Patent Publication Number: US-4650321-A

Title: Spatial/spectral real time imaging

Description:
TECHNICAL FIELD 
     This invention relates to optical surveillance. More specifically, the invention pertains to surveillance of a two dimensional scene to simultaneously determine, in real time, the identification of objects in a field and the wavelength of monochromatic radiation which may be emitted or reflected from the objects from nearby sources. 
     BACKGROUND ART 
     The closest known instrument to which this invention pertains is the slitless spectrometer. A typical slitless spectrometer consists of an astronomical, two mirror telescope with a plane grating placed between the secondary mirror and the photographic plate. When a star field is photographed, the images on the plate are the spectrum of each star placed at the position of the star. In this manner information is obtained on both the spatial location and the wavelength of many sources in the field simultaneously. 
     The detector array of a conventional imaging spectrometer in the diffracted order of the grating must provide M×N×W pixels (picture elements) where M×N is the number of spatial resolution elements in the two dimensional scene and W is the number of spectral resolution elements. For a system to provide a two dimensional field of view with both modest spatial resolution and wavelength resolution, this product, M×N×W, becomes prohibitively large. 
     DISCLOSURE OF INVENTION 
     This invention comprises a telescope system, which includes a diffracting component, capable of imaging a strip field of view onto an array of radiation detectors. Rather than using one large (M×N×W) array in the diffracted order, two smaller arrays are employed. One array, the spatial array, of size (M×N), is positioned at the image plane of the zero order (undiffracted) beam from the diffracting component. This array receives spatial information for the identification of the objects in the field of view. This array also displays the spatial locations of focused radiation incident on the objects from outside of the field of view. These spatial locations are independent of the wavelength of the source. A second array, the spectral array, of size (M×(N+W)), is positioned at the image plane of the first order diffracted beam, at the central wavelength of interest, which is spatially separated from the zero order image plane. This array provides information on the location of the same incident radiation as seen in the spatial array but the location of the sensed radiation in the spectral array is wavelength dependent. If the sources are monochromatic and unresolved, only (M×(2N+W) pixels are required rather than the (M×N×W) necessary to obtain the same information from a conventional imaging spectrometer. This substantial decrease in the required number of pixels can greatly increase the achievable resolution in spatial or wavelength information using current detector technology, which is inherently pixel number limited. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a telescope system embodying the present invention; 
     FIG. 2 is an illustration of the output of the spatial array of the invention; and 
     FIG. 3 is a diagram of the output of the spectral array of the invention, the spatial array being shown superimposed thereon. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Under certain circumstances, it might be desirable to be able to optically view a field and determine not only the spatial location and type of objects within the field but the wavelength of radiation incident on them from unknown sources. It might also be highly desirable to perform this function in real time. One application for such a function could be military wherein a battlefield is the field of view. If objects such as tanks within such a field were being irradiated by monochromatic laser light, it might be desirable to quickly determine the wavelength of the laser radiation in order to invoke suitable countermeasures. 
     With particular reference to FIG. 1, there is illustrated a Mersenne telescope arrangement positioned to receive the rays R from a field of view. It comprises a primary mirror 10 and secondary mirror 12, together with the usual field stop and Lyot stop (not shown). The radiation from the secondary mirror 12 is directed to a concave reflective optical grating 14. Two rectangular arrays of radiation detectors are positioned to receive the radiation reflected and diffracted from the grating 14. These may be, for example, arrays of charge-coupled devices (CCDs). 
     A spatial array 16 is positioned to receive the zero order (undiffracted) radiation reflected from grating 14. This would nominally be broadband. This array might be, for example, 500 (M) pixels wide and 50 (N) pixels high. A strip field (i.e., M≠N) is necessary to achieve the appropriate image quality. 
     A spectral array 18 of similar detectors is positioned to receive radiation diffracted in first order at the central wavelength of interest from the grating 14. As will be understood by those skilled in the art, the exact position of a source in the field imaged by the grating 14 is wavelength dependent. In a representative embodiment, the spectral array 18 may be 500 (M) pixels wide and 100 (N+W) pixels high. 
     The output signals from arrays 16 and 18 may be processed, displayed, or stored in a number of ways. FIG. 1 illustrates the output from the spatial array 16 being supplied to a cathode ray tube (CRT) 20 and the output of the spectral array 18 being supplied to the input of a CRT 22. However, the spectral information from the spectral array 18 may be combined with the spatial information, for example, by means of a signal processor 24 which may be utilized to combine the outputs into a single display. In such a case, one cathode ray tube, such as CRT 22 may be eliminated. 
     FIGS. 2 and 3 illustrate, in a simplified form, the manner in which the apparatus of the invention functions. FIG. 2 illustrates a 50×25 pixel portion of the spatial array 16. In this figure five different simultaneous detections of focused radiation on objects in the field of view are shown, labeled S 1  -S 5 , respectively. By &#34;unresolved&#34; focused irradiation it is meant that each source subtends only one pixel. In terms of a pixel numbered grid, the coordinates of S 1 , for example, would be 3, 16 and those of S 3  would be 24, 12. These locations are independent of the wavelength of the irradiating sources. 
     FIG. 3 illustrates a corresponding 50×50 pixel portion of the spectral array 18. On this array the pixel numbers at which the focused radiation is detected are wavelength dependent, as the radiation is diffracted before reaching the array. The difference between the detected location of a source in the spectral array and the detected location in the spatial array (shown as dashed in FIG. 3 for reference) provides the wavelength information. If each pixel corresponds to one (1%) percent of the wavelength λ c , then the wavelength of the unknown sources can be readily obtained from the following calculations: 
     
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                   Row  Column                                            
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S.sub.1 (λ.sub.1)                                                  
                     36      3                                            
S.sub.1 (λ.sub.c)                                                  
                     28.5    3                                            
λ.sub.1 = λ.sub.c + (36-28.5) × 1% λ.sub.c     
S.sub.2 (λ.sub.2)                                                  
                     18     15                                            
S.sub.2 (λ.sub.c)                                                  
                     22.5   15                                            
λ.sub.2 = λ.sub.c + (18-22.5) × 1% λ.sub.c     
S.sub.5 (λ.sub.5)                                                  
                     28     42                                            
S.sub.5 (λ.sub.c)                                                  
                     30.5   42                                            
λ.sub.5 = λ.sub.c + (28-30.5) × 1% λ.sub.c     
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     It will be understood that the foregoing description in connection with FIG. 3 is primarily for purposes of illustration. Although the undiffracted images and first order &#34;hits&#34; could be simultaneously displayed upon the same screen, many other methods of display and signal treatment could be employed. For example, the spatial image from array 16 could be pictorially displayed while the spectral information could be rapidly computed and printed out alongside each irradiated object. 
     As an example of an instrument which might be constructed in accordance with the invention, one might assume an aperture diameter of the primary mirror 10 of 600 millimeters and a field of view of 2.0×0.2 degrees. The spectral resolution might be one (1%) percent λ c  and the spatial resolution could be 0.1 milliradian. The pixel size could be 0.25×0.25 millimeter and the grating could have a bandpass of one octave. The array sizes could be 500 by 50 pixels for the zero order array and 500 by 100 pixels for the first order array. Note that, while this instrument would contain 75000 pixels, the equivalent imaging spectrometer would require 1,250,000 pixels to obtain the same information. 
     It is believed that the many advantages of this invention will now be apparent to those skilled in the art. It will also be apparent that a number of variations and modifications may be made therein without departing from its spirit and scope. Accordingly, the foregoing description is to be construed as illustrative only, rather than limiting. This invention is limited only by the scope of the following claims.