. Occasional overexposure and sunburn are probably greater risk factors for melanoma than long-term moderate exposure.
UV‑C is the highest-energy, most-dangerous type of ultraviolet
radiation, and causes adverse effects that can variously be mutagenic or
carcinogenic.
In the past, UV‑A was considered not harmful or less harmful than UV‑B, but today it is known to contribute to skin cancer via indirect DNA damage (free radicals such as reactive oxygen species).
UV‑A can generate highly reactive chemical intermediates, such as
hydroxyl and oxygen radicals, which in turn can damage DNA. The DNA
damage caused indirectly to skin by UV‑A consists mostly of
single-strand breaks in DNA, while the damage caused by UV‑B includes
direct formation of thymine dimers or cytosine dimers and double-strand DNA breakage.
UV‑A is immunosuppressive for the entire body (accounting for a large
part of the immunosuppressive effects of sunlight exposure), and is
mutagenic for basal cell keratinocytes in skin.
UVB photons can cause direct DNA damage. UV‑B radiation excites DNA molecules in skin cells, causing aberrant covalent bonds to form between adjacent pyrimidine bases, producing a dimer. Most UV-induced pyrimidine dimers in DNA are removed by the process known as nucleotide excision repair that employs about 30 different proteins. Those pyrimidine dimers that escape this repair process can induce a form of programmed cell death (apoptosis) or can cause DNA replication errors leading to mutation.
As a defense against UV radiation, the amount of the brown pigment melanin in the skin increases when exposed to moderate (depending on skin type) levels of radiation; this is commonly known as a sun tan. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, protecting the skin against both direct and indirect DNA damage
from the UV. UV‑A gives a quick tan that lasts for days by oxidizing
melanin that was already present and triggers the release of the melanin from melanocytes. UV‑B yields a tan that takes roughly 2 days to develop because it stimulates the body to produce more melanin.
Medical organizations recommend that patients protect themselves from UV radiation by using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors. However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells. The amount of sunscreen that penetrates into the lower layers of the skin may be large enough to cause damage.
Sunscreen reduces the direct DNA damage that causes sunburn, by blocking UV‑B, and the usual SPF rating indicates how effectively this radiation is blocked. SPF is, therefore, also called UVB-PF, for "UV‑B protection factor". This rating, however, offers no data about important protection against UVA,
which does not primarily cause sunburn but is still harmful, since it
causes indirect DNA damage and is also considered carcinogenic. Several
studies suggest that the absence of UV‑A filters may be the cause of the
higher incidence of melanoma found in sunscreen users compared to
non-users. Some sunscreen lotions contain titanium dioxide, zinc oxide, and avobenzone, which help protect against UV‑A rays.
The photochemical properties of melanin make it an excellent photoprotectant.
However, sunscreen chemicals cannot dissipate the energy of the excited
state as efficiently as melanin and therefore, if sunscreen ingredients
penetrate into the lower layers of the skin, the amount of reactive oxygen species may be increased. The amount of sunscreen that penetrates through the stratum corneum may or may not be large enough to cause damage.
In an experiment by Hanson et al. that was published in 2006, the amount of harmful reactive oxygen species
(ROS) was measured in untreated and in sunscreen treated skin. In the
first 20 minutes, the film of sunscreen had a protective effect and the
number of ROS species was smaller. After 60 minutes, however, the amount
of absorbed sunscreen was so high that the amount of ROS was higher in
the sunscreen-treated skin than in the untreated skin.
The study indicates that sunscreen must be reapplied within 2 hours in
order to prevent UV light from penetrating to sunscreen-infused live
skin cells.
The eye is most sensitive to damage by UV in the lower UV‑C band at
265–275 nm. Radiation of this wavelength is almost absent from sunlight
at the surface of the Earth but is emitted by artificial sources such as
the electrical arcs employed in arc welding. Unprotected exposure to these sources can cause "welder's flash" or "arc eye" (photokeratitis) and can lead to cataracts, pterygium and pinguecula formation. To a lesser extent, UV‑B in sunlight from 310 to 280 nm also causes photokeratitis ("snow blindness"), and the cornea, the lens, and the retina can be damaged.
Protective eyewear
is beneficial to those exposed to ultraviolet radiation. Since light
can reach the eyes from the sides, full-coverage eye protection is
usually warranted if there is an increased risk of exposure, as in
high-altitude mountaineering. Mountaineers are exposed to
higher-than-ordinary levels of UV radiation, both because there is less
atmospheric filtering and because of reflection from snow and ice.
