Image retention on topical thermograph
FIELD OF THE INVENTION 
     Stock material or miscellaneous articles, Liquid Crystal. 
     PRIOR ART 
     Furgason et al U.S. Pat. Nos. 3,114,836; Schranz et al 3,993,809; and Nagy et al 4,025,688. 
     BACKGROUND OF THE INVENTION 
     Topical thermography, wherein micro-encapsulated liquid crystals are used for medical diagnostic purposes, has been severely inhibited because of poor persistence of the thermal image created in the liquid crystals. When a conventional liquid crystal sheet is applied to a body surface, the display of color resulting from the variations in temperature along the body surface must be observed or photographed promptly then and there, because once the sheet is removed from the body surface, the heat loss from the liquid crystals causes the colors to change and produce a false reading. Not only does this impose undesired requirements for camera set-ups and for undue haste where deliberation may be called for, but it makes it virtually impossible to use topical thermography for observing temperature variations along surfaces of body cavities. 
     OBJECTS 
     The object of this invention is to provide means for retaining thermal images in liquid crystal sheets so that the color variations produced therein may be observed for a significant length of time after the sheet is removed from the body surface whose temperature is being measured. 
     A further object is to intensify the thermal image on a liquid crystal sheet so that the contrast in color resulting from thermal differences between adjacent areas will be more vivid and, therefore, more readable. 
    
    
     These and other objects will be apparent from the following specification and drawing, in which 
     FIG. 1 is an edge view of a topical thermographic assembly; 
     FIG. 2 is an exploded view of the assembly shown in FIG. 1; 
     FIG. 3 is an edge view of a modified form of the topical thermographic assembly; 
     FIG. 4 is an exploded view of the assembly shown in FIG. 3; and 
     FIG. 5 is an elevation, broken away to show the assembly in cross section of the FIG. 4 assembly which has been bent to cylindrical form for insertion into a body cavity. 
    
    
     Referring now to the drawings, in which like reference numerals denote similar elements, the topical thermographic assembly 2 shown in FIGS. 1 and 2 is composed of a thermographic film sheet 4 containing or having thereon temperature-sensitive liquid crystals. Such sheets are well known in the art, and they are usually made of a relatively thin plastic substrate, to obtain the maximum thermal conductivity between the liquid crystals and the body surface against which they are pressed. In conjunction with the thermographic film 4, there is provided a storage screen 6 which consists of a flexible sheet 8 of low thermal conductivity which may be, for example, a sheet of acrylic ploymer or other substance of low thermal conductivity. Other examples of storage media include polystyrene, polycarbonate, polyethylene, cellulose acetate and, if flexibility is not important, glass. The sheet 8, being flat, is adapted to have one side and surface engagement with the sheet 4. On the other side of sheet 8 is a mirror 10. This may simply be a reflective surface on sheet 8 or a sheet having a mirrored surface veneered to the flat side of sheet 8 opposite the side which engages against the thermographic film 4. This matrix 12 is presented to a body surface and exposed. The thermal image then persists because the mirror 10 reflects the thermal image back into the liquid crystals 1 and prevents radiation loss. A mirrored acrylic sheet 1/8 inch thick, for example, will retain a usable image for at least 30 seconds at room temperature. 
     A thinner storage screen 6a is shown in FIGS. 3, 4 and 5. These, together with the thermographic film sheet 4 from the topical thermographic assembly 12a. The storage screen 6a differs from screen 6 in that a second mirror 14, which is semi-transparent, semi-reflective, is interposed between the sheet 8 of low thermal conductivity and the thermographic film sheet 4. The two mirrors 10 and 14 isolate the thermal image between them and inhibit thermal radiation in either direction from sheet 8. The use of two mirrors remarkably increases image persistence and allows a reduction in the thickness of the storage medium. A two-mirror matrix as described with the combined thickness of only 1/32 of an inch will retain a thermal image for at least one minute at room temperature. 
     The mirror or mirrors in the storage screen serve a dual function. They prevent radiation loss, and they retroreflect the thermal image, which inhibits lateral degradation and subsequent loss of image resolution. 
     The storage screen may be fabricated in many shapes. A tubular storage screen is illustrated in FIG. 5. This is the assembly 12a shown in FIGS. 3 and 4. Such a screen is useful for introduction into body cavities. A flexible storage screen may be curved for introduction into the cavity and then flattened for easy photography upon removal. The liquid crystal sheet may be permanently adhered to the storage screen, or it may be removable; the storage matrix can then be contacted with the body separately and then rejoined with the liquid crystal sheet for viewing. Detachability of the elements of the storage screen allow for easy cleaning. 
     The storage screen can also be used to increase image contrast. Higher body temperatures expose the storage screen more rapidly. Variation of exposure time will therefore determine what thermal details are stored. Areas of elevated temperature can then be displayed more prominently and distinctly. Such increased contrast often greatly aids diagnosis.