Patent Publication Number: US-6339683-B1

Title: Standard measurement scale and markers for defining standard measurement scale

Description:
This application is a division of U.S. patent application Ser. No. 08/964,896, filed Nov. 5, 1997 now U.S. Pat. No. 6,108,497, the contents of which are expressly incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a standard measurement scale and markers for defining a standard measurement scale, which are used in photogrammetric analytical measurement systems. 
     2. Description of the Related Art 
     For example, photogrammetry measurement is carried out at a traffic accident site. Namely, the traffic accident site is photographed by a camera or cameras, and a survey map is established on the basis of pictures photographed by the camera. Before real distances and lengths can be reproduced on the survey map, a standard measurement scale must be recorded in the pictures. 
     Conventionally, the standard measurement scale is defined by at least two cone-shaped markers which are formed of, for example, a suitable plastic material. In particular, for example, at a traffic accident site, two markings are indicated on the ground with chalk, and a distance between the markings is obtained using a measuring tape. Then, the cone-shaped markers are positioned at the respective indications. Thereafter, the traffic accident site is photographed by the camera(s), such that the cone-shaped markers are included in the field of view to be photographed. 
     Before a survey map can be exactly drawn, the cone-shaped markers must be positioned with respect to the indications on the ground, such that an apex of each cone-shaped marker is just above the corresponding indication, because each of the apexes of the cone-shaped markers serves as a reference point for defining a standard measurement scale. 
     Nevertheless, it is difficult and troublesome to align the apex of the marker with the indication, because the indication is lost from sight due to the enlarged bottom of the cone-shaped marker during positioning. Further, when the ground is not horizontal, i.e., when the ground is sloped, the positioning of the cone-shaped marker is further complicate, because a fine positional adjustment of the cone-shaped marker is necessary before the apex of the cone-shaped marker can exactly coincide with the corresponding indication. 
     Also, conventionally, the apex of the cone-shaped marker is painted with a light-color, such as white, yellow or the like, so that the apex is conspicuous when recorded in a photographed picture. Nevertheless, the apex of the marker is not necessarily conspicuosly recorded in the picture, e.g., when the tone of color of the background is similar to the apex color. 
     Furthermore, it is troublesome to obtain a distance between the indications using the measuring tape. Especially, in photogrammetry, in which a reference plane must be defined by at least three reference points, it is necessary to measure the three distances between the three reference points by using the measuring tape. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a standard measurement scale, to be used in a photogrammetric measurement system, having at least three fixed reference points. 
     Another object of the present invention is to provide such a standard measurement scale as mentioned above, which is constituted such that the reference points can be conspicuously recorded in a photographed picture. 
     Yet another object of the present invention is to provide a marker for defining a standard measurement scale in a photogrammetric measurement system, which is constituted such that a reference point of the marker can be easily aligned with an indication on the ground. 
     Still yet another object of the present invention is to provide such a measurement-standard-scale-defining marker as mentioned above, which is constituted such that the reference point can be conspicuously recorded in a photographed picture. 
     In accordance with a first aspect of the present invention, there is provided a standard measurement scale, to be used in a photogrammetric measurement system, comprising a polygonal plate member having at least three apexes, which are arranged so as to define a reference plane, and each of which defines a reference point. Preferably, the three reference points are equally spaced apart from each other by a predetermined distance. Each of the triangular apexes, of the polygonal member, includes a reference point and may be marked with a conspicuous material. The conspicuous material may comprise a light color paint. Preferably, the light color paint is a fluorescent paint. The conspicuous material may comprise a piece of reflective sheet. 
     In accordance with a second aspect of the present invention, there is provided a standard measurement scale, to be used in a photogrammetric measurement system, comprising a light-guide plate member including a core layer containing fluorescent substances, said light-guide plate member having at least three light-emitting sites for defining respective reference points. Preferably, the three reference points are equally spaced apart from each other by a predetermined distance. Each of the light-emitting sites may be defined as a cone-shaped or polygonal-pyramidal-shaped recess formed in the light-guide plate member for emitting fluorescent radiation therefrom. Also, each of the light-emitting spots may be defined as a hemispherical projection attached to the light-guide plate member for emitting fluorescent radiation therefrom, or may be defined by at least two V-shaped grooves formed in the light-guide plate member and extending radially from the center thereof for emitting fluorescent radiation therefrom. 
     In accordance with a third aspect of the present invention, there is provided a standard measurement scale, to be used in a photogrammetric measurement system, comprising: a frame member, and at least three reference-point-forming elements, for defining respective reference points, arranged on the frame member so as to define a reference plane. The standard scale may further comprises a plate member mounted on the frame member. In this case, the three reference-point-forming elements are arranged on the plate member so as to define a reference plane. Preferably, the three reference points are equally spaced apart from each other by a predetermined distance. 
     Each of the reference-point-forming elements may be formed as a projection for defining the reference point thereof. In this case, the projection may be a light-emitting projection including an electrical lump or a light-emitting diode, and may be formed from at least two light-guide plate elements, including each a core layer containing fluorescent substances, which are arranged such that the reference point of the projection is defined by an emission of fluorescent radiation therefrom. 
     Also, each of the reference-point-forming elements may be formed as a cone-shaped projection or polygonal-pyramidal-shaped projection, an apex of which defines one of the reference points. In this case, the cone-shaped projection or polygonal-pyramidal-shaped projection may be coated with a fluorescent paint, or may be covered with a reflective sheet. 
     Further, each of the reference-point-forming elements may be formed as a polygonal-pyramidal-shaped projection constructed from at least two light-guide plate elements, including each a core layer containing fluorescent substances. Preferably, the light-guide plate elements are arranged such that the apex of the polygonal-pyramidal-shaped projection is defined by an emission of fluorescent radiation therefrom. 
     Furthermore, each of the reference-point-forming elements may be formed as a small circular-shaped element for defining the reference point thereof. Preferably, the small circular-shaped element is formed from a reflective sheet. 
     Yet further, each of the reference-point-forming elements may be formed as a circular-shaped plate element, a center of which defines one of the reference points. In this case, the circular-shaped plate element may be formed as a light-guide plate element including a core layer containing fluorescent substances. Preferably, the light-guide plate element may have: a cone-shaped recess formed at the center thereof for emitting light-rays therefrom; a polygonal-pyramidal-shaped recess formed at the center thereof for emitting fluorescent radiation therefrom; a hemispherical projection attached to the center thereof for emitting fluorescent radiation therefrom; or at least two V-shaped grooves formed therein, which extend radially from the center thereof, for emitting fluorescent radiation therefrom. 
     In accordance with a fourth aspect of the present invention, there is provided a standard measurement scale, to be used in a photogrammetric measurement system, comprising: a frame member, and a light-guide plate member mounted on the frame member and including a core layer containing fluorescent substances, the light-guide plate member having at least three light-emitting sites for defining respective reference points. Preferably, the three reference points are equally spaced apart from each other by a predetermined distance. 