Ordinary, untreated eyeglasses
give some protection. Most plastic lenses give more protection than
glass lenses, because, as noted above, glass is transparent to UV‑A and
the common acrylic plastic used for lenses is less so. Some plastic lens
materials, such as polycarbonate, inherently block most UV.
UV degradation is one form of polymer degradation that affects plastics exposed to sunlight.
The problem appears as discoloration or fading, cracking, loss of
strength or disintegration. The effects of attack increase with exposure
time and sunlight intensity. The addition of UV absorbers inhibits the
effect.
Sensitive polymers include thermoplastics and speciality fibers like aramids.
UV absorption leads to chain degradation and loss of strength at
sensitive points in the chain structure. Aramid rope must be shielded
with a sheath of thermoplastic if it is to retain its strength.
Many pigments and dyes absorb UV and change colour, so paintings
and textiles may need extra protection both from sunlight and
fluorescent lamps, two common sources of UV radiation. Window glass
absorbs some harmful UV, but valuable artifacts need extra shielding.
Many museums place black curtains over watercolour paintings
and ancient textiles, for example. Since watercolours can have very low
pigment levels, they need extra protection from UV. Various forms of picture framing glass, including acrylics (plexiglass), laminates, and coatings, offer different degrees of UV (and visible light) protection.
Applications
Because of its ability to cause chemical reactions and excite fluorescence in materials, ultraviolet radiation has a number of applications. The following table gives some uses of specific wavelength bands in the UV spectrum.
Photographic film responds to ultraviolet radiation but the glass
lenses of cameras usually block radiation shorter than 350 nm. Slightly
yellow UV-blocking filters are often used for outdoor photography to
prevent unwanted bluing and overexposure by UV rays. For photography in
the near UV, special filters may be used. Photography with wavelengths
shorter than 350 nm requires special quartz lenses which do not absorb
the radiation.
Digital cameras sensors
may have internal filters that block UV to improve color rendition
accuracy. Sometimes these internal filters can be removed, or they may
be absent, and an external visible-light filter prepares the camera for
near-UV photography. A few cameras are designed for use in the UV.
Photography by reflected ultraviolet radiation is useful for
medical, scientific, and forensic investigations, in applications as
widespread as detecting bruising of skin, alterations of documents, or
restoration work on paintings. Photography of the fluorescence produced
by ultraviolet illumination uses visible wavelengths of light.
In ultraviolet astronomy,
measurements are used to discern the chemical composition of the
interstellar medium, and the temperature and composition of stars.
Because the ozone layer blocks many UV frequencies from reaching
telescopes on the surface of the Earth, most UV observations are made
from space.
Electrical and electronics industry
Corona discharge
on electrical apparatus can be detected by its ultraviolet emissions.
Corona causes degradation of electrical insulation and emission of ozone and nitrogen oxide.
EPROMs (Erasable Programmable Read-Only Memory) are erased by exposure to UV radiation. These modules have a transparent (quartz) window on the top of the chip that allows the UV radiation in.
Fluorescent dye uses
Colorless fluorescent dyes that emit blue light under UV are added as optical brighteners
to paper and fabrics. The blue light emitted by these agents
counteracts yellow tints that may be present and causes the colors and
whites to appear whiter or more brightly colored.
UV fluorescent dyes that glow in the primary colors are used in
paints, papers, and textiles either to enhance color under daylight
illumination or to provide special effects when lit with UV lamps. Blacklight paints that contain dyes that glow under UV are used in a number of art and aesthetic applications.
Amusement parks often use UV lighting to fluoresce ride artwork
and backdrops. This often has the side effect of causing rider's white
clothing to glow light-purple.
To help prevent counterfeiting of currency, or forgery of important documents such as driver's licenses and passports, the paper may include a UV watermark or fluorescent multicolor fibers that are visible under ultraviolet light. Postage stamps are tagged with a phosphor that glows under UV rays to permit automatic detection of the stamp and facing of the letter.
UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). Some brands of pepper spray
will leave an invisible chemical (UV dye) that is not easily washed off
on a pepper-sprayed attacker, which would help police identify the
attacker later.
In some types of nondestructive testing
UV stimulates fluorescent dyes to highlight defects in a broad range of
materials. These dyes may be carried into surface-breaking defects by
capillary action (liquid penetrant inspection) or they may be bound to ferrite particles caught in magnetic leakage fields in ferrous materials (magnetic particle inspection).
Analytic uses
Forensics
UV
is an investigative tool at the crime scene helpful in locating and
identifying bodily fluids such as semen, blood, and saliva.
