Abstract:
The present invention provides within a ship, a passive electro-optic  dev for providing a measurement of the range, together with a positively identifiable image of an approaching ship or object day or night on a CRT, with the range displayed digitally on the CRT. The rangefinder capability provides a substitute or supplement to radar or active-gated laser beam rangefinders for collision avoidance and station-keeping.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to rangefinder systems and more particularly to passive optical rangefinder systems. 
     Frequently, both in commercial and in military situations, it is necessary to determine both the identity and location of an unknown object. The most common method to determine an object&#39;s location has been to use radar. The advantage of radar systems is that various other parameters of the object can be determined with great accuracy, such as, its speed and direction. The primary disadvantage, however, with such systems is that they have poor resolution rendering identification of the unknown object virtually impossible. Therefore, objects which can be located very closely with regard to position and speed normally cannot be otherwise identified since they appear to the radar viewer as no more than a blip or several blips on the face of a cathode ray tube. However, some advances have been made in radar technology. Complex and expensive monopulse radar systems have been developed which depend on extremely high frequency signals and intricate processing equipment to reconstruct radar images of the object to be identified. 
     Active optical rangefinding systems on the other hand make it quite easy to identify unknown objects but require great skill to obtain correct range information. In addition, expensive mechanical devices are needed to move the optics with precision sufficient to determine range. 
     Other active optical rangefinder systems have been developed which make use of gated laser beams, such as gated gallium arsenide lasers, in combination with a gated low level light television camera to provide simultaneous range and optical image information. However, these systems require liquid nitrogen cooling to 77° K with refills of liquid nitrogen about every 10  hours which is expensive and inconvenient to operate. In addition, these systems are somewhat less effective during the day when bright background skies limit the sensitivity and therefore the range of the system. 
     SUMMARY OF THE INVENTION 
     The present system overcomes the disadvantages and limitations of the prior art by providing a passive optical rangefinder system. The system makes use of optics similar to a standard optical rangefinder configuration, but fixes both mirrors or prisms on infinity. This creates a double image of the unknown object which is detected by a low level light television (LLLTV) camera. The signal from the LLLTV camera is processed such that only the vertical portions of the image are left. The electronic signal is then time scanned horizontally to determine the spacing of the images, their spacing being inversely proportional to their distance from the rangefinder. 
     It is therefore the object of the present invention to provide an improved rangefinder. 
     It is also an object of the present invention to provide a rangefinder which supplies range and image information about an unknown object. 
     Another object of the invention is to provide a rangefinder which supplies image information of an unknown object along with its range and which is inexpensive and convenient to operate. 
     Another object of the present invention is to provide a rangefinder which supplies image as well as range information of an unknown object in an efficient manner during both day and night conditions of luminance. 
     Another object of the present invention is to provide a rangefinder which supplies both image and range information to a remote location. 
     Yet another object of the present invention is to provide a passive electro-optical device for supplying information about the range and image of a distance object to a remote display. 
     Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 discloses the optical portion of one embodiment of the present invention. 
     FIG. 2 discloses the electronic time scanner portion of one embodiment of the present invention. 
     FIG. 3 illustrates typical information produced on monitor A. 
     FIG. 4 illustrates typical information produced on monitor B. 
     FIG. 5 discloses an alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the optical portion of the preferred embodiment for supplying range and image information of an unknown object 10 located at a distance R from the rangefinder. The image of object 10 is reflected by reflective surfaces 12 and 14 which are fixed at an infinite range and located at a distance X from each other. Light from the object 10 thus forms an angle θ from the object 10 to each of the reflective surfaces 12 and 14. Right angle prism 16 reflects light from reflective surfaces 12 and 14 through imaging lenses 18 and 20 to the LLLTV camera tube 22. The image displacement is indicated by a distance I while the photosurface size of the camera is indicated by a distance H. From a purely geometrical standpoint, it is clear that; ##EQU1## From the optics shown in FIG. 1; ##EQU2## where f = the effective focal length of the objective lens. Thus ##EQU3## and ##EQU4## Since Xf is set for the device, it can be considered a constant, and as the face of camera tube 22 is located past the focal point the range R is thus inversely proportional to the image displacement (distance between dual images of the object) on the LLLTV camera tube 22. The image displacement I can be calculated by the time T it takes LLLTV camera 22 to scan across I. By simple proportions, if it takes 50μ sec to scan across the distance H, then; ##EQU5## and ##EQU6## inserting equation (6) into equation (4); ##EQU7## Thus by knowing T (the time it takes to scan across I) the distance R can be found since x, f, and H are all fixed for the device. 
     FIG. 2 discloses the apparatus for determining T and therefore the range R. The dual image of object 10 formed by the reflective surfaces 12, 14 spaced at a distance X is detected by LLLTV camera 22 to form a video signal representative thereof. That signal is amplified by video amplifier 24 and passed through high pass filter 26. Both the filtered and unfiltered video signal are combined in mixer 28 to enhance the resolution of the display on monitor A30. The filtered signal is amplified in amplifier 32 for display on monitor B34. 
     FIGS. 3 and 4 show displays of monitors A and B, 30, 34 respectively. The unknown object 10 appears on the monitor A30, as a dual image as shown in FIG. 3, one image represented as a solid line and the other as a dotted line. The range information is displayed in digital form at the right side of monitor A as shown in FIG. 3. Monitor B34, as shown in FIG. 4 displays only the data passed through high pass filter 26 so that only the vertical sections 68, 70 of the image remain to better enable the device to measure the image displacement. Turning back to FIG. 2, the manner in which the device performs these functions is disclosed. 
     A window generator 54, shown in FIG. 2, is an operator controlled device for selecting a window 66 on the display of monitor 34 as shown in FIG. 4. This window 66 precludes all but the two best vertical edges 68, 70 for timing measurement. The window information is coupled to the J-K flip flop 36 for the generation of a reset signal. The reset pulse from the window generator enables the J-K flip flop 36 only for scans across monitor B which fall within the window area 66. Upon receipt of the first input image edge 68 from amplifier 56, the J-K flip flop 36 changes state and activates the timing counter 44 via output Q 38 from the J-K flip flop and enable input 42 of the counter 44. The counter then accumulates counts from clock oscillator 46. Upon receipt of the second image edge 70 from amplifier 56 the J-K flip flop 36 is reset and the counter 44 is disabled. The accumulated count is transferred to a memory and time to range translator 48 which is enabled by output 40 of J-K flip flop 36. Translator 48 in turn drives the range readout 50 and auxiliary readouts 52. The range data is fed back to monitor A for display via mixer 28. 
     FIG. 5 discloses an alternative embodiment of the present invention wherein a LLLTV 60 is used in conjunction with a standard optical rangefinder 58. A time scanning mechanism 62 such as the one shown in FIG. 2 is connected to the LLLTV 60 for determining the distance between images and therefore the range. The electronic time scanning and readout system can be calibrated to known distances for any particular setting of the standard optical range finder 58. 
     The present invention therefore provides positively identifiable images of nearby ships or objects along with range information about the object on a CRT monitor with the ship. The system is simple, inexpensive, and permits more effective range measurements during daylight than active optical laser rangefinders. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example, any suitable camera tube, LLLTV or standard day camera, including an image converter tube (i.e., a tube having a far infrared or ultra-violet image input), can be used with the device if properly sensitive to the light from the optics of the device. In addition, reflective or refractive optics as well as prisms for mirrors or vice versa can be used in the optical portion of the device. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.