FLIR Concept

1
FLIR Concept
Prepared by Ernest Grimberg - Opgal chief
scientist
2
Table of contain

• General background.
• Physical Constants.
• Basic radiometric concepts.
• Black body radiation.
• Optics - introduction.
• IR Detectors.
• Spatial resolution and thermal resolution.
• Signal processing block diagram.

3
General Background electromagnetic waves
4
General Background electromagnetic waves
Plane polarized EM wave Speed of an EM wave
Link to a more detailed paper
5
General Background electromagnetic waves

• ENERGY TRANSPORTED BY AN EM WAVE
• The B and E fields of an electromagnetic wave
  contain energy. e.g Heat from a light bulb
• The rate of energy flow per unit frontal area
  (Energy flux) ,
• (watts/m2)
• In general, the energy flux or POYNTING VECTOR .
• Notice how the vector product gives the travel
  direction of an EM wave.

6
General Background electromagnetic waves
INTENSITY OF AN EM WAVE Consider a point in
space. Take x 0 for convenience. Hence the
average energy flux Wave Intensity I

7
General Background electromagnetic waves

8
General Background electromagnetic waves
propagation

9
Physical Constants

10
Angle definitions
Planar angle ?(arc length)/radius radians

Solid angle ? (surface area)/radius steradians
11
Angle approximations formulas

???², (? in rad), for ?lt0.4 rad (23),
Max. Error 1.5 ??sin ²(?) (? in rad), for
?lt0.4 rad (23), Max. Error 1.5
12
Radiometric quantities and formulas

13
Blackbody Radiation

The spectral radiant emittance formula is
T is the absolute temperature in degrees Kelvin.
Spectral radiance L(?) is equal to M(?)/? because
blackbodies are Lambertian sources
14
Blackbody Radiation

15
Blackbody Radiation

16
Blackbody Radiation

17
Optics, F/number

F/number (f) or speed of a lens is a measure
of the angular acceptance of the lens.
f represents the focal length d represents the
entrance pupil diameter of the lens For small ?
angles the numerical aperture is approximately
equal to 0.5F.
18
Optics, F/number

When an optical lens is used to image a scene, of
radiance equal Lsc, on a detector faceplate or on
film the faceplate radiance may be obtain from
the following formula
Lfp represents detector faceplate radiance in
W/(mmsteradian) Lsc represents scenery
radiance in W/(mmsteradian) Tr represents
the lens transmittance m represents the
magnification from scene to detector faceplate
19
Optics, Diffraction limit

Diffraction, poses a fundamental
limitation on any optical system.
Diffraction is always present, although its
effects may be masked if the system has
significant aberrations. When an optical
system is essentially free from
aberrations, its performance is limited solely
by diffraction, and it is referred to as
diffraction limited. In calculating
diffraction, we simply need to know
the focal length(s) and aperture diameter(s)
we do not consider other lens-related
factors such as shape or index of
refraction. Since diffraction increases
with increasing f-number, and aberrations
decrease with increasing f-number,
determining optimum system performance often
involves finding a point where the combination
of these factors has a minimum effect.
20
Optics, Diffraction limit continue
Fraunhofer diffraction at a circular aperture
dictates the fundamental limits of performance
for circular lenses. It is important to remember
that the spot size, caused by diffraction, of a
circular lens is where d is the diameter
of the focused spot produced from
plane-wave illumination and ? is the
wavelength of light being focused. The
diffraction pattern resulting from a uniformly
illuminated circular aperture is shown in the
image below. It consists of a central bright
region, known as the Airy disc, surrounded by a
number of much fainter rings.

21
Optics, Diffraction limit continue
Each ring is separated by a circle of zero
intensity. The irradiance distribution in this
pattern can be described by where I0 peak
irradiance in the image. J1(x) is a Bessel
function of the first kind of order unity, and
where ? is the wavelength, D is the
aperture diameter, and ? is the angular radius
from pattern maximum.

