Theory of night vision
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Transcript of Theory of night vision
1/19/2014 1
By NS Wickramasinghe BSc (Eng)
CONTENTS INTRODUCTION
HISTORY
GENERATIONS OF NVDs
BASIC WORKING PRINCIPLE
IMAGE INTENSIFIER TUBES
BASIC COMPONENTS
MAINTENANCE OF NVDs
NIGHT VISION DEVICES USED IN SRI LANKA NAVY
INTRODUCTION HUMAN VISION IS COMPACKED TO VISIBLE RANGE.
ELECTROMAGNETIC SPECTRUM .
ENHANCED SPECTRAL RANGE TECHNIQUES USES TO VIEW INVISIBLE THINGS.
ENHANCED SPECTRAL RANGE ALLOWS THE VIEWER TO TAKE ADVANTAGE OF NON-VISIBLE SOURCES OF ELECTROMAGNETIC RADIATION
NIGHT VISION TECHNOLOGIES USED
IMAGE ENHANCEMENT
THERMAL IMAGING
IMAGE ENHANCEMENT COLLECTS THE TINY AMOUNTS OF LIGHT .
INCLUDES THE LOWER PORTION OF THE INFRARED LIGHT SPECTRUM .
AMPLIFYING IT TO THE POINT THAT WE CAN EASILY OBSERVE THE IMAGE .
THERMAL IMAGING
CAPTURES THE UPPER PORTION OF THE INFRARED LIGHT SPECTRUM.
IR EMITTED AS HEAT BY OBJECTS INSTEAD OF SIMPLY REFLECTED AS LIGHT .
INFRA RED
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INVENTION OF NIGHT VISION
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“SEEING WITHOUT BEING SEEN” IT IS THE BASIC TACTIC OF ANY MILITARY FORCE IN THE WORLD.
NIGHT VISION SERVES FOR THAT PURPOSE.
NIGHT VISION DEVICE INVENTED BASICALLY FOR MILITARY USE.
SCIENTIFIC / CIVILIAN USE SOMETIMES LIMITED OR EVEN FORBIDDEN BY LAW
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HISTORY / FUTURE OF NVDs 1930’S - IMAGE CONVERTER TUBE – Basis for NVDs
LATER NVDs WERE CLASSIFIED AS PER THE DEVELOPMENT STAGE OF USED IMAGE INTENSIFIER TUBE (IIT) , AS GENERATION OF NVD
GENERATION 0,1, 2, 2+, 3
GENERATION 4 (ONLY IN US)
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SPECTRUM OF NVDs
NIGHT VISION MONOCULARS/ BINOCULARS
NIGHT VION WEAPON SIGHTS
NIGHT VISION GOGGLES
LONG RANGE NIGHT VISION
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HOW NIGHT VISION WORKS
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ARRANGEMENT OF UNITS IN NVD
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AN OBJECTIVE LENS (CONVEX TYPE) FOCUSING AN IMAGE INTO A VACUUM TUBE WITH A PHOTOCATHODE AT ONE END
IT RELEASES ELECTRONS BY THE PHOTOELECTRIC EFFECT ON THE INCIDENCE OF INCOMING PHOTONS.
FROM THERE, THE PHOTOELECTRON IS ACCELERATED THROUGH AROUND 5000 VOLTS INTO A TILTED MICROCHANNEL PLATE.
THE HIGH ENERGY ELECTRON RELEASES SOME MICROCHANNEL PLATE ELECTRONS, WHICH FURTHER RELEASE OTHER ELECTRONS, IN A PROCESS CALLED SECONDARY CASCADED EMISSION.
THE MCP IS TILTED TO ENCOURAGE MORE ELECTRON COLLISIONS
HOW NIGHT VISION WORKS
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FOM FIGURE OF MERIT OF A NIGHT VISION DEVICE
IT’S A MEASURE OF THE QUALITY OF THE IIT
FOM= RESOLUTION X SIGNAL TONOISE RATIO (S/N)
GEN 3 – APPROX 1600 FOM
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IITs IN NVDs
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GENERATION 0 SO LITTLE, LOW LIGHT AMPLIFICATION
ADDITIONAL IR –ILLUMINATORS MUST BE USED FOR OBSERVATION.
(THAT’S WHY , THEY ARE CALLED ACTIVE NIGHT VISION DEVICES )
IMAGE CONVERTER TUBE WAS USED, NOT THE IMAGE INTENSIFIER TUBE.
