TELEVISION SYSTEM WITH IMPULSE LASER LIGHTING
The television system with impulse laser lighting is a new TV system enabling the operation in hard meteorological conditions.
The most promising fields of application are:
Fig. 1 presents the view of the prototype of the system (TV monitor with control unit and the impulse laser lighting and receiver unit).
Figs. 2-8 present the fragments of pictures demonstrating system operation under various conditions. 6 laser of LPI 114 type with average power of 200 W were used as light source (impulse duration of t = 30 ns, impulse repetition frequency of F = 3 kHz). Opto-electronic transducer “STEK” with light amplifier was used as receiver.
Videotape presents the operation principle and pictures taken from the monitor of the system. This is the result of direct record without any additional computer treatment.
The operation principle is based on the impulse principle suggested in 1936 by academician A.A. Lebedev. This method is based on the illumination of the object by short light impulses. The duration of impulses is much shorter than the time necessary for light propagation to and from the object. An optical instrument equipped with fast shutter synchronized with light source records the light. The shutter of the instrument opens for a certain time interval in the instant, when light returns from a selected object. In this case, it is possible to see only the selected object and not the background, parasitic light, etc.
The spatial (distance) resolution of the system is determined by the opening time of the shutter and by the duration of light impulse.
To manufacture this system, it is necessary to fabricate impulse lighting instrument generating rather short light impulses and light receiver with fast shutter. These could be, respectively, powerful semiconductor lasers with electro-beam pumping (SLEP) or injection lasers, which enable uniform lighting of the object with spatial resolution of 1 - 30 m, and fast opto-electronic transducers.
The prototype of television system with impulse laser lighting (TS ILL) includes:
Receiver records the laser beam reflected by a selected object in the instant, when reflected light returns from the object, during the opening time of the receiver. This operation scheme excludes the influence of light scattered in the path from the laser to the object and back. Therefore, it enables the application of the system under hard meteorological conditions (rain, fog, snow, etc.), under water, and under intensive background illumination.
Spatial resolution is determined by the duration of laser impulse, the distance is determined by time delay between the laser impulse and the instant of receiver opening.
In our system, the duration of laser impulse is equal to 5 - 30 ns. This assures the spatial resolution of 1.5 - 10 m and maximal distance of 3 - 5 km.
· Maximum distance, adjustable, m ~800
Spatial resolution, m 20 - 350
Angular field of vision, degrees 1.5 - 10
Laser wavelength 0.82; 1.1
Laser impulse energy, mJ 1 - 5
Sensitivity of the receiver, lx 10-4
· Resolution of the monitor, TV lines 350
Power consumption, W ~60
Dimensions, m3 0.15
Weight, kg 25
Technical specification of the prototype of TS ILL based on SLEP
· Maximum distance in atmosphere, adjustable, m ~3000 - 5000
· Maximum distance in muddy water, m ~50
Spatial resolution, m 3 - 10
Angular field of vision, degrees 1.5 - 5
Laser wavelength 0.51; 0.82
Laser impulse power, MW 10 - 15
· Resolution of the monitor, TV lines 350
Power consumption, W ~60
Dimensions, m3 0.3
1. The application of TS ILL enables an increase in working distance due to an increase in contrast of the picture observed. This results from the cutting of back-scattered light and the reducing of background (“natural”) light. This last is reduced by a factor equal to reverse duty cycle of the laser.
In usual active TV systems based on OET, the back-scattered light masks the image of the object and reduces the contrast of the picture, especially under hard meteorological conditions. This makes usual systems useless in fume, fog, rain, and snow, when TV ILL can be applied.
The application of SLEP, which is characterized by almost two order of magnitude higher impulse energy compared to injection laser, enables the application of TS ILL even under water.
2. TS ILL enables the visibility of low-contrast objects due to the suppression of background and the visibility of narrow spatial range. Usual passive and active vision systems do not permit the observation of such objects even under daylight conditions. For example, TS ILL enables the observation of snow-made constructions or human in white cloth on snow background. This property of TS ILL makes it very useful for rescue services.