     Each of the light-emitting sites may be defined as a cone-shaped recess or a polygonal-pyramidal-shaped recess formed in the light-guide plate member for emitting fluorescent radiation therefrom. Also, the light-emitting site may be defined as a hemispherical projection attached to the light-guide plate member for emitting fluorescent radiation therefrom, or may be defined by at least two V-shaped grooves formed in the light-guide member and extending radially from the center thereof for emitting fluorescent radiation therefrom. 
     In accordance with a fifth aspect of the present invention, there is provided a marker, to be used in a photogrammetric measurement system for defining a standard measurement scale, comprising a light-guide plate member including a core layer containing fluorescent substances and having a light emitting site for defining a reference point. 
     In accordance with a sixth aspect of the present invention, there is provided a marker, to be used in a photogrammetric measurement system for defining a standard measurement scale, comprising a polygonal-pyramidal-shaped optical assembly, formed from at least two light-guide plate elements, including each a core layer containing fluorescent substances, such that an apex of the polygonal-pyramidal-shaped optical assembly is defined by an emission of fluorescent radiation therefrom. 
     The polygonal-pyramidal-shaped optical assembly may be produced as a generally-triangular-pyramidal-shaped optical assembly, from three isosceles-triangular light-guide plate elements, in such a manner that an inner triangular-pyramidal space is defined therewithin. Preferably, two contiguous slanting side faces of two adjacent isosceles-triangular light-guide plate elements form a V-shaped trough extending along a corresponding ridgeline of the inner triangular-pyramidal space, and the predominant emission of fluorescent radiation occurs from the side faces. 
     Also, the polygonal-pyramidal-shaped optical assembly may be produced as a generally-quadrilateral-pyramidal-shaped optical assembly, by crosswisely interlinking two isosceles-triangular light-guide plate elements. Preferably, the predominant emission of fluorescent occurs the slanting side faces of each isosceles-triangular light-guide plate element. 
     Preferably, each of the isosceles-triangular light-guide plate elements has a slit formed therein, whereby the light-guide plate elements are detachably and crosswisely interlinked via the slits formed therein. In this case, the isosceles-triangular light-guide plate elements preferably have the same isosceles-triangular shape. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and other objects of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
     FIG. 1 is a conceptual perspective view showing a photogrammetric measurement system using a standard measurement scale, according to the present invention; 
     FIG. 2 is a conceptual view showing a picture photographed at a first photographing position in the measurement system of FIG. 1; 
     FIG. 3 is a conceptual view showing another picture photographed at a second photographing position in the measurement system of FIG. 1; 
     FIG. 4 is a conceptual view showing a relative-positional relationship between the standard scale and the first and second pictures from FIGS. 2 and 3 respectively; 
     FIG. 5 is a flowchart showing a photogrammetric measurement routine for producing a survey map on the basis of the first and second pictures from FIGS. 2 and 3 respectively; 
     FIG. 6 is a conceptual view showing a three-dimensional coordinate system for producing the survey map; 
     FIG. 7 is a perspective view of a first embodiment of the standard scale, according to the present invention; 
     FIG. 8 is a perspective view of a second embodiment of the standard scale, according to the present invention; 
     FIG. 9 is a perspective view of a third embodiment of the standard scale, according to the present invention; 
     FIG. 10 is a partial perspective view of a fourth embodiment of the standard scale, according to the present invention; 
     FIG. 11 is a sectional view taken along line XI—XI of FIG. 10; 
     FIG. 12 is a partial perspective view of a fifth embodiment of the standard scale, according to the present invention; 
     FIG. 13 is a partial perspective view of a sixth embodiment of the standard scale, according to the present invention; 
     FIG. 14 is sectional view taken along line XIV—XIV of FIG. 13; 
     FIG. 15 is a partial perspective view of a seventh embodiment of the standard scale, according to the present invention; 
     FIG. 16 is a plan view of a generally-triangular-pyramidal-shaped optical projection, shown in FIG. 15; 
     FIG. 17 is a partial perspective view of an eighth embodiment of the standard scale, according to the present invention; 
     FIG. 18 is a plan view of a generally-quadrilateral-pyramidal-shaped optical projection, shown in FIG. 17; 
     FIG. 19 is an elevational view of two isosceles-triangular light-guide plate elements for assembling the generally-quadrilateral-pyramidal-shaped optical projection, shown in FIG. 17; 
     FIG. 20 is a partial perspective view of a ninth embodiment of the standard scale, according to the present invention; 
     FIG. 21 is a plan view of a cross-shaped optical projection, shown in FIG. 20; 
     FIG. 22 is an elevational view of two parallelepiped-shaped light-guide plate elements for assembling the cross-shaped optical projection, shown in FIG. 20; 
     FIG. 23 is a perspective view of a tenth embodiment of the standard scale, according to the present invention; 
     FIG. 24 is an enlarged perspective view showing a circular-shaped light-guide plate element, shown in FIG. 23; 
     FIG. 25 is an enlarged perspective view, similar to FIG. 24, showing a modification of the circular-shaped light-guide plate element; 
     FIG. 26 is an enlarged perspective view, similar to FIG. 24, showing another modification of the circular-shaped light-guide plate element; 
     FIG. 27 is a perspective view of an eleventh embodiment of the standard scale, according to the present invention; 
     FIG. 28 is a conceptual plan view showing a stereo-photogrammetric measurement system using markers for defining a standard measurement scale, according to the present invention; 
     FIG. 29 is a perspective view of a first embodiment of the standard-scale-defining marker, according to the present invention; 
     FIG. 30 is a plan view of the standard-scale-defining marker of FIG. 29; 
     FIG. 31 is a sectional view taken along line XXXI—XXXI of FIG. 30; 
     FIG. 32 is a perspective view of a second embodiment of the standard-scale-defining marker, according to the present invention; 
     FIG. 33 is a diametrically-sectional view of the standard-scale-defining marker, shown in FIG. 32; 
     FIG. 34 is a plan view of a third embodiment of the standard-scale-defining marker, according to the present invention; 
     FIG. 35 is an enlarged view of eight V-shaped troughs formed in the standard-scale-defining marker, shown in FIG. 34; 
     FIG. 36 is a sectional view taken along line XXXVI—XXXVI of FIG. 35; 
     FIG. 37 is a perspective view of a fourth embodiment of the standard-scale-defining marker, according to the present invention; 
     FIG. 38 is a plan view of the standard-scale-defining marker, shown in FIG. 37; 
     FIG. 39 is a perspective view of a fifth embodiment of the standard-scale-defining marker, according to the present invention; 
     FIG. 40 is a plan view of the standard-scale-defining marker, shown in FIG. 39; and 
     FIG. 41 is a perspective view of two isosceles-triangular light-guide plate elements for assembling the standard-scale-defining marker, shown in FIGS.  39  and  40 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 conceptually shows a photogrammetric measurement system, using a standard measurement scale  10 , constructed according to the present invention. The standard measurement scale  10  is placed beside a cubic object  12  to be measured, and the standard scale  10  and the cubic object (three-dimensional)  12  are photographed in two different directions by an electronic still video camera  14 . Namely, as shown in FIG. 1, the standard scale  10  and the cubic object  12  are photographed by the camera  14  placed at a first photographing position M 1 , shown by a solid line, and are then photographed by the camera  14  placed at a second photographing position M 2 , shown by a dashed line. At the first photographing position M 1 , an optical axis of the camera  14  is indicated by reference O 1 , and, at the second photographing position M 2 , the optical axis of the camera  14  is indicated by reference O 2 . 
