Chapter 1
• Optoelectronics use IR-Visible-UV
• Ultraviolet (UV) light
– Highest frequency
– Smallest period
– Very energetic, so bumps electrons to higher
orbit
• Infrared (IR) light
– Slightly vibrates atoms
UV - Negatives
• UV can cause skin cancer and eye
damage
• Fades paint on a car
• Degrades rubber on tires
– Tires have coating to absorb UV and dissipate
it as heat instead of allowing it to combine
with polymer’s molecules
– Tires need to be used or dry rot occurs or
cracking in tires
UV - Positives
• UV kills germs
• UV disinfects bacteria by altering DNA in
germs
• UV sterilizes city waste water
• Helps to detect bogus Benjis ($100)
• Helps to analyze stamps, rocks, &
minerals
UV Classes
• UV-V visible 395 to 445 nm
• UV-A 320 to 395 nm
– Least damaging, but passes through ozone
• UV-B 280 to 320 nm
– Very harmful, but absorbed in stratosphere
• UV-C 200 to 280 nm
– Most damaging, but oxygen in stratosphere
absorbs most of it
• Vacuum UV (VUV) 100 to 200 nm
– Man made
– More destructive than UV-C
• Stratosphere
– Ozone layer resides here
• Troposphere
– Region directly above earth
• Sun light hits equator most, so also
receives most UV
• UV Experimental Index
– Used by National Weather Service
• Forecasts amount of UV reaching earth’s surface
– Index is from 0 to 15 in summer
– Also used by Environmental Protection
Agency (EPA)
Dobson Unit
• Ozone layer
– Thickest at the equator and thinnest at poles
– Acts as natural shield of UV light
– Antarctic ozone hole
• Dobson Unit
– Measure of ozone
– 1 DU is ozone thickness of 0.01 mm at 0o C
and 1 atm
– Average thickness in U.S. is 300 DU
– Uses spectrophotometer to measure.
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Ozone (O3)
Colorless
Harsh acrid smell
90% occurs in ozone layer
Chlorine and Bromine hasten ozone
destruction
• Single chlorine atom can destroy 100,000
ozone molecules before it leaves
stratosphere
• Halo carbons destroy ozone.
– can remain in atmosphere for up to 120 years
Ozone Destroyers
• 80% of ozone destruction occurs from
CFCs (Chlorofluorocarbons
• Volcanoes also destroy ozone
– Mount Pinatubo in Philippines erupted in 1991
and reduced ozone levels by 20%
• UV intensity is reduced by the following
– Clouds
– Rain
– Smog
• UV affects many animals
– Birds detect UV and select mate as to how UV
reflects off them
– Animals can see UV. Hunters who have
washed cloths in laundry detergent are very
visible to their prey!
– Iguanas use UV to detect edible plants and
see territories staked out by other iguanas
Flame Detectors
http://www.reliablefire.com/firealarm/flame_detectors.html
http://www.flame-detection.net
UV detectors respond to hydrogen fires, but IR detectors don’t.
Due to their fast detection capabilities, flame
detectors are generally used only in highhazard areas, such as fuel-loading platforms.
UV Flame Detectors
• A UV flame radiates in the 1850 to 2450 angstrom
range. (or 185 to 245 nm)
• Virtually all fire emit radiation in this band, while the
sun's radiation at this band is absorbed by the Earth's
atmosphere. (remember the ozone layer)
• UV detector is solar blind, so won’t have false alarm due
to sun.
• Can be used indoors or outdoors.
• UV detectors are sensitive to most fires, including
hydrocarbon, metals, sulfur, hydrogen, hydrazine, and
ammonia.
• They detect flames at high speed (3-4 milliseconds) due
to the UV high-energy radiation emitted by fires and
explosions at the instant of their ignition.
IR Flame Detectors
• An infrared (IR) detector basically is composed
of a filter used to screen out unwanted
wavelengths and focus only on IR.
• Main problem is false alarm from radiation of the
sun. If inside and shielded from the sun, then
works well. Good for bank vaults.
• IR detectors are sensitive to most hydrocarbon
fires (liquid, gases and solids).
• IR detectors respond in less than 50 msec.
• IR detectors are less affected by smoke, oil, and
certain gases and vapors than UV Detectors.
UV/IR Flame Detector
• An ultraviolet/infrared flame detector consists of
an UV and single-frequency IF sensor, paired
together to form one unit.
• A fire alarm is produced only when both sensors
detect a fire.
• Better false alarm rejection
• UV and IR spectral band detectors compare the
threshold signal in two spectral ranges and their
ratio to each other to confirm the reliability of the
fire signal.
3
IR Flame Detectors
3
• IR or triple IR spectral band detectors
compare three specific wavelength bands
within the IR spectral region.
• Mathematical techniques are used to
correlate the three bands to discriminate
between fire and false alarm.