22
Optics, Diffraction limit continue
Energy Distribution in the Diffraction Pattern of
a Circular Aperture Ring or Band Position
(x) Relative Intensity (Ix/I0)
Energy in Ring () Central Maximum 0.0
1.0
83.8 First Dark
1.22? 0.0 First Bright
1.64? 0.0175
7.2 Second Dark 2.23?
0.0 Second Bright 2.68?
0.0042
2.8 Third Dark 3.24?
0.0 Third Bright 3.70?
0.0016
1.5 Fourth Dark 4.24?
0.0 Fourth Bright 4.71?
0.0008
1.0 Fifth Dark 5.24?
0.0

23
Optics, Diffraction limit continue
The graph below shows the form of both circular
and slit aperture diffraction patterns when
plotted on the same normalized scale. Aperture
diameter is equal to slit width so that patterns
between x values and angular deviations in the
far field are the same.

24
Optics, Diffraction limit continue
The graph below shows the diameter of the first
circular bright disc versus optics f for two
different wavelengths 4 microns and 10 microns
respectively.

25
Optics Detector relations
Assuming that the detector is a two
dimensional matrix of n_x by n_y
elements, and that each detector element size is
d_x by d_y meters, and that the optics focal
length is f meters, the instantaneous field of
view (IFOV), on X and Y directions, are given by
the following relations

26
Optics Detector relations continue

Assuming that the detector is a two
dimensional matrix of n_x by n_y
elements, and that each detector element size is
d_x by d_y meters, and that the optics focal
length is f meters, the field of view, on X
and Y directions, are given by the following
relations
27
Detection, Orientation, Recognition, and
Identification

Task Line Resolution per Target
Minimum Dimension Detection
1.0 0.25 line pairs Orientation
1.4 0.35 line pairs
Recognition 4.0 0.8
line pairs Identification
6.4 1.5 line pairs
28
IR Detectors Quantum noise limit

The quantum noise difference in temperature
(QNETD) for cooled detectors is limited by the
signal quantum noise.
n represents the amount of photoelectrons
collected from the scenery.
29
IR Detectors Quantum noise limit continue

The quantum noise difference in temperature
(QNETD) for cooled detectors is limited by the
signal quantum noise.
30
IR Detectors Quantum noise limit continue

The quantum noise difference in temperature
(QNETD) for cooled detectors is limited by the
signal quantum noise.
31
IR Detectors technology

There are two very distinctive detector
technologies the direct detection (or photon
counting ), and thermal detection. Direct
detection technology (photon counting)
translates the photons directly into
electrons. The charge accumulated, the
current flow, or the change in conductivity is
proportional to the scenery view radiance.
This category contains many detectors, like
PbSe, HgCdTe, InSb, PtSi etc. Except for FLIRs
working in the SWIR range, all the FLIRs
based on the direct detection technology are
cooling the detectors to low temperatures, close
to 200 degrees Celsius.
32
IR Detectors technology

Thermal detection technology. These detectors are
using secondary effects, like the relation
between conductivity, capacitance, expansion and
detector temperature. The following detectors are
classified in this category Bolometers,
Thermocouples, Thermopiles, Pyroelectrics etc.
Usually these detectors do not require cryogenic
temperatures.
33
IR Detectors description

• Any IR detector (except for the near IR
  spectra) is an assembly that contains
• A Focal Plane Array (FPA),
• A dewar or a vacuum package,
• A cooler or a temperature stabilization device,
• and in most of the cases a cold shield or a
  radiation shield.