     Note, each of the first and second photographing positions M 1  and M 2  may be defined as a back principal point of the photographing lens system of the camera  14 . 
     The standard measurement scale  10  is shaped as an equilateral-triangular plate member, and has three reference points P 1 , P 2  and P 3  positioned in the vicinity of the apexes of the equilateral-triangular plate member, such that an equilateral triangle is defined by the reference points P 1 , P 2  and P 3 , as shown by a hatched area in FIG.  1 . The sides of the equilateral triangle, defined by the reference points P 1 , P 2  and P 3 , have a length of L. 
     FIG. 2 shows a first picture photographed by the camera  14  at the first photographing position M 1 . As is apparent from this drawing, a rectangular x 1 -y 1  coordinate system is defined on the first picture, and an origin c 1  of the x 1 -y 1  coordinate system is at the photographing center of the first picture. In this coordinate system, the reference points P 1 , P 2  and P 3  are represented by coordinates p 11 (px 11 , py 11 ), p 12 (px 12 , py 12 ) and p 13 (px 13 , py 13 ), respectively. 
     FIG. 3 shows a second picture photographed by the camera  14  at the second photographing position M 2 . As is apparent from this drawing, a rectangular x 2 -y 2  coordinate system is defined on the second picture, and an origin c 2  of the x 2 -y 2  coordinate system is at the photographing center of the second picture. In this coordinate system, the reference points P 1 , P 2  and P 3  are represented by coordinates p 21 (px 21 , py 21 ) p 22 (px 22 , py 22 ) and p 23 (px 23 , py 23 ), respectively 
     FIG. 4 shows a relative-positional three-dimensional relationship between the standard scale  10 , the camera  14 , and the first and second pictures. In this case, the standard scale  10  is relatively reproduced on the basis of the first and second pictures placed at the first and second photographing positions M 1  and M 2 , but a size of the standard scale  10  is relative. Thus, a length of the sides of the equilateral triangle, defined by the reference points P 1 , P 2  and P 3 , is indicated by L′. 
     In order to calculate three-dimensional coordinates of the cubic object  12 , it is necessary to define an X-Y-Z three-dimensional coordinate system, as shown in FIG. 4, and the reference points P 1 , P 2  and P 3  of the standard scale  10 , recorded on each of the first and second pictures, must be positionally determined with respect to the three-dimensional coordinate system. 
     As shown in FIG. 4, an origin of the three-dimensional coordinate system is at the first photographing position M 1 . Namely, the first photographing position M 1  is represented by the origin coordinates (0, 0, 0) of the three-dimensional coordinate system. Also, a Z-axis of the three-dimensional coordinate system coincides with the optical axis O 1  of the camera  14  placed at the first photographing position M 1 . The second photographing position M 2  is represented by coordinates (X 0 , Y 0 , Z 0 ), and the optical axis O 2  of the camera  14 , placed at the second photographing position M 2 , is represented by angular coordinates (α, β, γ). Namely, the optical axis O 2  of the camera  14  defines angles of α, β and γ with the X-axis, Y-axis and Z-axis of the three-dimensional coordinate system, respectively. 
     The reference points P 1 , P 2  and P 3  of the standard scale  10  are represented by three-dimensional coordinates P j (PX j , PY j , PZ j ) (j=1, 2, 3). As shown in FIG. 4, each of the reference points [P 1 (PX 1 , PY 1 , PZ 1 ) P 2 (PX 2 , PY 2 , PZ 2 ) and P 3 (PX 3 , PY 3 , PZ 3 )], the image point [p 11 (px 11 , py 11 ) p 12 (px 12 , py 12 ), p 13 (px 13 , py 13 )] of the corresponding reference point recorded on the first picture, and the back principal point (M 1 ) of the camera  14  are aligned with each other on a straight axis. Similarly, each of the reference points [P 1 (PX 1 , PY 1 , PZ 1 ), P 2 (PX 2 , PY 2 , PZ 2 ) and P 3 (PX 3 , PY 3 , PZ 3 )], the image point [p 21 (px 21 , py 21 ), p 22 (px 22 , py 22 ), p 23 (px 23 , py 23 )] of the corresponding reference point recorded on the second picture, and the back principal point (M 2 ) of the camera  14  are aligned with each other on a straight axis. 
     Accordingly, the three-dimensional coordinates P j (PX j , PY j , PZ j ) can be determined by the following collinear equations:                PX   j     =         (       PZ   j     -     Z   0       )                           a   11          px   ij       +       a   21          py   ij       -       a   31        C             a   13          px   ij       +       a   23          py   ij       -       a   33        C           +     X   0                     PY   j     =         (       PZ   j     -     Z   0       )                           a   12          px   ij       +       a   22          py   ij       -       a   32        C             a   13          px   ij       +       a   23          py   ij       -       a   33        C           +     Y   0                             
     Herein: 
     a 11 =cos β*sin γ 
     a 12 =−cos β*sin γ 
     a 13 =sin β 
     a 21 =cos α*sin γ+sin α*sin β*cos γ 
     a 22 =cos α*cos γ+sin α*sin βsin γ 
     a 23 =−sin α*sin β 
     a 31 =sin α*sin γ+cos α*sin β*cos γ 
     a 32 =sin α*cos γ+cos α*sin β*sin γ 
     a 33 =cos α*cos β 
     Note that, in these equations, “C” indicates a principal focal length of the camera  14 , which is defined as a distance between the back principal point (M 1 ) and the photographing center (c 1 ) of the first picture, and a distance between the back principal point (M 2 ) and the photographing center (c 2 ) of the second picture. Also note, “i” corresponds to a number of the pictures; and “j” corresponds to a number of the reference points P 1 , P 2  and P 3  of the standard scale  10 . 
     FIG. 5 shows a flowchart for a photogrammetric measurement routine, in which a survey map is made on the basis of the first and second pictures, shown in FIGS. 2 and 3. This routine is executed by a computer (not shown). Before the execution of the routine, the video data of the first and second pictures is fed from the electronic still video camera  14  to the computer, and the first and second pictures are simultaneously displayed on a TV monitor connected to the computer, as shown in FIGS. 2 and 3. 
