the present invention is limited to the optical target itself . there is an abundance of optical and electronic technology relating to reflected light impulses . the description provided herein will , therefore , concentrate on target details . however , as an aid to comprehension of how the target may be positioned on a container or other vehicle , one exemplary use will be briefly described and explained . fig1 shows an array of three - dimensional segments or solid prisms mounted on a plane base . some of the prisms are rectangular in cross - section , for example , the prism defined by the points ( 1 , 2 , 3 , 4 , 5 , 7 and other points hidden in the drawing ). the prism comprising points 1 , 2 , 3 , 4 , 5 , 6 , 7 form a &# 34 ; planar segments &# 34 ;. other prisms are quadrilateral in cross - section , for example , the prism defined by the points ( 3 , 4 , 5 , 6 , 8 , 9 , 11 and another point hidden in the drawing ). the prism formed by 3 , 4 , 5 , 6 , 8 , 9 , 11 , 13 , and hidden points 12 and 14 , form a &# 34 ; sloping segment &# 34 ;. the dimensions of the prisms are determined in the following manner : the distances between the point pairs ( 5 , 7 ), ( 5 , 13 ), ( 13 , 19 ), ( 25 , 29 ) etc . are equal but arbitrary , but might in practice range from one - half to one inch in width . the distances between the point pairs ( 1 , 7 ), ( 3 , 5 ), ( 13 , 15 ), ( 29 , 35 ), etc . are also arbitrary , but might in practice range from one inch to one and one half inches . the distances , between point pairs ( 9 , 11 ), ( 23 , 27 ), and ( 31 , 33 ) are preferably one - half those of ( 3 , 5 ), ( 21 , 25 ) etc ., while the distances between point pairs ( 5 , 11 ), ( 25 , 27 ) and ( 29 , 31 ) are one - half those of point pairs ( 5 , 13 ), ( 13 , 19 ), ( 25 , 29 ), etc . there are spaces between some of the prisms . these spaces are outlined by such point groups ( 3 , 4 , 6 , 8 , 9 , 10 , 11 , 13 , 15 ) and another point hidden in the drawing ), etc . the lengths of the prisms , i . e ., the distance between point pairs ( 1 , 2 ), ( 3 , 4 ), ( 6 , 9 ), ( 10 , 15 ) etc . are also arbitrary but equal , and might in practice range from 6 to 12 inches . the prisms may be constructed of any durable material , typically steel . the method of their attachment to the base is arbitrary . the parts of the prisms that are of interest are the outer , lateral , reflective surfaces defined by the planes spanned by point groups ( 1 , 2 , 3 , 4 ), ( 3 , 4 , 6 , 9 ), ( 6 , 8 , 9 , 11 ) etc . the boundary planes of the spaces are , of course defined by the outer , lateral planes of the prisms and the base , for example , the planes spanned by the point group ( 6 , 9 , 8 , 11 ) and ( 8 , 11 , 13 and a point hidden in the drawing ). it is not necessary that these reflective surfaces be defined by prisms . fig2 shows an equivalent set of surfaces defined by a sequence of corrugations of a sheet of material , typically steel . the reflective surfaces defined by the configuration shown in fig2 have the same spatial relation as their equivalently numbered counterparts in fig1 . fig3 shows the method by which the target achieves selective directional reflectivity . the number pairs ( 1 , 3 ) correspond to the plane defined by the number group ( 1 , 2 , 3 , 4 ) in fig1 and 2 . similarly , the number pairs ( 3 , 9 ) correspond the plane defined by the number group ( 3 , 4 , 6 , 9 ) and so forth . the target is illuminated by a light source , 36 . light rays fall on the target at an incidence angle θ , being the angle between the ray and a line , c , perpendicular to the base plane . the angle of incidence is also equal to the angle between the lines defined by the point pairs ( 3 , 9 ) and ( 3 , 5 ). all rays , a , falling on the surface ( 1 , 3 ) will be reflected at an angle θ towards a light sensor , 37 . all rays , b , falling in the space ( 3 , 15 ) will be reflected back toward the source . because the target is three - dimensional and because the space between the reflecting planes are optically black to the sensors , accumulations of dirt , soot , and other contaminants will not confound the operation of the target . rays incident on the upper surfaces , e . g ., that surface defined by points ( 1 , 2 , 3 , 4 ), even if that surface is soot covered , will be reflected specularly towards the sensor with sufficient intensity to be read , especially if the light source is very intense . furthermore , because the light source and light sensor are on the same side of the target there is no problem related to placement of the target on the side or other lateral surface of the vehicle . if the light source , 36 , is a narrow pencil of beams , narrow with respect to the angle subtended by points ( 36 , 1 , 3 ) or ( 36 , 3 , 15 ), and the target is in motion , say from right to left , then the sensor will in time perceive a sequence of alternating reflections and nonreflections of some time duration . if the received reflections are interpreted as &# 34 ; ones &# 34 ; ( 1 ) and the nonreflections as &# 34 ; zeros &# 34 ; ( 0 ), then the target segment shown above could be read as 1 , 0 , 1 , 1 , 0 base two , or equivalently as 22 base ten . the target array can be made as long as desired to accommodate any character length , and check bits may be incorporated as well as start and end sequences . in this hypothetical illumination scheme there may arise the complication of distinguishing between repetitive sequences , such as 1 , 1 , 1 , 1 or 0 , 0 , 0 , 0 etc . in this case the target can comprise two side - by - side arrays , one containing the code , the other a timing sequence of alternating zeros and ones . if the two outputs are read simultaneously and added modulo 2 , a simple logic is available to separate repetitive sequences and determine the content of the code . an alternative illumination method is for the light source 36 to be a flash unit having beam width sufficient to illuminate the entire target with an approximately collimated burst of light ( i . e ., one wherein the individual rays do not substantially depart from the incidence angle θ . if the light flash is triggered by the presence of the rail car , container , etc ., and the target is in a prearranged standard position with respect to the triggering device the sensor will be presented with a still picture of the target . this picture can be scanned electronically by the sensor to determine the coded sequence . other illuminating and sensing systems are extant , but in any event the light falling on the target must be of sufficiently intensity to overcome casual light from other sources . it is theoretically possible that the sensor , 37 , be co - located with the source , 36 . in this case the rays reflected back to the sensor could be interpreted as zeros . this arrangement is not likely to be as feasible , however , since each such ray would undergo four reflections , rather than one in the above illustrated manner , and would thus be subject to greater attenuation . fig4 shows a perspective view of an ocean container , 38 , with the present target 39 , mounted with its principal axis parallel to the earth . it is not necessary that it be thus mounted . it could be mounted perpendicular to the earth if the light source and sensor were similarly arranged . fig5 is a plan view of the ocean container seen from above . the light source 36 and sensor , 37 , are shown in relationship to the target 28 . needless to say the target , 28 , will need to be mounted on both sides of the vehicle . while several embodiments of the invention have been described , it will be understood that it is capable of still further modifications and this application is intended to cover any variations , uses , or adaptations of the invention , following in general the principles of the invention and including such departures from the present disclosure as to come within knowledge or customary practice in the art to which the invention pertains , and as may be applied to the essential features hereinbefore set forth and falling within the scope of the invention or the limits of the appended claims .