3. The TS ILL enables the precise measurement of the distance to the object by the measurement of time delay between the laser impulse and the instant of OET opening. In the system existing now, the precision is equal to 5 - 10 m, but can be easily improved, if necessary, by one order of magnitude. The precision of distance measurement does not depend on distance, but only on duration of laser impulse and opening time of OET. In contrast to usual laser range finder, the TS ILL excludes the false determination of distance due to the reaction of usual laser range finder to light reflected by non-target objects (trees, wires, etc.). In TS ILL, this light is cut by time delay system.
4. The TS ILL reduces any permanent light source (daylight, headlight of car, fire, etc.). The reduction factor is equal to the reverse duty cycle of the laser lighting. Additional background suppression can be obtained by the application of optical filter transparent at laser emission wavelength. Background suppression at the application of both spectral and temporal selection can reach 105 - 107. This suppression is sufficient to enable normal operation of TS ILL under the background illumination with searchlight (4×106 cd) or under good weather daylight conditions (10 lx). Therefore, the TS ILL can be used under both day and night conditions.
5. The TS ILL can operate in passive, active continuous, or active impulse mode depending on ambient conditions. The so-called “hybrid” mode, when different modes are used for different distances, can also be used. This last possibility is useful for the application in car driving.
The variation in time delay permits us to illuminate only the object or only the background behind the object. The first possibility enables the observation of illuminated object on dark background; the second possibility enables the observation of dark silhouette on illuminated background. This last possibility is important for the observation of “stealth” objects.
From the point of view of energy, the observation of dark silhouette is better because natural background has usually higher reflectance than artificial objects, and therefore, it is possible to use lower lighting intensity at the same distance to the object. However, some important information is lost in this case. In addition, it is complicated to observe the objects using this method if the reflectance of background is low, for example, at the observation on sky background. Therefore, the illumination of the object is more universal, and all data presented in this description were obtained using this operation mode.
6. TS ILL can distinguish objects using light reflected by some elements of these objects (for example, human’s or animal’s eyes, optical or opto-electronic instruments, etc.). This enables good possibility for rescue operations, hunting, navigation and driving, as well as for the remote determination of instrument position (for example, coil mining machines). Wire lines also reflect light, this is important, for example, for helicopter landing under hard meteorological conditions in rescue operations.
7. TS ILL is very efficient for the application under sea conditions. Reflectance of water surface is high; therefore the contract of objects on water surface is low. This complicates the discovery of objects on water surface with searchlight lighting and with usual night vision instruments. The application of space/temporal selection allow high contrasts and disclosure of objects on water surface in the night, in fog, fume, as well as under background lighting conditions.
The TS ILL can be placed in a hermetic box for underwater application. This enables the monitoring of underwater constructions and pipelines, the application for underwater rescue and maintenance operations, for underwater construction, etc. The deepness is determined by the design of hermetic box; the maximum distance to the object in muddy water is about 50 m.
8. The application of the TS ILL enables the remote control of instruments and mechanisms, the remote monitoring of moving object with precision of ~0.1 m. The distance to the mechanism and the picture of the mechanism is depicted by a monitor. The picture can be recorded, computer treated, transmitted by radio, optical, or other signal.
The application of several TS ILL systems allows the collection of information about an accident in one center, to monitor and control the situation.
Recent state of art
The prototypes of the TS ILL for underwater, car, ship application were fabricated and tested under real conditions.
Recently, we continue the upgrade of the system and its elements, first of all, short impulse semiconductor lasers working at wavelength from 0.51 mm (green light, water transparency) to 8 -12 mm (atmosphere transparency window) and impulse OET.
Underwater test showed that the maximal distance increases by a factor of 2-3 compared to usual TV system. In atmosphere, maximal distance exceeds “meteorological” visibility by a factor of 3-5. The contrast of picture increases by one order of magnitude.
The application of semiconductor lasers makes possible the fabrication of vibration and shock prove apparatus.