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IR & thermal Image guidance

Pt 1

M S Prasad

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Infrared laws

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IR radiations from a Typical aircraft

= 0.85

Egt = exhaust Gas temp

Tail Pipe behaves as a gray body

with emissivity of 0.9

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Typical seeker head

1.Optical System : to collect IR radiation

2. Reticule : Optical Modulator

3. Detector electronics

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IR Homing

Atmospheric Attenuation = where = a+ . a is due to absorption and

Y is due to scattering. Scattering causes change in direction of the beam.

Rayleigh Scattering : for smaller particles .

However, rain, fog particles and aerosols are larger and consequently scatter IR and

visible radiation equally being independent of the wavelength.

= 1 + 1

2

Aircraft Skin Temp ( aerodynamic Heating is :-

Ta ambient temp , Ts is skin temp and is recovery factor ( 0.75 to 0.98)

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Detectors

QuantumIntrinsic ( PV) InGaAs , Hg Cd Te

Extrinsic Si :As , Ga:As

Photo EmissiveQuantum Effect Ga As Al

Therrmal

thermocouple

Bolometers : Change in internal resistance is measuredPyroelectric : Change In polarisation

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D*

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Figure of merit

Detectivity.*

This parameter varies inversely with the square root of both the

detectors sensitive area, A, and the electrical bandwidth, f.

Detectors

where e is the spectral radiant incident power. D* is defined as the

rms signal-to-noise ratio (SNR)ina1Hz bandwidth per unit rms incident

radiation power per square root of detector area. D* is expressed in

cmHz1/2W1. Expressed as D* ( 500,1000)

Responsivity

ofan IR detector is defined as the ratio of the root mean square(rms) value of the electrical output signal of the detector to the rms

value of the input radiation power.

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NEP ( Noise Equivalent Power )

The NEP is the incident power on the detector generating a signal output equal to the

rms noise output. OR the NEP is the signal level that produces a signal-to-noise ratio

(SNR) of 1.

The NEP is also quoted for a fixed reference bandwidth, which is often assumed

be 1 Hz. This NEP per unit bandwidth has a unit of watts per square root hertz

W/Hz1/2).

Detectivity

The detectivity D is the reciprocal of NEP:

For many detectors, the NEP is proportional to the square root of the detector

signal that is proportional to the detector area, Ad .This means that both NEPand detectivity are functions of the electrical bandwidth and

detector area . A normalized detectivity D

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Cooling Techniques IR thermal detectors

4 stage thermo elestric Cooler ( Peltier effect )

Joule Thomson cooler

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Sterling cycle Cooler

Cold Shields impacts the D * by a factor ( pi/) ^ .

D** = ( /pi) ^1/2 D*

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LOCK ON Range

Tail pipe : as a gray body with emissivity 0.9 ( assumed )

Plume temperature = 0.85 of exhaust temp

Assumption

Thermal radiations are emanating from One point ( radiating in Hemisphere)

Parameters are : a. Transmittance thru atmosphere

b. Transmittance thru IR optics

The total received IR radiance at Missile seeker should be equal to SNR required.