     At step  501 , as coordinate data (X 0 , Y 0 , Z 0 ) of the second photographing position M 2  and as angular coordinate data (α, β, γ) of the optical axis O 2 , suitable initial values (except for zero) are inputted to the computer through, for example, a keyboard. Then, at step  502 , the respective reference points P ij  (px ij , py ij ) are successively designated, on the first and second pictures displayed on the TV monitor, with a cursor manipulated by a mouse. Namely, the two sets of coordinates P 11 (px 11 , py 11 ) and P 21 (px 21 , py 21 ), the two sets of coordinates P 12 (px 12 , py 12 ) and P 22 (px 22 , py 22 ), and the two sets of coordinates P 13 (px 13 , py 13 ) and P 23 (px 23 , py 23 ) are retrieved by a central processing unit (CPU) or of the computer. 
     After the designation of the reference points P ij  (px ij , py ij ) and P ij  (px ij , py ij ), at step  503 , a counter k is made to be “1”. Then, at step  504 , a suitable point Q 1(k=1)  of the cubic object  12  is selected, and image points q ik  (FIGS. 2 and 3) of the point Q 1 , displayed on the first and second pictures of the TV monitor, are designated with the cursor manipulated by the mouse. Namely, the two sets of coordinates q 11 (qx 11 , qy 11 ) and q 21  (qx 21 ) of the image point Q 1  is retrieved by the central processor of the computer. 
     At step  505 , the above-mentioned collinear equations are solved on the basis of the retrieved coordinates, and three-dimensional coordinates P j  (PX j , PY j , PZ j ) of the reference points P 1 , P 2  and P 3 , and three-dimensional coordinates Q 1 (QX 1 , QY 1 , QZ 1 ) of the object point Q 1  are determined. Then, primary-approximate data of the three-dimensional coordinates (X 0 , Y 0 , Z 0 ) of the second photographing position M 2  and the angle coordinates (α, β, γ) of the optical axis O 2  are determined, i.e. the initial coordinate data (X 0 , Y 0 , Z 0 ) and the initial angular coordinate data (α, β, γ), inputted at step  501 , are revised by the primary-approximate data. 
     At step  506 , a coefficient “m” is calculated as follows: 
     
       
         
           m←L/L′ 
         
       
     
     Note, “L” is the real length between the reference points P 1 , P 2 , and P 3  and “L” is the relative length obtained from the determined three-dimensional coordinates P j (PX j , PY j , PZ j ). 
     At step  507 , scaling is executed, using the coefficient “m”, between the determined three-dimensional coordinates P j (PX j , PY j , PZ j) and Q 1 (QX 1 , QY 1 , QZ 1 ), so as to obtain a real spatial relationship therebetween. Then, at step  508 , the X-Y-Z three-dimensional coordinate system is transformed into an X′-Y′-Z′ three-dimensional coordinate system defined as shown in FIG.  6 . 
     As is apparent from FIG. 6, an origin of the X′-Y′-Z′ three-dimensional coordinate system is at the reference point P 1 , and the X′-axis thereof is defined by the reference points P 1  and P 2 . Also, The X′- and Z′-axes of the coordinate system define a plane “Ps”, which includes a hatched triangular plane area defined by the reference points P 1 , P 2  and P 3 . Note, in the example of FIG. 6, although the origin of the X′-Y′-Z′ three-dimensional coordinate system coincides with the reference point P 1 , the origin may be at any location included in the plane “Ps”. 
     At step  509 , for example, the X′-Z′ plane or plane “Ps”, on which the reference points P 1 , P 2  and P 3  and the object point Q 1  are recorded, is displayed as a survey map on another TV monitor. Nevertheless, the displayed survey map is not accurate, because the revised coordinate data (X 0 , Y 0 , Z 0 ) and angular coordinate data (α, β, γ) are not sufficiently approximated. 
     At step  510 , it is determined whether or not another set of points q 1k  and q 2k  should be designated with respect to the cubic object  12 . When the other set of points q 1k  and q 2k  should be further designated, i.e. when the renewed coordinate data (X 0 , Y 0 , Z 0 ) and angular coordinate data (α, β, γ) are not sufficiently approximated, at step  511 , the counter k is incremented by “1”. Thereafter, the routine comprising steps  504  to  510  is again executed. 
     At step  510 , when a further set of points q 1k  and q 2k  should not be designated, i.e. when the revised coordinate data (X 0 , Y 0 , Z 0 ) and angular coordinate data (α, β, γ) are sufficiently approximated, this routine is completed. 
     Before the approximation of the coordinate (X 0 , Y 0 , Z 0 ) and angular coordinate data (α, β, γ) is acceptable, it is necessary to designate at least two sets of points q 1k  and q 2k  with respect to the cubic object  12 , i.e. the approximation calculation should be repeated at least twice. Preferably, more than two sets of object points q 1k  and q 2k  should be designated, i.e., the approximation calculation should be repeated more than twice. 
     FIG. 7 shows a first embodiment of the standard measurement scale  10  according to the present invention. In this embodiment, the standard measurement scale  10  comprises an equilateral-triangular plate  16 , a thickness of which may be from about 2 mm to about 3 mm. Although it is preferable to form the triangular plate  16  of a suitable resin material, such as acrylic resin, the triangular plate  16  may be formed of another material, such as a wood, a suitable metal and so on. Three respective apexes  18 ,  20  and  22  of the triangular plate  16  define the points P 1 , P 2  and P 3  of the standard scale  10 , and a distance between the reference points P 1 , P 2  and P 3  may be 1 m. 
     Preferably, the small triangular area including each of the reference points P 1 , P 2  and P 3  is marked with a suitable material, such as a reflective paint, a fluorescent paint, a piece of reflective sheet and so on, because the reference points P 1 , P 2  and P 3  are required to be conspicuously recorded on a photographed picture. Thus, the designation of the reference points P 1 , P 2  and P 3  with a cursor on a TV monitor, as mentioned above, can be easily carried out. 
     FIG. 8 shows a second embodiment of the standard measurement scale  10  according to the present invention. In this second embodiment, the standard measurement scale  10  comprises an equilateral-triangular frame  24 , and three respective projections  26 ,  28  and  30  securely mounted on the apex areas of the frame  24 . The triangular frame  24  may be assembled from rectangular wood lumbers having, for example, a width of about 30 mm to about 50 mm and a thickness of about 50 mm. Of course, the frame  24  may be formed of another material such as a suitable resin, a suitable metal and so on. Similarly, the projections  26 ,  28  and  30  may be shaped from a wood, a suitable resin, a suitable metal and so on. 
     In this embodiment, although each of the projections  26 ,  28  and  30  is formed as a quadrangular pyramid, each projection may be shaped into another form such as a circular cone, a triangular-base pyramid, a polygonal-base, a hemisphere or the like. Three respective apexes  26 A,  28 A and  30 A of the projections  26 ,  28  and  30  define the reference points P 1 , P 2  and P 3  of the standard scale  10 , and a distance between the points P 1 , P 2  and P 3  may be 1 m. Note, each of the bottom sides of the quadrangular-pyramidal-shaped projection ( 26 ,  28 ,  30 ) may have a length of about 50 mm. 
