Unit 3

Infrared Basics and Limitations

Objectives:

The student will be able to explain in layman’s terms four basic elements that affect thermal IR sensing and interpretation

The student will be able to identify the limitations of all infrared systems.

Unit Contents1. Objectives

2. IR Basic Physics

3. IR Limitations

Infrared Basics

The four basic elements to consider in thermal IR sensing and interpretation are:

The source (the fire)Attenuation Sensor or detectorThe remote sensing analyst and/or image interpreter

Source (the fire)The energy from fires, heat, is emitted as electromagnetic (EM) energy within specific wavelength bands.Energy from fires is emitted at long wavelengths.Most fire mapping applications utilize long-wavelength thermal remote sensors.

Source Temperature (C) Nominal Wavelength (μm)

Background 25 10

Fuel ignition 275 5

Glowing 550 4

Cool fire 725 3

Hot fire 1200 2

We are dealing with Thermal Infrared

Not all IR is heat

Reflective

Thermal

Attenuation

The scattering or absorption of electromagnetic energy by the atmosphere.

The atmosphere is a good transmitter of IR energy in two regions (called windows) 3-5 µm and 8-14 µm

Sensor or detector

Ideal sensor design considers

The sources it is designed to measure

The atmospheric windows for those wavelengths

The most suitable materials to build the detector

Thermal Bands

VisibleNear- and Mid-IR

Thermal IR

Concurrent Burn Imagery

Thermal imagery from an low level UASMulti-spectral EO/IR imagery from theNASA AMS Wildfire sensor

USDA Forest Service MODIS Active Fire Mapping Program

http://activefiremaps.fs.fed.us

October 26, 2003 18:35 UTC Terra MODIS True & False Color Composite

Remote Sensing Analyst or Image Interpreter

Training

Experience

Time

Situation

Customers expectations

Infrared Limitations

AttenuationSolar RadiationHeat Source TemperatureSaturation

ALL infrared sensors are limited by the physics of thermal infrared energy

AttenuationThermal IR energy can be emitted or reflected. During fire/heat-mapping missions we are usually looking for emitted energy (fires)

Water and water vapor absorb IR energy (e.g., clouds or fog)

The atmosphere is a good transmitter of IR energy in certain regions (remember the two windows)

Image Credit: FLIR

Infrared will not pass through water.

Marine Layer (Fog) Mendocino June 29, 2008

High Clouds – Battle Creek Complex July 27, 2007, Eastern OR

White-hot vs. Black-hot

Phoenix Imagery is black-hot.

Solar RadiationReflected sunlight may mix with emitted IR energy. Highly reflective surface may appear hot in IR imagery.

Creates false positives.Long-wavelength (8-14 µm) data is collected to portray background areas.IR missions scheduled to maximize the thermal difference between fires and background.

Energy Detection

Heat Source TemperatureThe amount of energy emitted by a fire or heat source depends on the temperature of the object.

Fires are not perfect emitters of IR energy

Emitted energy can be absorbed by heavy canopy or overstory

Thermal remote sensing can only detect heat on the surface of targets.

Fire Engine

Road Heat from burning grass

Photo Point B1:00 PM

Fire Engine

Road Burn Area

Photo Point B

Fire Engine

Road Heat from burning grass

Photo Point B1:00 PM

Other hot objects

Other hot objects

Other hot objects

Other hot objects

SaturationVery hot object or heat sources may saturate a sensor, creating low contrast in the image.Blooming is caused by hot gasses when a fire flares up. Gasses in the convective fire column can be hot enough to saturate the sensor and render portions of the imagery unusable.

Roundabout - 5/01/07, Southeastern GA

Heat from the engines is saturating the sensor

Crewmember can be seen through the open hatch

Pilot cannot be seen through windshield

Objectives:

The student will be able to explain in layman’s terms four basic elements that affect thermal IR sensing and interpretation

The student will be able to identify the limitations of all infrared systems.

Questions?