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Cont …
• S-1 photocathode, coated with
silver shifted cesium oxide
• Anode cone, ensures over high
voltage acceleration focusing and
turning the image up-side-down
• P1 phosphor screen, coating out of
zinc and cadmium transferred
phosphor
GENERATION 0 -DISADVANTAGES HAD A VERY DISTORTED PICTURE
SHORT TUBE LIFE DUE TO THE HIGH ELECTRICAL VOLTAGE.
HEAVY AND BIG
POWER SOURCE
AN AUTOMATIC
PROTECTION OF
THE PHOTO-
SENSITIVE TUBE
DOES NOT EXIST
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GENERATION 1 IMAGE INTENSIFIER TUBE USED FOR THE 1ST TIME (WITH MULTI -
ALKALI PHOTO CATHODE - S-20, coating from potassium, sodium, cesium, silver)
LUMINOUS GAIN BETTER THAN GEN. 0
(P-20 phosphor screen)
SERVICE LIFE OF GEN 1 IIT- 1000-2000 HRS
NO AUTOMATIC PROTECTION AGAINST BRIGHT LIGHT
WORKING RANGE 750-800 nm OF IR WAVE LENGTH
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Sectional view Gen1, single-
stage NVD, low gain
Sectional view Gen1, 3-stage
NVD, generally high gain at low
contrast
Cont …
Cont …
GENERATION 2
MICRO CHANNEL PLATE (MCP) USED IN IIT INSTEAD OF ANODE CONE.
PRINCIPLE CHANGED FROM
ACCELERATION MULTIFICATION (electrons)
THE LIFE SPAN INCREASED TO APPROX. 2500 -5000 H
WORKS WITHIN THE RANGE BETWEEN 780 AND 850 NM WAVELENGTH
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STRUCTURE OF IIT (2ND GEN)
PHOTO CATHODE
MICRO CHANNEL
PHOSPER SCREEN
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ADVANTAGES OF 2ND GEN NVD NO IR-ILLUMINATOR NEEDED
SMALLER IN DIMENTION
LOW IN WEIGHT
LIFE SPAN OF IIT - 2500- 5000 HRS
AUTOMATIC BRIGHTNESS CONTROL (ABC)
HIGH IMAGE RESOLUTION THAN EARLY GENs.
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GENERATION 2+(SUPPER GEN)
MORE IMPROVED MCP
OVER 4 MILLION MICRO CHANNELS
PHOTO CATHODE-MORE SENSITIVE TO IR
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NEW COATING FOR PHOTO CATHODE – Ga As
(Gallium Asernic)
IMPROVED IMAGE RESOLUTION
6-12 MILLIONs OF MICRO CHANNELS IN MCP
AN THIN ALUMINIUM LAYER ON THE PC (ION BARRIER) IS USED FOR AN ENHANCED SERVICE LIFE SPAN OF IIT- 10000 HRS
IR WAVE LENGTH- 780-920 µm
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GENERATION 3
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FILMLESS OR THINFILM TUBES IS USED
ONLY USED BY US ARMED FORCES
15000 HRS OF LIFE SPAN
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GENERATION 4
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GEN 1 GEN 2
GEN 3
LENSES USED IN NVD
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CONVEX LENSES
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LENSES IN LONG RANGE NVDs
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CONVEX LENS
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PRISMS USED IN NVD
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BATTERY COVER
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DAY LIGHT COVER
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DAY LIGHT COVER
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EYE GUARD
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TYPE OF BATTERY USED IN NVDs
SIZE AAA
SIZE AA
SIZE 9V
LITHIUM 3V
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‘O’ RINGS
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CARING CASE
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MAINTENANCE
SHOULD NOT OPERATE AT BAD WEATHER
MUST NOT OPERATE AT DAY TIME
KEEP NIGHT VISION BINOCULER CLEAN
CLEAN LENS WITH 99% ETHIL ALCOHOL
SHOULD NOT DROP DOWN
SHOULD NOT COLLIED WITH ANYTHING
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Contd.
MUST NOT APPLY GREAS OR OIL ON LENSES
THE N/V BINOCULER HAS TO BE IN THE CARRING CASE WHILE MOVING/TRANSPORT
REMOVE THE BATTERY BEFORE STORED.
STORED IT AT COOL /DRY PLACE.
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Night vision devices are dependent on every little light information available
The task of the objective lens to collect as much as possible light and to focus an image on the image intensifier tube's input window
Tube can strengthen only the low light passed and collected by the objective lens
Appropriate tuning between the optical and electronic part should be take place
PHOTOELECTRIC EFFECT
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MICRO CHANNEL PLATE
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