     In order to conspicuously record the reference points P 1 , P 2  and P 3 , defined by the projections  26 A,  28 A and  30 A, on a photographed picture, each of the projections  26 ,  28  and  30  may be coated with a reflective paint, a fluorescent paint or the like. Also, the surfaces of each projection  26 ,  28 ,  30  may be covered with a piece of reflective sheet and so on. 
     FIG. 9 shows a third embodiment of the standard measurement scale  10  according to the present invention. In this third embodiment, the standard measurement scale  10  comprises an equilateral-triangular frame  32 , an equilateral triangular-plate  34  securely attached to the frame  32 , and three respective projections  36 ,  38  and  40  securely mounted on the apex areas of the plate  34 . The triangular frame  32  may be assembled from elongated wood boards, and the triangular plate  32  may be shaped from a wood board. Of course, the frame  32  and the plate  34  may be formed of another material such as a suitable resin, a suitable metal and so on. The projections  36 ,  38  and  40  may be made and shaped in substantially the same manner as the projections  26 ,  28  and  30  of the second embodiment. 
     Similar to the second embodiment, three respective apexes  36 A,  38 A and  40 A of the projections  36 ,  38  and  40  define the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 , and a distance between the points P 1 , P 2  and P 3  may be 1 m. Further, in order to conspicuously record the reference points P 1 , P 2  and P 3  on a photographed picture, each of the projections  36 ,  38  and  40  also may be coated with a reflective paint, a fluorescent paint or the like, or the surfaces of each projection  36 ,  38 ,  40  may be covered with a piece of reflective sheet and so on. 
     FIGS. 10 and 11 show a fourth embodiment of the standard measurement scale  10 , according to the present invention. In this fourth embodiment, the standard measurement scale  10  comprises an equilateral-triangular light-guide plate  42 . Note, in FIG. 10, only an apex area of the triangular light-guide plate  42  is illustrated. 
     As shown in FIG. 11, the light-guide plate  42  is constituted from a core layer  42 A containing fluorescent substances uniformly distributed therein, a first clad layer  42 B formed over an upper surface of the core layer  42 A, and a second clad layer  42 C formed over a lower surface of the core layer  42 A. In this embodiment, the core layer  42 A is made of an acrylic resin material, and the first and second clad layers  42 B and  42 C are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer  42 A. 
     Although light rays, which become incident upon the clad layers  42 B and  42 C at a right angle with respect to the surfaces thereof, can pass through the light-guide plate  42 , light rays, which become incident upon the clad layers  42 B and  42 C at a slanting angle with respect to surfaces thereof, are trapped in the light-guide plate  42 . Also, light-rays, which become incident upon the core layer  42 A via the peripheral side faces of the light-guide plate  42 , cannot be substantially emitted from the core layer  42 A through the first and second clad layers  42 B and  42 C. 
     When the fluorescent substances, contained in the core layers  42 A, are subjected to the light rays, the fluorescent substances generate fluorescent radiation as visible light. The generated fluorescent radiation is trapped between the first and second clad layers  42 B and  42 C, i.e. the fluorescent radiation cannot be emitted from the core layer  42 A through the first and second clad layers  42 B and  42 C. Note, of course, the fluorescent radiation can be emitted from the peripheral side faces of the plate  42 . 
     As representatively shown in FIGS. 10 and 11, three small cone-shaped recesses  44  are respectively formed at the apex areas of the upper surface of the light-guide plate  42 , and define the reference points P 1 , P 2  and P 3  of the standard scale  10 , where a distance between the points P 1 , P 2  and P 3  may be 1 m. As best shown in FIG. 11, each of the cone-shaped recesses  44  penetrates the core layer  42 A, so that a part of the fluorescent radiation is emitted from each of the cone-shaped recesses  44 . Thus, when the standard scale  10  of the fourth embodiment is photographed by the camera  14 , the reference points P 1 , P 2  and P 3 , which are defined by the cone-shaped recesses  44 , are conspicuously recorded on a photographed picture. 
     Note, in the fourth embodiment, it should be understood that polygonal-pyramidal-shaped recesses such as triangular-pyramidal-shaped recesses, quadrangular-pyramidal-shaped recesses, or the like, may be substituted for the cone-shaped recesses  44 . 
     FIG. 12 shows a fifth embodiment of the standard measurement scale  10 , according to the present invention. Note, in this drawing, only an apex area of the standard plate  10  is illustrated. The fifth embodiment is substantially similar to the third embodiment (FIG.  9 ), except that three pieces of reflective sheet  46  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3 . Of course, the reflective pieces  46 , defining the reference points P 1 , P 2  and P 3 , are conspicuously recorded on a photographed picture due to the reflectivity thereof. 
     FIGS. 13 and 14 show a sixth embodiment of the standard measurement scale  10 , according to the present invention. Note, in this drawing, only an apex area of the standard plate  10  is illustrated. This sixth embodiment also is substantially similar to the third embodiment (FIG.  9 ), except that three hemispherical lenses  48  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3  of the standard scale  10 . 
     In the sixth embodiment, each of the hemispherical lenses  48  is associated with an electrical light source  50 , such as an electrical lamp, a light emitting diode (LED) or the like. As shown in FIG. 14, the electrical light source  50  is connected to an electric power source  52  through an ON/OFF switch  54 . When the switch is turned ON, the light source  50  is electrically energized by the power source  52 , resulting in emitting light rays from the light source  50 . The light rays are radiated in all directions from the hemispherical lens  48 . Accordingly, the hemispherical lenses  48 , defining the reference points P 1 , P 2  and P 3 , are conspicuously recorded on a photographed picture due to the emission of the light rays from the light source  50 . 
     FIGS. 15 and 16 show a seventh embodiment of the standard measurement scale  10 , according to the present invention. Note, in FIG. 15, only an apex area of the standard plate  10  is illustrated. This seventh embodiment also is similar to the third embodiment (FIG.  9 ), except that three generally-triangular-pyramidal-shaped optical projections  56  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3  of the standard scale. 
     In the seventh embodiment, each of the optical projections  56  comprises three isosceles-triangular light-guide plate elements  58  having, for example, a thickness of about 2 mm, a bottom length of about 50 mm, and a height of about 50 mm. Each of the light-guide plate elements  58  has the same optical structure as the light-guide plate  42  of the fourth embodiment (FIGS.  10  and  11 ). Namely, the light-guide plate element  58  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. The core layer is made of an acrylic resin material, and the first and second clad layers are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer. 
     Each of the optical projections  56  is assembled from the three light-guide plate elements  58 , in such a manner that an inner triangular-pyramidal space is defined therewithin. As best shown in FIG. 16, two contiguous side faces  58 A of two adjacent light-guide plate elements  58  form a V-shaped trough extending along a corresponding ridgeline of the inner triangular-pyramidal space, and an apex  58 B of the inner triangular-pyramidal space defines one of the reference points P 1 , P 2  and P 3 . 
