Scientific Research Institute for Comprehensive Testing of Optoelectronic Devices and Systems, Sosnovy Bor,Leningrad Oblast, Russia�Submitted November 18, 2009�Opticheski� Zhurnal 77, 67–72 �March 2010�
One of the main development trends of modern airbornesystems for remote probing of the earth’s surface is the cre-ation of multispectral complexes that provide digital imagesin several spectral ranges simultaneously, including the vis-ible and the IR regions.1–7 Modern optical and electronic setsof components make it possible to create multispectral opto-electronic devices with significantly reduced mass and sizewhile maintaining high technical characteristics. It was re-ported earlier6 that a compact airborne optoelectronic scan-ning device had been developed that was capable of simul-taneously recording two images of the same field ofobservation in the visible ��=0.4–0.9 �m� and the IR ��=8–12.5 �m� regions. This device was able to reduce themass and size by about an order of magnitude by comparisonwith the earlier Vesuvius-EC multispectral optoelectroniccomplex, with the other technical characteristics beingcomparable.1,6
This paper is devoted to the further development of acompact multispectral scanning device intended for round-the-clock monitoring of the earth’s surface by obtaining digi-tal images in the wavelength interval from 0.3 to 12.5 �m.The number of working spectral ranges has been increasedand can reach six if the appropriate photodetector devices�PDDs� are incorporated. Two versions of the device are con-templated, one with a single optical window and one withtwo windows to separately detect solar radiation �UV, vis-ible, and near-IR ranges� and thermal radiation �the mid- andfar-IR ranges�. The advantages and disadvantages of the twoversions are discussed, along with the possibility of improv-ing the characteristics achieved in the thermal channel. Wepresent a calculation of the parameters of the detector chan-nels when the device is equipped with specific PDDs that usetwo optical windows. The results of terrestrial and airborneexperiments are presented, illustrated by images obtained bythe devices.
In the version of the device with a single optical window,the input radiation flux arrives at a two-sided scanning mir-ror, after which it is divided into two parts �Fig. 1�. The firstpart of the flux is directed to a parabolic mirror objective,whose focal plane contains a one-color ��=8–12.5 �m� ortwo-color ��=3–5.5 and 8–12.5 �m� linear array for the IRrange. The second part of the flux, by means of a system offixed mirrors that includes a two-sided mirror, passes through
207 J. Opt. Technol. 77 �3�, March 2010 1070-9762/2010/03
the corresponding filters to lens objectives of the same typein the visible and near-IR regions. As a rule, the objective forthe visible range is equipped with a “three-color” PDD,which is based on linear CCD arrays and allows operation inan isopanchromatic regime. The objective for the near-IRregion is equipped with a “one-color” �black-and-white� lin-ear CCD array based on CdHgTe. The option is provided ofreplacing the channel for the near-IR region with a UV chan-nel. This replacement is most easily done by using a narrow-band filter �for example, one made from UFS-2 glass� thatreflects radiation with wavelengths greater than 400 nm anda PDD based on CCD structures with increased sensitivity inthe UV region.
An advantage of this version is the pixel-by-pixel regis-tration of the images recorded in different wavelengthranges. This simplifies their joint processing, which is di-rected to extracting additional information concerning theobjects under be observation.6 The recording of images ofthe same field of observation in different sections of the op-tical spectrum is synchronized with the rotation of the two-sided scanning mirror and is carried out at the same time.The physical pixel-by-pixel registration of the images re-corded simultaneously in the IR and visible regions ensuresthat the choice of the parameters of the objectives and PDDsbeing used is consistent. The need to use linear CCD arrayswith a high polling frequency can be a definite disadvantageif the number of photosensitive elements �PSEs� in the linearCCD array exceeds the corresponding number of PSEs in theIR detector by more than an order of magnitude. The sensi-tivity of the short-wavelength channels accordingly de-creases in this case because the accumulation time is re-duced. Another drawback is that the sensitivity of thethermal channel is reduced because of the decreased effec-tive size of the entrance pupil �Fig. 1�.
The second version involves using a device of virtuallythe same size and mass, but with individual optical windowsfor solar radiation �visible, near-IR, or UV ranges� and ther-mal radiation �far-IR or mid- and far-IR ranges� �Fig. 2�. Thethermal channel, as in the first version, uses a two-sidedscanning mirror, from which the arriving flux is directed tothe IR objective. The solar channel uses fixed mirrors, in-cluding a two-sided mirror for separating the arriving lightflux, filters, and lens objectives with the appropriate PDDs.
The thermal and solar channels operate independently ofeach other in this version. There is no limitation from usinglinear arrays based on CCD structures with high polling fre-quency, which reduce the sensitivity by reducing the accu-mulation time. It becomes possible to increase the sensitivityof the thermal channel by increasing the effective size of theentrance pupil.