For example, ejaculated fluids or saliva can be detected by high-power
UV sources, irrespective of the structure or colour of the surface the
fluid is deposited upon. UV–vis microspectroscopy is also used to analyze trace evidence, such as textile fibers and paint chips, as well as questioned documents.
Other applications include the authentication of various
collectibles and art, and detecting counterfeit currency. Even materials
not specially marked with UV sensitive dyes may have distinctive
fluorescence under UV exposure or may fluoresce differently under
short-wave versus long-wave ultraviolet.
Enhancing contrast of ink
Using multi-spectral imaging it is possible to read illegible papyrus, such as the burned papyri of the Villa of the Papyri or of Oxyrhynchus, or the Archimedes palimpsest.
The technique involves taking pictures of the illegible document using
different filters in the infrared or ultraviolet range, finely tuned to
capture certain wavelengths of light. Thus, the optimum spectral portion
can be found for distinguishing ink from paper on the papyrus surface.
Simple NUV sources can be used to highlight faded iron-based ink on vellum.
Sanitary compliance
Ultraviolet helps detect organic material deposits that remain on
surfaces where periodic cleaning and sanitizing may have failed. It is
used in the hotel industry, manufacturing, and other industries where
levels of cleanliness or contamination are inspected.
Perennial news features for many television news organizations
involve an investigative reporter using a similar device to reveal
unsanitary conditions in hotels, public toilets, hand rails, and such.
In pollution control applications, ultraviolet analyzers are used
to detect emissions of nitrogen oxides, sulfur compounds, mercury, and
ammonia, for example in the flue gas of fossil-fired power plants. Ultraviolet radiation can detect thin sheens of spilled oil
on water, either by the high reflectivity of oil films at UV
wavelengths, fluorescence of compounds in oil, or by absorbing of UV
created by Raman scattering in water.
UV absorbance can also be uesd to quantify contaminants in wastewater.
Most commonly used 254 nm UV absorbance is genrally used as a surrogate
parameters to quantify NOM.
Another form of light-based detection method uses a wide spectrum of
excitation emission matrix (EEM) to detect and identify contaminants
based on their flourense properties.
EEM could be used to discriminate different groups of NOM based on the
difference in light emission and excitation of fluorophores. NOMs with
certain molecular structures are reported to have fluorescent properties
in a wide range of excitation/emission wavelengths.
Ultraviolet lamps are also used as part of the analysis of some minerals and gems.
In general, ultraviolet detectors use either a solid-state device, such as one based on silicon carbide or aluminium nitride,
or a gas-filled tube as the sensing element. UV detectors that are
sensitive to UV in any part of the spectrum respond to irradiation by sunlight and artificial light. A burning hydrogen flame, for instance, radiates strongly in the 185- to 260-nanometer range and only very weakly in the IR
region, whereas a coal fire emits very weakly in the UV band yet very
strongly at IR wavelengths; thus, a fire detector that operates using
both UV and IR detectors is more reliable than one with a UV detector
alone. Virtually all fires emit some radiation in the UVC band, whereas the Sun's radiation at this band is absorbed by the Earth's atmosphere.
The result is that the UV detector is "solar blind", meaning it will
not cause an alarm in response to radiation from the Sun, so it can
easily be used both indoors and outdoors.
UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays
used in nondestructive metal testing equipment (though this is highly
unlikely), and radioactive materials can produce levels that will
activate a UV detection system. The presence of UV-absorbing gases and
vapors will attenuate the UV radiation from a fire, adversely affecting
the ability of the detector to detect flames. Likewise, the presence of
an oil mist in the air or an oil film on the detector window will have
the same effect.
Photolithography
Ultraviolet radiation is used for very fine resolution photolithography,
a procedure wherein a chemical called a photoresist is exposed to UV
radiation that has passed through a mask. The exposure causes chemical
reactions to occur in the photoresist. After removal of unwanted
photoresist, a pattern determined by the mask remains on the sample.
Steps may then be taken to "etch" away, deposit on or otherwise modify
areas of the sample where no photoresist remains.
Electronic
components that require clear transparency for light to exit or enter
(photovoltaic panels and sensors) can be potted using acrylic resins
that are cured using UV energy. The advantages are low VOC emissions and
rapid curing.
Certain inks, coatings, and adhesives are formulated with photoinitiators and resins. When exposed to UV light, polymerization occurs, and so the adhesives harden or cure, usually within a few seconds. Applications include glass and plastic bonding, optical fiber coatings, the coating of flooring, UV coating and paper finishes in offset printing, dental fillings, and decorative fingernail "gels".