     The fluorescent radiation, generated and trapped in the core layer of each light-guide plate element  58 , cannot be substantially emitted from a triangular surface  58 C thereof, but a part of the fluorescent radiation can be emitted from the side faces  58 A thereof. Thus, the V-shaped troughs of the optical projection  56  are conspicuously recorded on a photographed picture, whereby the convergent center  58 B of the V-shaped troughs can be easily located from the photographed picture. 
     FIGS. 17,  18  and  19  show an eighth embodiment of the standard measurement scale  10 , according to the present invention. Note, in FIG. 17, only an apex area of the standard plate  10  is illustrated. This eighth embodiment also is substantially similar to the third embodiment (FIG.  9 ), except that three generally-quadrilateral-pyramidal-shaped optical projections  60  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3  of the standard scale. 
     In the eighth embodiment, each of the optical projections  60  comprises two isosceles-triangular light-guide plate elements  62  and  64  having each a thickness of about 2 mm. Each of the light-guide plate elements  62  and  64  has the same optical structure as the light-guide plate  42  of the fourth embodiment (FIGS.  10  and  11 ). Namely, each of the light-guide plate elements  62  and  64  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. The core layer is made of an acrylic resin material, and the first and second clad layers are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer. 
     The optical projection  60  is assembled from the two light-guide plate elements  62  and  64  into the generally-quadrilateral-pyramidal-shape, as shown FIGS. 17 and 18. To this end, as shown in FIG. 19, the light-guide plate element  62  has an upper half slit  62 A formed therein and extended from the apex thereof to the middle position of the height thereof, and the light-guide plate element  64  has a lower half slit  64 A, formed therein and extended from the center of the bottom side thereof to the middle position of the height thereof. Thus, the generally-quadrilateral-pyramidal-shaped projection  60  is obtained from the light-guide plate elements  62  and  64  by crosswisely interlinking them via the upper and lower half slits  62 A and  64 A. Note, a width of each of the slit  62 A and  64 A is approximately 2 mm, which is equal to the thickness of the light-guide plate elements  62  and  64 . 
     As shown in FIGS. 17 and 18, an apex of the light-guide plate element  64  is shaped as a small square area  65 , which defines one of the reference points P 1 , P 2  and P 3  of the standard scale  10 . 
     A part of the fluorescent radiation, generated and trapped in the core layer of each light-guide plate element ( 62 ,  64 ), is predominantly emitted from the slanting side faces ( 62 B,  64 B) thereof, but the fluorescent radiation cannot be substantially emitted from the triangular surfaces  62 C,  64 C thereof. Thus, the slanting side faces  62 B and  64 B of the light-guide plate elements  62  and  64  are conspicuously recorded on a photographed picture, whereby the apex or small square area  65  of each optical projection  60  can be easily located from the photographed picture. 
     FIGS. 20,  21  and  22  show a ninth embodiment of the standard measurement scale  10 , according to the present invention. Note, in FIG. 20, only an apex area of the standard measurement plate  10  is illustrated. This ninth embodiment also is substantially similar to the third embodiment (FIG.  9 ), except that three cross-shaped optical projections  66  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     In the ninth embodiment, each of the cross-shaped optical projections  66  comprises two parallelepiped-shaped light-guide plate elements  68  and  70  having each a suitable thickness. Each of the light-guide plate elements  68  and  70  has the same optical structure as the light-guide plate  42  of the fourth embodiment (FIGS.  10  and  11 ). Namely, each of the light-guide plate elements  68  and  70  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. The core layer is made of an acrylic resin material, and the first and second clad layers are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer. 
     The optical projection  66  is assembled from the two light-guide plate elements  68  and  70  into the cross-shape, as shown FIGS. 20 and 21. To this end, as shown in FIG. 22, the light-guide plate element  68  has an upper half slit  68 A, formed therein and extended from the center of the top side thereof to the middle position of the width thereof, and the light-guide plate element  70  has a lower half slit  70 A, formed therein and extended from the center of the bottom side thereof to the middle position of the-width thereof. Thus, the cross-shaped projection  66  is obtained from the light-guide plate elements  68  and  70  by crosswisely interlinking them via the upper and lower half slits  68 A and  70 A. Note, a thickness of each slit  68 A,  70 A is equal to each other. Note, a width of each of the slit  68 A and  70 A is equal to the thickness of the light-guide plate elements  68  and  70 . 
     As shown in FIGS. 20 and 21, a central area  72  of the light-guide plate element  70  defines one of the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     A part of the fluorescent radiation, generated and trapped in the light-guide plate elements  68  and  70 , can be predominately emitted from only the top side face  68 B and  70 B and end side faces  68 C and  70 C thereof, but the fluorescent radiation cannot be substantially emitted from the side wall surfaces  68 D and  70 D thereof. Thus, the top side faces  68 B and  70 B and the end side faces  68 C and  70 C of the light-guide plate elements  68  and  70  are conspicuously recorded on a photographed picture, and the central area  72  of the light-guide plate element  70  can be easily located from the photographed picture. 
     In the ninth embodiment, preferably, the two light-guide plate elements  68  and  70  are identical to each other. Namely, only one kind of light-guide plate elements ( 68 ,  70 ) is produced, and the cross-shaped optical projection  66  is obtained from two light-guide plate elements by interlinking them crosswise via the slits thereof. Accordingly, the cross-shaped optical projection  66  can be obtained at a low cost. 
     FIGS. 23 and 24 show a tenth embodiment of the standard measurement scale  10 , according to the present invention. This tenth embodiment also is substantially similar to the third embodiment (FIG.  9 ), except that three circular-shaped light-guide plate elements  74  are respectively substituted for the projections  36 ,  38  and  40  for defining the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     In the tenth embodiment, each of the circular-shaped light-guide plate elements  74  has the same optical structure as the light-guide plate  42  of the fourth embodiment (FIGS.  10  and  11 ). Namely, each of the light-guide plate elements  74  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. The core layer is made of an acrylic resin material, and the first and second clad layers are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer. Note, for example, the light-guide plate element  74  has a diameter of about 100 mm and a thickness of abut 50 mm. 
     As best shown in FIG. 24, each of the light-guide plate elements  74  has a hemispherical projection  76  attached to and placed at the center thereof, and the three projections  76  define the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     Each of the projections  76  may be formed of a suitable transparent resin material, having an index of refraction different from that of the clad layers of the light-guide plate element  74  or that of air, so that a part of the fluorescent radiation, generated and trapped in the core layer of the light-guide plate element  74 , is emitted from the projection  76 . Of course, another part of the fluorescent radiation can be emitted from the peripheral side face of the light-guide plate element  74 , but the fluorescent radiation cannot be emitted from the circular surface  78  (FIG. 24) of the plate element  74 . Accordingly, the three hemispherical projections  76  of the plate elements  74  defining the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 , are conspicuously recorded on a photographed picture. 
     Preferably, the hemispherical projection  76  is formed of a transparent plastic material exhibiting softness, so that the hemispherical projection  76  can be detachably adhered to the center of the light-guide plate element  74 , due to the softness of the projection  76 . 
