Ultraviolet
(UV) radiation is defined as all radiations with wavelengths
between 10 and 400nm and can be divided into four basic categories
in terms of wavelength: UVA, UVB, UVC and vacuum UV, as shown
in Fig. 1. Ultraviolet detectors are playing a more and more
important role in modern science and technology. But what
are mainly sought after are visible-blind and solar-blind
UV detectors. Visible-blind ultra-violet photodetectors have
high UV responsitivity but high discrimination against visible
and infrared radiation. Solar-blind photodetectors, as defined
for applications near the Earth surface, can offer even a
shorter cutoff wavelength and respond only to UV radiation
at wavelengths shorter than those of solar radiation that
can penetrate the atmosphere of the Earth. Due to the stratospheric
ozone layer absorption, the solar radiation near the earth’s
surface has a short-wavelength limit at 290nm. Thus UV detectors
with a long-wavelength cut-off below 280nm are usually called
solar-blind. Visible/solar-blind UV detectors can find applications
in industrial equipment (fire detection, flame combustion
control), scientific research (UV astronomy, biological and
medical applications) as well as consumer market (a number
of personal health-care products).
(Fig.1 The four UV categories
and the solar spectrum)
In the following, several specific applications of visible/solar-blind
UV detection technology are introduced:
Visible-blind UV detectors are very handy when
detecting any oil spilled into the sea from ships, which can
cause devastating pollution to marine life. Near real-time
detection of oil spills is vital because their area can multiply
in just minutes (See figure 2). Therefore we need a device
which is not just able to detect any occurrence of a spill,
but immediately tell us exactly to what extent the spill has
spread. In other words, we need a UV camera i.e. a mapping
UV detector. The physics behind is that oil, with a refractive
index higher than water, reflects light to a much greater
extent than clean sea water does; and this difference happens
to be the greatest for UV radiation. So we can detect oil
spills simply by detecting the UV radiation, coming from the
sun, reflected by the sea surface, which can be done by satellites
or aircrafts flying above the sea.
(Fig. 2 Oil spread over a 20-minute
period, photographed by a UV-visible scanner (PNNL Sensors
and Electronics))
A solar-blind UV detector is particularly useful
in outdoor, where even a small flame would be hazardous. This
is why petrol filling stations usually have a flame detector
installed, which is in the form of a solar-blind UV camera.
The camera does not only detect flames, but also maps the
area where a flame is detected so it can be located easily
(See figure 3), and appropriate safety actions can be carried
out quickly. Solar-blind flame detectors can also be used
for other outdoor applications such as locating faults on
power lines and forest fire.
(Fig. 3 Gasoline fire: 20 cm in diameter,
from 450 meter distance (Ofil company))
Detecting missiles is considered as another novel application
for solar-blind UV cameras. With the solar-blind characteristics,
these cameras are able to avoid the disturbance from the sunlight
and to ensure an accurate tracking of a missile. This is because
fumes are the hottest when they have just been ejected and
carry the most intensive far UV radiation. The reason why
the solar-blind characteristic is required in this application
is that the hot fume ejected from a missile not only releases
far UV radiation, but also leaves a track with near UV, visible
and infrared radiation. Non-solar-blind UV detectors that
also detect near UV would be detecting the whole track of
the missile, and not the missile itself.
Have you ever wondered how scientists find
out what elements exist in the atmospheres of other stars?
Or how they obtain their temperatures there? Thanks toˇ@UV
detectors, the scientists can carry out measurements to achieve
these tasks. The technology is called UV astronomy. It is
the study of extraterrestrial objects by examining their UV
spectrum. Actually the spectra of other electromagnetic waves
apart from UV are also analyzed, but it is performed by analysing
the UV spectra that enables us most effectively to tell the
relative composition of different elements in the atmosphere
of a particular star. This is how: the hot core of a star
emits all types of electromagnetic waves, and as a result
giving a full electromagnetic spectrum. But if waves of certain
frequencies are absorbed, say by some atoms, dark lines will
be left on the original spectrum. It happens that there is
a deep minimum in night sky background (1600-2500 )
and many strong transitions(often
resonance) of important species such as H, D, H2, He, C, N,
O, Mg, Si, S & Fe occur in the UV spectral region. So
by analysing the dark lines in the UV region of the electromagnetic
spectrum emitted by the hot core of a star, we can find out
what elements are presented in its atmosphere. Also, the radiation
of hot stars over 10,000 K peaks in UV so we can determine
their temperatures by studying their UV spectra.
So we have seen that visible-blind and solar-blind UV detectors
have their applications in safety-related products, military
defense and even basic scientific research. What we have not
yet mentioned is their role in our daily life. The UV radiation
coming from the Sun could be quite harmful to us, and in fact
is the ˇ§villainˇ¨ that causes painful sunburns, premature ageing
of the skin and even skin cancer. Over exposure of our eyes
to strong UV radiation may also cause cataracts. This is why
we must make sure that the sunscreens we put on and the sunglasses
we wear provide the essential protection. A visible-blind
detector is the perfect device for this - it directly measures
how well sunscreens or sunglasses block UV radiation and not
just blocking visible light. The development of visible- or
solar-blind UV detectors has indeed been a great contribution
to many fields and even our health. As technology becomes
more advanced every day, they will likely serve even more
areas of interest in the future.
(Fig. 4
The Hong Kong Observatory's broadband UV sensor
at its King's Park Meteorological Station-picture is provided
by Hong Kong Observatory)
Traditionally, photomultiplier tubes (PMT) and gas-ionization
chambers are used in combination with blocking filters and
phosphor down-conversion techniques as the UV detector units
for applications requiring visible/solar-blind characteristics.
However they suffer from high cost, fragility, short life-time
and the need for high operating voltages. Solid-state semiconductor
photo detectors are preferred in many applications because
of their low cost, compact size, long life time and low voltage
operation. In recent years, several wide-bandgap semiconductor
UV detectors capable for visible-blind and solar-blind UV
detection applications are commercially available. The active
materials of these novel UV detectors include the diamond,
SiC, GaAlN alloys and the two novel alloys of ZnSSe and ZnMgS
developed at the Physics Department of HKUST in Hong Kong.
The working principle of these thin film devices is usually
based on the photovoltaic effect: the production of a voltage
when EM radiation falls on certain materials coated with another
substance. The effect can be detected by connecting the two
materials through an external circuit to generate a current.
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