34
IR Detectors description continue

35
IR Detectors, DEWARS Description

36
IR Detectors, InSb spectral band description


320?256 InSb FOCAL PLANE ARRAY DETECTOR
37
Microbolometer detector basic concept
The original design disclosed by Honeywell.
38
Microbolometer detector basic concept
The original design disclosed by Honeywell.
39
Microbolometer detector basic concept
Real picture. Sofradirs detector.
40
Spatial resolution and thermal resolution.
41
Spatial resolution and thermal resolution.
The spatial resolution and the thermal resolution
will be analyzed Assuming that the thermal
cameras can be described by linear models.
42
Spatial resolution and thermal resolution continue
Thermal camera response to any input signal is
given by
T represents cameras transfer function.
Recoll T depends on x,y only, therefore assuming
linearity
43
Spatial resolution and thermal resolution continue
Therefore the thermal camera response to any
input signal is given by
h represents cameras impulse response
function. The camera impulse response is given by
convolving its subsystems.
represents the convolution operator.
44
Spatial resolution and thermal resolution continue

• Example. Estimate the MTF of a FLIR camera based
  on a the uncooled
• microbolometer detector manufactured by Sofradir.
• The input data for performance estimation is
• 1. Optics focal length 0.1 m,
• 2. Optics f number 1.17 ,
• 3. Optics transfer function at 1.1
  cycles/milliradian 0.75
• Gimbals line of site stabilization standard
  deviation equals 100 microradian.

45
Spatial resolution and thermal resolution continue
Assuming diffraction limit optics performances

But according to the input data Optics transfer
function at 1.1 cycles/milliradian 0.75
46
Spatial resolution and thermal resolution continue
Assuming geometrically limited optics

47
Spatial resolution and thermal resolution continue
Assuming that the detector impulse response is
geometrically limited

48
Spatial resolution and thermal resolution continue
Stabilization impulse response for a standard
deviation of 100 µrad

49
Spatial resolution and thermal resolution continue
The electronics is model as a low pass filter on
horizontal direction therefore

50
Spatial resolution and thermal resolution continue
Entire system impulse response is estimated by
the following process

51
Spatial resolution and thermal resolution continue
The horizontal and vertical modulation transfer
function are defined by the following relations

52
Spatial resolution and thermal resolution continue
The Fourier transform of systems impulse
response is presented in the following Two
dimensional graph.

53
Spatial resolution and thermal resolution continue
The MTF on horizontal direction is presented in
the following graph.

54
Spatial resolution and thermal resolution continue
The MTF on vertical direction is presented in the
following graph.

55
Spatial resolution and thermal resolution continue
The thermal resolution is defined by the
following two values NEDT Noise equivalent
temperature difference, MRTD Minimum resolvable
temperature difference. The NEDT is the minimum
temperature difference, at the FLIR input,
required in order to overcame the noise. The
NEDT is defined for the zero spatial
frequency, therefore NEDT is independent of
spatial frequencies. The MRTD is a two
dimensional function of spatial frequency,
defined as the minimum input temperature
required for any spatial frequency in order to be
visible at the FLIR output.

56
Spatial resolution and thermal resolution continue

• The dominant noise sources that affect cooled
  FLIR performances are
• The Shot noise caused by the discreteness of
  electronic charge. The current Id
• flowing through the responsive element is the
  result of current pulses produced by
• the individual electrons and or holes.
• The Readout noise caused by the electronic
  circuits that manipulates the signal
• in order to reduce the number of video output
  lines between 1 to 8 although the
• number of detector elements is much higher.
• The 1/f noise characterized by a noise power
  spectrum
• The fixed pattern noise caused by the
  insufficient correction of detector signal
• non uniformity.

57
Spatial resolution and thermal resolution continue

• The dominant noise sources that affect uncooled
  FLIR performances are
• The Johnson noise caused by the random motion of
  charge carriers in thermal
• equilibrium.
• The Readout noise caused by the electronic
  circuits that manipulates the signal
• in order to reduce to one (1) the number
  of video output lines although the
• number of detector elements is much higher.
• The 1/f noise characterized by a noise power
  spectrum.
• The fixed pattern noise caused by the
  insufficient correction of detector signal
• non uniformity.

58
Spatial resolution and thermal resolution continue
The MRTD on horizontal direction for the example
presented before is described by the following
graph

59
EVS signal processing block diagram