     FIG. 25 shows a modification of the circular-shaped light-guide plate element  74 . In this modification, eight V-shaped grooves  80  are formed in the light-guide plate element  74 , so as to radially extend from the center thereof. Each of the V-shaped grooves  80  has a width of about 2 mm, and penetrates the core layer of the light-guide plate element  74 , so that a part of the fluorescent radiation is predominantly emitted from the grooves  80 . Of course, the convergent center of the grooves  80  defines one of the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     FIG. 26 shows another modification of the circular-shaped light-guide plate element  74 . In this modification, a cone-shaped recess  82  is formed in the light-guide plate element  74 , at the center thereof, and the cone-shaped recess  82  has a diameter of about 6 mm and a depth of about 3 mm, penetrating the core layer of the light-guide plate element  74 , so that a part of the fluorescent radiation can be emitted from the cone-shaped recess  82 . Of course, the center of the recess  82  defines one of the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 . 
     FIG. 27 shows an eleventh embodiment of the standard measurement scale  10 , according to the present invention. In this eleventh embodiment, the standard measurement scale  10  comprises an equilateral-triangular frame  84 , and an equilateral-triangular light-guide plate  86 , securely attached to the frame  84 . The triangular frame  84  may be assembled in the same manner as the triangular frame  32  of the third embodiment (FIG.  9 ). The equilateral-triangular light-guide plate  86  has the same optical structure as the light-guide plate  42  of the fourth embodiment (FIGS.  10  and  11 ). Namely, the light-guide plate  86  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. The core layer is made of an acrylic resin material, and the first and second clad layers are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer. 
     As shown in FIG. 27, three small hemispherical projections  88  are respectively attached to the apex areas of the upper surface of the light-guide plate  86 , which define the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 , where a distance between the points P 1 , P 2  and P 3  may be 1 m. 
     Similar to the projection  76  of the tenth embodiment (FIG.  24 ), the projection  88  may be formed of a suitable transparent resin material, having an index of refraction different from that of the clad layers of the light-guide plate  86 , so that a part of the fluorescent radiation, generated and trapped by the light-guide plate  86 , is predominantly emitted from the projections  88 . Of course, another part of the fluorescent radiation can be emitted from the peripheral side faces of the light-guide plate  86 , but the fluorescent radiation cannot be substantially emitted from the upper and lower surfaces of the light-guide plate  86 . Accordingly, the three hemispherical projections  88  of the light-guide plate  86 , defining the reference points P 1 , P 2  and P 3  of the standard measurement scale  10 , are conspicuously recorded on a photographed picture. 
     In the eleventh embodiment, each of the hemispherical projections  88  may be formed of a transparent soft plastic material exhibiting softness, so that each projection  88  can be detachably adhered to the light-guide plate  86 , due to the softness of the projection  88 . 
     FIG. 28 conceptually shows a stereo-photogrammetric measurement system, using markers for defining a standard measurement scale, constructed according to the present invention. In this drawing, an intersection point, at which a traffic accident has occurred, is illustrated. Two cameras “L” and “R” are positioned at a suitable location, so as to be spaced apart from each other by a predetermined distance of d. Two respective broken lines, indicated by references “l” and “r”, are optical axes of the cameras “L” and “R”. The camera “L” has an angle of view indicated by reference θ 1 , and the camera “R” has an angle of view indicated by reference θ 2 . 
     As shown in FIG. 28, a photographing area of the camera “L” is defined by the view angle of θ 1 , and a photographing area of the camera “R” is defined by the view angle of θ 2 . The photographing areas of the camera “L” and “R” overlap with each other, as shown by a hatched area in FIG.  28 . This hatched area or overlapped area is used in the stereo-photogrammetric measurement. Then, the standard-scale-defining markers must be positioned within the overlapped area. Namely, as shown in FIG. 28, for example, the standard-scale-defining markers are positioned at locations A and B, and a distance between the markers, at the locations A and B, is measured. 
     FIGS. 29,  30  and  31  show a first embodiment of the marker for defining a standard measurement scale, according to the present invention. The marker comprises a circular-shaped light-guide plate  90  having a diameter of about 100 mm, and a hemispherical projection  92  placed at the center of the light-guide plate  90 . In the stereo-photogrammetric measurement, as shown in FIG. 28, the two markers are prepared, and are positioned at the locations A and B, respectively, such that the hemispherical projection  92  of each marker coincides with an indication marked at the location (A, B). Namely, the hemispherical projection  92  of the marker serves as a reference point for defining the standard measurement scale. 
     As shown in FIG. 31, the light-guide plate  90  is constituted from a core layer  90 A containing fluorescent substances uniformly distributed therein, a first clad layer  90 B formed over an upper surface of the core layer  90 A, and a second clad layer  90 C formed over a lower surface of the core layer  90 A. In this embodiment, the core layer  90 A is made of an acrylic resin material, and the first and second clad layers  90 B and  90 C are made of an acrylic resin material exhibiting an index of refraction smaller than that of the acrylic resin material of the core layer  90 A. Namely, the light-guide plate  90  has the same optical structure as the light-guide plate  42  shown in FIGS. 10 and 11. 
     On the other hand, the hemispherical projection  92  may be formed of a suitable transparent resin material, having an index of refraction different from that of the clad layers  90 B and  90 C of the light-guide plate  90  or that of air, so that a part of the fluorescent radiation, generated and trapped in the core layer  90 A is predominantly emitted from the projection  92 . Note, another part of the fluorescent radiation can be emitted from the peripheral side face of the light-guide plate  90 , but the fluorescent radiation cannot be substantially emitted from the first and second clad layers  90 B and  90 C of the light-guide plate  90 . Accordingly, the hemispherical projection  92  of the marker is conspicuously recorded on a photographed picture. Namely, the designation of the hemispherical projection or reference point  92  of the marker, with a cursor on a TV monitor, can be easily carried out. 
     Further, the marker comprising the light-guide plate  90  can be easily positioned, such that the reference point  92  thereof exactly coincides with the indication marked at the location (A, B) due to the flatness of the marker or light-guide plate  90  and the transparency of the marker per se. 
     In the first embodiment of the maker according to the present invention, the hemispherical projection  92  may be formed of a transparent plastic material exhibiting softness, so that the projections  92  can be detachably adhered to the light-guide plate  90 , due to the softness of the projection.  92 . 
     FIGS. 32 and 33 show a second embodiment of the marker for defining a standard measurement scale, according to the present invention. This marker also comprises a circular-shaped light-guide plate  94  having the same optical structure of the light-guide plate  90  of the first embodiment (FIGS. 29,  30  and  31 ). Namely, as shown in FIG. 33, the light-guide plate  94  is constituted from a core layer  94 A containing fluorescent substances uniformly distributed therein, a first clad layer  94 B, formed over an upper surface of the core layer  94 A, and a second clad layer  94 C, formed over a lower surface of the core layer  94 A. 