UV sources for UV curing applications include UV lamps, UV LEDs, and excimer
flash lamps. Fast processes such as flexo or offset printing require
high-intensity light focused via reflectors onto a moving substrate and
medium so high-pressure Hg (mercury) or Fe
(iron, doped)-based bulbs are used, energized with electric arcs or
microwaves. Lower-power fluorescent lamps and LEDs can be used for
static applications. Small high-pressure lamps can have light focused
and transmitted to the work area via liquid-filled or fiber-optic light
guides.
The impact of UV on polymers is used for modification of the (roughness and hydrophobicity) of polymer surfaces. For example, a poly(methyl methacrylate) surface can be smoothed by vacuum ultraviolet.
UV radiation is useful in preparing low-surface-energy polymers for adhesives. Polymers exposed to UV will oxidize, thus raising the surface energy
of the polymer. Once the surface energy of the polymer has been raised,
the bond between the adhesive and the polymer is stronger.
Biology-related uses
Air purification
Using a catalytic chemical reaction from titanium dioxide and UVC exposure, oxidation of organic matter converts pathogens, pollens, and moldspores
into harmless inert byproducts. However, the reaction of titanium
dioxide and UVC is not a straight path. Several hundreds of reactions
occur prior to the inert byproducts stage and can hinder the resulting
reaction creating formaldehyde, aldehyde, and other VOC's en route to a
final stage. Thus, the use of titanium dioxide and UVC requires very
specific parameters for a successful outcome. The cleansing mechanism of
UV is a photochemical process. Contaminants in the indoor environment
are almost entirely organic carbon-based compounds, which break down
when exposed to high-intensity UV at 240 to 280 nm. Short-wave
ultraviolet radiation can destroy DNA in living microorganisms. UVC's effectiveness is directly related to intensity and exposure time.
UV has also been shown to reduce gaseous contaminants such as carbon monoxide and VOCs.UV lamps radiating at 184 and 254 nm can remove low concentrations of hydrocarbons and carbon monoxide
if the air is recycled between the room and the lamp chamber. This
arrangement prevents the introduction of ozone into the treated air.
Likewise, air may be treated by passing by a single UV source operating
at 184 nm and passed over iron pentaoxide to remove the ozone produced
by the UV lamp.
Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Commercially available low-pressure mercury-vapor lamps
emit about 86% of their radiation at 254 nanometers (nm), with 265 nm
being the peak germicidal effectiveness curve. UV at these germicidal
wavelengths damage a microorganism's DNA/RNA so that it cannot
reproduce, making it harmless, (even though the organism may not be
killed).
Since microorganisms can be shielded from ultraviolet rays in small
cracks and other shaded areas, these lamps are used only as a supplement
to other sterilization techniques.
UV-C LEDs are relatively new to the commercial market and are gaining in popularity. Due to their monochromatic nature (±5 nm)
these LEDs can target a specific wavelength needed for disinfection.
This is especially important knowing that pathogens vary in their
sensitivity to specific UV wavelengths. LEDs are mercury free, instant
on/off, and have unlimited cycling throughout the day.
Disinfection using UV radiation is commonly used in wastewater treatment applications and is finding an increased usage in municipal drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water. Solar water disinfection has been researched for cheaply treating contaminated water using natural sunlight. The UV-A irradiation and increased water temperature kill organisms in the water.
Ultraviolet radiation is used in several food processes to kill unwanted microorganisms. UV can be used to pasteurize
fruit juices by flowing the juice over a high-intensity ultraviolet
source. The effectiveness of such a process depends on the UV absorbance of the juice.
Pulsed light
(PL) is a technique of killing microorganisms on surfaces using pulses
of an intense broad spectrum, rich in UV-C between 200 and 280 nm. Pulsed light works with xenon flash lamps that can produce flashes several times per second. Disinfection robots use pulsed UV.
The antimicrobial effectiveness of filtered far-UVC (222 nm)
light on a range of pathogens, including bacteria and fungi showed
inhibition of pathogen growth, and since it has lesser harmful effects,
it provides essential insights for reliable disinfection in healthcare
settings, such as hospitals and long-term care homes.
Biological
Some
animals, including birds, reptiles, and insects such as bees, can see
near-ultraviolet wavelengths. Many fruits, flowers, and seeds stand out
more strongly from the background in ultraviolet wavelengths as compared
to human color vision. Scorpions glow or take on a yellow to green
color under UV illumination, thus assisting in the control of these
arachnids. Many birds have patterns in their plumage that are invisible
at usual wavelengths but observable in ultraviolet, and the urine and
other secretions of some animals, including dogs, cats, and human
beings, are much easier to spot with ultraviolet. Urine trails of
rodents can be detected by pest control technicians for proper treatment
of infested dwellings.
