Theory of night vision

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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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