Table I illustrates an example of the matched choice ofthe characteristics and calculated parameters of the detectorchannels of a device �version 2, with two separate windows�when it is equipped with specific PDDs. It is assumed that allthe detector channels form images of the same field of ob-servation. The IR channel was the foremost one when theparameters were calculated. As can be seen from the opticallayout of the device shown in Fig. 2, mechanical scanningwith the two-sided mirror �in a direction perpendicular to thetrajectory of the camera carrier� is used only for forming the
F
FD
1
2
3
4
5
6
7
8
9
10
FIG. 1. Optical layout of a device with a single optical window.1—scanning mirror, 2—mirror objective, 3—IR photodetector device,4—two-sided mirror, 5 and 6—filters, 7 and 8—lens objectives, 9—lineararray of visible-range CCDs, 10—linear array of near-IR CCDs, FD is theflight direction, and F is the single input flux.
F1 F2
FD
1
2
3
45 6
7 8
9 10
11 12
FIG. 2. Optical layout of a device with separate optical windows.1—scanning mirror, 2—mirror objective, 3—IR photodetector device,4—two-sided mirror, 5 and 6—flat mirrors, 7 and 8—filters, 9 and 10—lensobjectives, 11—linear array of CCDs for the visible region, 12—linear arrayof CCDs for the near-IR region. FD is the flight direction, F1 is the inputflux of the IR region, and F2 is the input flux of the visible and near-IRregions.
208 J. Opt. Technol. 77 �3�, March 2010
IR images. Linear CCD arrays �in which the number of PSEsis usually one or two orders of magnitude larger than in aCdHgTe-based IR linear array� are used to form the imagesin the short-wavelength region, and these arrays lie in thefocal plane perpendicular to the flight direction. Electronicline-by-line scanning is accordingly used, unlike the me-chanical scanning in the IR channel. Table I covers the fol-lowing linear photodetector arrays: a CdHgTe-based IR PDDdeveloped at NPO Orion �spectral range �=8–12.5 �m, for-mat 1�24 elements, element size 35�35 �m, detectivityD*=5�1010 cm Hz1/2 W−1�, an ILX535 three-color linearCCD array made by Sony Corp. �spectral ranges �=0.4–0.5 �m, 0.5–0.6 �m, and 0.6–0.8 �m, format 3�5348 elements, element size 7�7 �m�, and anMPL6144H black-and-white linear CCD array made by theMikron Factory �spectral range �=0.35–1.1 �m, format 1�6144 elements, element size 7�7 �m�. The specified in-stantaneous field of view �angular resolution� is 0.4 mrad forthe long-wavelength IR channel and 0.2 mrad for the visibleand near-IR channels.
A definite step in developing the device was that, insteadof mirror optics, the thermal channel used a lens objectivewith a large linear field, designed for a CdHgTe-based PDDwith format 4�288. Estimates show that, because this in-creased the accumulation time by more than an order ofmagnitude, it made it possible to reach a threshold sensitivityof 0.05 K or less. The probability of detecting small andlow-contrast objects in the IR region is accordingly in-creased. A lens-based IR objective is naturally heavier andmore expensive to fabricate than its mirror counterpart. Thelabor associated with fabricating the aspheric surfaces of thelens elements and the complexity of adjusting the objectiveare increased, and the mass of the device is 1.5–2.0 kggreater.
Both versions of the device with mirror optics in the IRchannel are currently being fabricated in the form of testsamples of the Etna-M1 �with a single optical window� andthe Etna-M2 �with two separate windows for solar and ther-mal radiations�. Photographs of these devices are shown inFig. 3, while the technical characteristics are given in TableII. Structurally, the device consists of an optomechanical unitand an electronic matching device �the controller�, with anon-board computer and a hardware-software complex. Theoptomechanical unit includes a two-sided scanning mirrorwith an electric drive, a scanning-rate sensor, and a sensor ofthe beginning of a row; an IR objective based on an off-axisparabolic metallic mirror with a linear PDD array, integratedwith a micro-Stirling cooling system; a system of splittingand rotating mirrors, and similar lens objectives, with filtersand PDDs based on linear CCD arrays. The electronicmatching device contains a multiple switch, an analog-to-digital converter, and a controller of the universal serial bus�USB�. A brief description of the hardware-software complexis given in Ref. 6.