Butterflies use ultraviolet as a communication system for sex recognition and mating behavior. For example, in the Colias eurytheme
butterfly, males rely on visual cues to locate and identify females.
Instead of using chemical stimuli to find mates, males are attracted to
the ultraviolet-reflecting color of female hind wings. In Pieris napi
butterflies it was shown that females in northern Finland with less
UV-radiation present in the environment possessed stronger UV signals to
attract their males than those occurring further south. This suggested
that it was evolutionarily more difficult to increase the UV-sensitivity
of the eyes of the males than to increase the UV-signals emitted by the
females.
Many insects use the ultraviolet wavelength emissions from
celestial objects as references for flight navigation. A local
ultraviolet emitter will normally disrupt the navigation process and
will eventually attract the flying insect.
The green fluorescent protein (GFP) is often used in genetics
as a marker. Many substances, such as proteins, have significant light
absorption bands in the ultraviolet that are of interest in biochemistry
and related fields. UV-capable spectrophotometers are common in such
laboratories.
Ultraviolet traps called bug zappers
are used to eliminate various small flying insects. They are attracted
to the UV and are killed using an electric shock, or trapped once they
come into contact with the device. Different designs of ultraviolet
radiation traps are also used by entomologists for collectingnocturnal insects during faunistic survey studies.
Ultraviolet radiation is helpful in the treatment of skin conditions such as psoriasis and vitiligo. Exposure to UVA, while the skin is hyper-photosensitive, by taking psoralens is an effective treatment for psoriasis. Due to the potential of psoralens to cause damage to the liver, PUVA therapy may be used only a limited number of times over a patient's lifetime.
UVB phototherapy does not require additional medications or
topical preparations for the therapeutic benefit; only the exposure is
needed. However, phototherapy can be effective when used in conjunction
with certain topical treatments such as anthralin, coal tar, and vitamin A and D derivatives, or systemic treatments such as methotrexate and Soriatane.
Herpetology
Reptiles need UVB for biosynthesis of vitamin D, and other metabolic processes. Specifically cholecalciferol
(vitamin D3), which is needed for basic cellular / neural functioning
as well as the utilization of calcium for bone and egg production.
The UVA wavelength is also visible to many reptiles and might play a
significant role in their ability survive in the wild as well as in
visual communication between individuals.
Therefore, in a typical reptile enclosure, a fluorescent UV a/b source
(at the proper strength / spectrum for the species), must be available
for many captive species to survive. Simple supplementation with cholecalciferol (Vitamin D3) will not be enough as there's a complete biosynthetic pathway that is "leapfrogged" (risks of possible overdoses), the intermediate molecules and metabolites also play important functions in the animals health.
Natural sunlight in the right levels is always going to be superior to
artificial sources, but this might not be possible for keepers in
different parts of the world.
It is a known problem that high levels of output of the UVa part
of the spectrum can both cause cellular and DNA damage to sensitive
parts of their bodies – especially the eyes where blindness is the
result of an improper UVa/b source use and placement photokeratitis.
For many keepers there must also be a provision for an adequate heat
source this has resulted in the marketing of heat and light
"combination" products.
Keepers should be careful of these "combination" light/ heat and UVa/b
generators, they typically emit high levels of UVa with lower levels of
UVb that are set and difficult to control so that animals can have their
needs met.
A better strategy is to use individual sources of these elements and so
they can be placed and controlled by the keepers for the max benefit of
the animals.
Evolutionary significance
The evolution of early reproductive proteins and enzymes is attributed in modern models of evolutionary theory to ultraviolet radiation. UVB causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand that reproductive enzymes cannot copy. This leads to frameshifting during genetic replication and protein synthesis, usually killing the cell. Before formation of the UV-blocking ozone layer, when early prokaryotes
approached the surface of the ocean, they almost invariably died out.
The few that survived had developed enzymes that monitored the genetic
material and removed thymine dimers by nucleotide excision repair enzymes. Many enzymes and proteins involved in modern mitosis and meiosis
are similar to repair enzymes, and are believed to be evolved
modifications of the enzymes originally used to overcome DNA damages
caused by UV.
Photobiology is the scientific study of the beneficial and harmful
interactions of non-ionizing radiation in living organisms,
conventionally demarcated around 10 eV, the first ionization energy of
oxygen. UV ranges roughly from 3 to 30 eV in energy. Hence photobiology
entertains some, but not all, of the UV spectrum.