     In the second embodiment, the marker features a cone-shaped recess  96 , formed at the center of the upper surface of the light-guide plate  94 . As best shown in FIG. 33, the cone-shaped recess  96  penetrates the core layer  94 A, so that a part of the fluorescent radiation is emitted from the cone-shaped recess  96 . Of course, the cone-shaped recess  96  of the marker serves as a reference point for defining the standard measurement scale. 
     Similar to the first embodiment of the marker (FIGS. 29,  30  and  31 ), the cone-shaped recess  96  of the marker is conspicuously recorded on a photographed picture due to the emission of fluorescent radiation therefrom. Also, the marker comprising the light-guide plate  94  can be easily positioned, such that the reference point  96  thereof exactly coincides with the indication marked at the location (A, B), due to the flatness of the marker or light-guide plate  94  and the transparency of the marker per se. 
     FIGS. 34,  35  and  36  show a third embodiment of the marker for defining a standard measurement scale, according to the present invention. This marker also comprises a circular-shaped light-guide plate  98 , having the same optical structure as the light-guide plate  90  of the first embodiment (FIGS. 29,  30  and  31 ). Namely, as shown in FIG. 36, the light-guide plate  98  is constituted from a core layer  98 A containing fluorescent substances uniformly distributed therein, a first clad layer  98 B, formed over an upper surface of the core layer  98 A, and a second clad layer  98 C, formed over a lower surface of the core layer  98 A. 
     In the third embodiment, as best shown in FIG. 35, the marker features eight V-shaped grooves  100 , formed in an upper surface of the light-guide plate  98 , radially extending from the center  102  thereof. Each of the V-shaped grooves  100  penetrates in the core layer  98 A of the light-guide plate  96 , as shown in FIG. 36, so that a part of the fluorescent radiation is predominantly emitted from the V-shaped grooves  100 . The convergent center  102  of the eight V-shaped grooves  100  serves as a reference point for defining the standard measurement scale. 
     Similar to the first and second embodiments of the marker (FIGS. 29,  30  and  31 ; and FIGS.  32  and  34 ), the V-shaped grooves  100  of the marker are conspicuously recorded on a photographed picture due to the emission of fluorescent radiation therefrom. Also, the marker comprising the light-guide plate  100  can be easily positioned, such that the center or reference point  102  thereof exactly coincides with the indication marked at the location (A, B) due to the flatness of the marker or light-guide plate  98  and the transparency of the marker per se. 
     FIGS. 37 and 38 show a fourth embodiment of the marker for defining a standard measurement scale, according to the present invention. This marker comprises a generally-triangular-pyramidal-shaped optical assembly  104 , constructed from three isosceles-triangular light-guide plate elements  106 , a bottom side of which may have a length of about 100 mm. Each of the light-guide plate elements  106  has the same optical structure as the light-guide plate  90  of the first embodiment (FIGS. 29,  30 , and  31 ). Namely, each of the light-guide plate elements  106  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. 
     The generally-triangular-pyramidal-shaped optical assembly  104  is assembled from the three light-guide plate elements  106  in such a manner that an inner triangular-pyramid space is defined therewithin. As best shown in FIG. 38, two contiguous slanting side faces  106 A of two adjacent light-guide plate elements  106  form a V-shaped trough extending along a corresponding ridgeline of the inner triangular-pyramid space, and an apex  106 B of the inner triangular-pyramidal space serves as a reference point for defining a standard measurement scale. 
     The fluorescent radiation, generated and trapped in the core layer of each light-guide plate element  106 , cannot be substantially emitted from a triangular surface  106 C thereof, but a part of the fluorescent radiation can be emitted from the side faces  106 B thereof. Thus, the V-shaped troughs of the optical assembly  104  are conspicuously recorded on a photographed picture, due to the predominant emission of fluorescent radiation therefrom, whereby the center  106 B of the V-shaped troughs can be easily located from the photographed picture. Also, the marker comprising the optical assembly  104  can be easily positioned, such that the apex or reference point  106 B thereof exactly coincides with the indication marked at the location (A, B) due to the transparency of the marker per se. 
     FIGS. 39,  40 , and  41  show a fifth embodiment of the marker for defining a standard measurement scale, according to the present invention. This marker comprises a generally-quadrilateral-pyramidal-shaped optical assembly  108  constructed from two isosceles-triangular light-guide plate elements  110  and  112 , a bottom side of which may have a length of about 100 mm. Each of the light-guide plate elements  110  and  112  has the same optical structure as the light-guide plate  90  of the first embodiment (FIGS. 29,  30 , and  31 ). Namely, each of the light-guide plate elements  110  and  112  is constituted from a core layer containing fluorescent substances uniformly distributed therein, a first clad layer formed over an upper surface of the core layer, and a second clad layer formed over a lower surface of the core layer. 
     The optical assembly  108  is assembled from the two light-guide plate elements  110  and  112  into the generally-quadrilateral-pyramidal-shape as shown FIGS. 39 and 40. To this end, as shown in FIG. 41, the light-guide plate element  110  has a lower half slit  110 A, formed therein and extended from the center of the bottom side thereof to the middle position of the height thereof, and the light-guide plate element  112  has an upper half slit  112 A, formed therein and extended from the apex thereof to the middle position of the height thereof. Thus, the generally-quadrilateral-pyramidal-shaped optical assembly  108  is obtained from the light-guide plate elements  110  and  112  by crosswisely interlinking them via the lower and upper half slits  110 A and  112 A. Note, of course, a width of each slit  110 A and  112 A is equal to the thickness of the light-guide plate element  110 ,  112 . 
     As is apparent from FIGS. 39,  40  and  41 , an apex of the light-guide plate element  110  is shaped as a small square area  114 , which serves as a reference point for defining a standard measurement scale. 
     A part of the fluorescent radiation, generated and trapped in the core layer of each light-guide plate element ( 110 ,  112 ), are predominantly emitted from both slanting side faces ( 110 B,  112 B) thereof, but the fluorescent radiation cannot be substantially emitted from both triangular surfaces ( 110 C,  112 C) thereof. Thus, the slanting side faces  110 B and  110 B of the light-guide plate elements  110  and  112  are conspicuously recorded on a photographed picture, whereby the apex or small squar area  114  of the optical assembly  108  can be easily located from the photographed picture. Also, the marker comprising the optical assembly  108  can be easily positioned, such that the apex or reference point  114  thereof exactly coincides with the indication marked at the location (A, B) due to the transparency of the marker per se. 
     In the fifth embodiment of the marker shown in FIGS. 39,  40  and  41 , it is preferable to detachably and crosswisely interlink the light-guide plate elements  110  and  112 , because the disassembled light-guide plate elements  110  and  112  can be compactly stored, and can be carried without bulkiness. 
     Finally, it will be understood by those skilled in the art that the foregoing description is of preferred embodiments of the device and assembly, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof. 
     The present disclosure relates to subject matter contained in Japanese Patent Applications No. 8-310029 (filed on Nov. 6, 1996), No. 8-310030 (filed on Nov. 6, 1996), and No. 9-276546 (filed on Sep. 24, 1997) which are expressly incorporated herein, by reference, in their entireties.