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Modeling the performances
of
infrared scanners and cameras in POVRAY
Only since about 2003 has it
been
possible to make thermal cameras with 'staring' focal planes.
Prior to that,
it was necessary to scan a
very small number of detector
elements through the field of view with either a
1-dimensional scanner
or a 2-dimensional scanner.
This is a model of a
1980s 2-dimensional infrared scanner which scanned a 23
element
linear detector through a 600,000 pixel field. The hexagon
rotates at
9,333 rpm and the pair of yoke mirrors at 25Hz
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This shows the type
of image
which early ( 1980s) thermal imagers made. The camera is
mounted
on an anti-tank missile control post and present the image to
the
gunner who is controlling the missile attacking the tank
target.
The tank detects the approaching missile and deploys smoke and an
inflatable decoy.
0.6MB
I made
this
movie with an
ideal camera and then added off-line the defects created by the
scanning geometry and the detector non-uniformity and temporal noise.
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As detectors improved
only 1-D scanning was required. Such a camera was
shown in
the POVRAY scenes page. Radiometric errors were still present and
appear as the
horizontal streaking.
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The ultimate performance of
modern thermal imaging cameras is generally determined by the MTF of
the complete system and the random noise from the detector.
In a
'Background Limited' camera, the noise is set by the noise in the
photons rate from the scene. Generally the output is observed
by
a
human eye and brain which acts as a matched filter to the image
modulation and the noise. The performance as a device capable of
imaging temperature modulation in the scene is characterized by the
'Minimum Resolveable Temperature Difference' as a function of the
temperature contrast in a 7-bar
chart object in the scene.
In the
following
movies I've modelled a complete thermal camera with
bar charts
in the scene and post-processed to apply noise and MTF. Notice how the
visibility of the bar charts increases with the moving image. These
were
used to predict the performance of the real camera.
Excellent
agreement with the real camera was obtained.
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The ability to visualise stray
light is one of POVRAY's greatest features. Stray light is
critical in the assessment of infrared lenses. It manifests
itself in two ways. Firstly as false images created by bright
objects in the scene, and secondly by the radiation from the lenses and
metalwork reaching the focal surface by either/both
refraction/reflection/diffraction in and from the lenses and
metalwork. This latter is known as the Narcissus
effect.
Generally a cold spot appears in the centre of the image.
This is
removed by 'Non-Uniformity correction' algorithms, but if the scene or
camera temperature changes the correction balance is
disturbed.
Bear in mind that a thermal camera is required to detect contrasts
typically less than 0.5% and is sensitive to contrast of a few
hundredths of a Kelvin at a distance of several kilometres - even
miniscule changes in temperature of
the scene or the camera can become visible as image defects. These defects
are well known
but difficult to model, although there are many programs ( I've used
Nathan Kopp's POVRAY version in Ken Colefax's 'cities' model
)
for the cosmetic cases when 'cheating' is allowed.
Cheating
at Straylight 2.6MB
But for
proper scientific analysis, photon mapping is essential.
This is an
example. I used the awesome routines
created by Jaime
Vives Piqueres at
ignorancia.org/latest.php
to create the 'optics lab' from my desk position in the office to
contain the optical bench and electronics.
It
shows an 8-element F/1 lens with a 150degree field of view on
the
optical bench.
The lens has three
aspherical surfaces and one diffractive. The lens surfaces have angle
dependent reflectivity according to the designs of the anti-reflection
coatings. The cryogenically cooled
focal plane is shown glowing 'blue'. The interior of the lens is shown
glowing 'green'.
1.3MB
This following movie is a
photon-mapping rendering of the image formed by the lens at the
diagonal of a square focal plane as an object moves through the
150degree field. This again confirms the validity of the
photon
mapping algorithm implementation algorithm in
POVRAY. And
notice that the image is not a collection of 'spots' in the spotty
diagrams typical of traditional ray-tracing programs - it has the
appearance of a real image i.e of a limitless number of spots.
Image
quality
0.4MB
By
adjusting the
'assumed gamma' setting and using a heightfield tga image it
was
possible to create a dynamic range of 10^7. I then read the
tga
files with the 'jet' palette to cover that range. Notice how
the
very wide dynamic range has increased the visibility of the aberrations
increasing with field angle.
6.1MB
There
was
little difference between these images and those calculated in the
top-of-the-range optics modeling program ASAP.
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This shows a ZEMAX
raytrace
of the causes of the external stray light images. A point light source
to the left is collimated by a lens. The test lens rotates around the
entrance pupil created by a crygenic stop just prior to the focal
plane. As the lens rotates a myriad of unintended paths are
formed by reflections at imperfect 'anti-reflection' coatings and total
internal reflection inside the lens elements.
1.1MB
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This is a model of one section
of a
submarine periscope known to have a weak but still noticeable
straylight problem. The layout includes a number of lenses
and
prisms. We believed that the straylight was caused by a
Double
Dove prism used to steer the sightline in elevation.
In the
first
movie, a screen plane moves
through the top components to arrive at the first focused
intermediate image surface. There are five 'suns' in the
field
As the suns move
across
the field of view the effects of light following unanticipated paths
creates multiple images of the suns.
Suns
move in
azimuth
1.0MB
and as the suns
move in elevation and the steering prism rotates, more straylight
images appear.
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Glint from lenses
can also be
important. In this scene a lens is illuminated by an axial
source
and light is reflected from the lenses.
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