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


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.

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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.
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.

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.

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
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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'.

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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
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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.

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There was little difference between these images and those calculated in the top-of-the-range optics modeling program ASAP.

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.


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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

Suns
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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
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and as the suns move in elevation and the steering prism rotates, more straylight images appear.
                                             

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.

Glint
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©Don Barron