In the Etna-M1 device, the focal length and the effectivediameter of the parabolic mirror are 100 mm. The circle ofconfusion within the linear field does not exceed 30 �m. Theobjective is equipped with a CdHgTe-based PDD with the
208N. I. Pavlov and G. I. Yasinski
Note: PSE is a photosensitive element, and W and H are the flight spe
209 J. Opt. Technol. 77 �3�, March 2010
following characteristics: number of PSEs in the linear array16, size of the PSEs 50�50 �m, D*=4�1010 cm Hz1/2 W−1. The PDD, integrated with a miniaturegas-cryogenic �micro-Stirling� cooling system and also in-cluding electronic control units and pre-amps with voltagestabilizers, weighs 1 kg. The lens objective has a focal lengthof 28 mm with a relative aperture of 1:2 and a resolvingpower within the limits of the linear field of at least100 mm−1. The objective is equipped with a black-and-whiteILX751B linear CCD array made by Sony Corp. with thefollowing characteristics: number of PSEs 2048, PSE size14�14�, and maximum polling frequency 5 MHz. Thespectral sensitivity of the linear array allows it to be usedboth for recording radiation in the wavelength range0.4–0.8 �m and for UV radiation in the spectral intervalfrom 300 to 400 nm by including a narrow-band filter thatcuts out the long-wavelength radiation.
An experimental sample of the Etna-M1 device has un-dergone flight testing on board the AN-30 airplane and theMi-8 helicopter and was also used in terrestrial experiments,placed on a single-coordinate rotating platform. Figure 4shows images of segments of the earth’s surface, obtainedwhile the device was being tested on board the AN-30 air-plane in the IR ��=8–12.5 �m� and visible ��=0.4–0.8 �m� spectral regions. Figure 5 shows images ob-tained by the Etna-M1 device in the IR ��=8–12.5 �m� andUV ��=0.3–0.4 �m� regions during terrestrial imaging ofairstrip-servicing objects located within direct visibility. Thepictures were taken in the summer at noon after a severe
ls of device equipped with specific photodetectors �the Etna-2M�.
ed and height of the carrier of the observation camera.
TABLE I. Initial data and calculated parameters of the detector channe
ParametersIR pho
�NPO
Spectral range, �, �m 0Size of PSE along a row and along a frame, ar�af, �m 35Number of PSEs along a row and along a frame, mr�mf 1Maximum polling frequency of a PSE, Fmax, MHzNumber of spectral channelsAngular resolution, �0, mradCapture angle along a frame, � f =mf�0, mradNumber of rows in a frame, Zf =� f /�0
Focal length of objective, mmWorking field of view, �0, rad �deg� � /Number of working pixels in the field of view,Z0=�0 /�0
2
Relative flight speed of carrierW /H, sec−1
Scanning rate of two-sided mirror,�= �W /H� / �2mf�0��103, sec−1
Mirror-scanning period, T=1 /�, msTime to form a row, tr=T /Zf, ms
Exposure time of pixel, te=T / �� /�0�
tr /Z0
, �s
Polling frequency of PSE, Fpoll=1 / te, MHz
(a)
(b)
FIG. 3. External view of optomechanical units with separate optical win-dows �a�, and with a single window �b�.
209N. I. Pavlov and G. I. Yasinski
TABLE II. Characteristics of the Etna-M1 and Etna-M2 multispectral scanning devices.
Characteristics
Device
Etna-M1 Etna-M2
Spectral sensitivity ranges, �m 0.4–0.8 or0.3–0.4
0.8–12.5 0.4–0.50.5–0.60.6–0.8
0.8–1.0 or0.3–0.4
0.8–12.5
Viewing angle, deg to 120 60
Instantaneous angle of view, mrad 0.5 0.2 0.4Radiometric digital resolution,
bit/element12 12
Detectable temperature difference, K � 0.15 � 0.10
Interface USB 2.0 USB 2.0
Information data:digital card, formatdata-recording medium, type
BMPHDD, CD
BMPHDD, CD
Application software
Flight height of carrier:minimum, mmaximum �without IR port�, m
503000
503000
Size of optomechanical unit, cm 35�Ø20 38�Ø20Mass of optomechanical unit, kg 8.5 9.5Required power for the dc circuit
�voltage 27 V�, W30.0 50.0
Note: USB is a universal serial bus, HDD is a computer hard disk, and CD is a compact laser disk.
(а) (b)
FIG. 4. Segments of images in the IR �a� and visible �b� regions, obtained with a Etna-M1 compact scanning device from an altitude of 1500 m..
210 210J. Opt. Technol. 77 �3�, March 2010 N. I. Pavlov and G. I. Yasinski
storm accompanied by a torrential downpour. The distance tothe objects was from 10 to 400 m.
These terrestrial and flight tests with test objects con-firmed that spatial-resolution and temperature-sensitivitycharacteristics have been incorporated in the development ofthe device. Two test samples of compact multispectral scan-ners suitable for installation on aircraft have been fabricated�the Etna-M1 and Etna-M2, whose characteristics are givenin Table II�.
The authors are grateful to the specialists of the section“Engineering problems of stability and conversion” of theRussian Engineering Academy, St. Petersburg, who partici-pated in the airborne and ground testing of the device: A. V.Markov, V. F. Mochalov, and L. I. Chapurski�.
FIG. 5. Segments of images obtained in the IR �a� and the UV �b� regi
211 J. Opt. Technol. 77 �3�, March 2010
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(а)
(b)
y ground-based photography using a one-coordinate rotating platform.
