Abstract:
A beam splitting optical system for an automatic focusing apparatus includes a telescopic system having an objective optical system and a viewing optical system, a beam splitter which splits object-carrying light transmitted through the objective optical system from the telescopic optical system by a splitter surface, and a focus detection optical system having a pair of light receivers which receive beams of the object-carrying light split by the splitter surface. The focus detection optical system is arranged so that beams of the object-carrying light to be respectively received by the light receiver are incident upon the splitter surface of the beam splitter at different incident angles. The transmittance and reflectance of the splitter surface of the beam splitter is not uniform in distribution and changes depending on the incident angle of the object-carrying light incident thereupon.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a beam splitting optical system for an auto-focus sensor, which can be utilized in an optical instrument, and in particular with a surveying instrument. 
     2. Description of the Related Art 
     In a conventional automatic focusing device for a surveying instrument having a collimator telescope (such as a total station), a light path of a focus detection optical system is split from a light path of a collimating optical system by a beam splitting optical system to detect the focus state on a surface (referred to as a reference focusing surface) which is optically equivalent to the focusing surface of the collimating optical system by means of a phase difference detection type auto-focus sensor module having a pair of CCD sensors, in order to calculate the amount of defocus of a focusing lens. Consequently, the focusing lens is moved to an in-focus position in accordance with the defocus amount to complete an automatic focusing (AF) operation. The principle of the AF function in which the phase difference is detected is known in the art, and is used in an AF single lens reflex camera. 
     In a conventional beam splitter system for an auto-focus sensor, the beam splitter and the auto-focus sensor are arranged so that the beams of light received by the pair of CCD sensors are made incident upon a coated splitter surface of the beam splitting optical system at different incident angles. In this arrangement, since the transmittance of the splitter surface which is made of a multi-layered dielectric film varies depending on the incident angle, there is a difference in the level between the quantities of light received by the pair of CCD sensors. To prevent a level difference occurring, it is necessary to arrange the optical elements so that the incident angles of the beams incident upon the split surface are identical. This reduces the freedom of design of the layout of the optical elements, hindering any possible enhancement in operational efficiency, hindering miniaturization and reduces the weight of the optical system. 
     The difference in the quantity of light caused due to the above-mentioned arrangement is usually corrected using a correction coefficient in the determination of the focus during the automatic focusing operation. However, if the difference is large or if electric noise is produced in the auto-focus sensor itself, the noise is increased according to the correction coefficient, thus resulting in a failure to perform a precise auto-focusing operation. This tends to occur when the quantity of light is small, for example, at dusk. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to eliminate the above-mentioned drawbacks of the prior art, by reducing the difference in the quantity of light received by a pair of CCD sensors within the auto-focus sensor to thereby reduce the restriction of the arrangement of the components in the beam splitting optical system. 
     In order to achieve the above-mentioned aim, there is provided a beam splitting optical system for an automatic focusing apparatus including: a telescopic system having an objective optical system and a viewing optical system; a beam splitter provided between the objective optical system and the viewing optical system, the beam splitter being provided with a splitter surface to split object-carrying light transmitted through the objective optical system; a focus detection optical system having a pair of light receiving elements which respectively receive an object-carrying light beam split by and transmitted through the splitter surface of the beam splitter, the focus detection optical system being arranged so that the object-carrying light beam to be respectively received by the respective light receiving elements is incident upon the splitter surface at different incident angles; and wherein the transmittance and reflectance of the splitter surface of the beam splitter is set so that the transmittance and reflectance changes depending on the incident angle of the object-carrying light incident thereupon to compensate the difference of distribution quantity of the object-carrying light beam that is passed through the splitter surface, said difference being caused due to the difference of the incident angle of the object-carrying light beam incident upon the splitter surface. 
     According to another aspect of the present invention, there is provided a beam splitting optical system for an automatic focusing apparatus including: a telescopic system having an objective optical system and a viewing optical system through which an object image formed on a predetermined focusing surface by the objective optical system can be viewed; a beam splitter provided between the objective optical system and the viewing optical system, the beam splitter being provided with a splitter surface to split object-carrying light transmitted through the objective optical system; and an auto-focus sensor module which detects a focus state on a reference focusing surface which is optically equivalent to the predetermined focusing surface, on the light path of the object-carrying light split by the beam splitter, the auto-focus sensor module being provided with a pair of optical sensors, the auto-focus sensor module and the beam splitter being arranged so that an object-carrying light beam to be respectively received by the pair of optical sensors is incident upon the splitter surface of the beam splitter at different incident angles; wherein the transmittance and reflectance of the splitter surface of the beam splitter is set so that the transmittance and reflectance changes depending on the incident angle of the object-carrying light incident thereupon to compensate the difference of distribution quantity of the object-carrying light beam that is passed through the splitter surface, said difference being caused due to the difference of the incident angle of the object-carrying light beam incident upon the splitter surface. 
     Preferably, the transmittance and reflectance of the splitter surface is determined so that the quantities of the object-carrying light beams transmitted therethrough or reflected thereby are substantially identical. 
     According to the above mentioned structures, since the difference of the distribution quantity of the object-carrying light beam that is passed through the splitter surface is compensated, a precise focusing operation can be carried out. 
     Preferably, the splitter surface includes a multi-layered dielectric film. 
     Preferably, the splitter surface is divided into at least two areas which have different transmittance and reflectance. Preferably these two areas are multi-layered dielectric films that have different film structures depending on the transmittance and reflectance thereof. 
     Preferably, the splitter surface includes a reflection surface of an optical element which constitutes an image erecting optical system provided in the telescopic optical system. Preferably, the optical element is a Porro prism. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-126233 (filed on May 8, 1998) which is expressly incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below in detail with reference to the accompanying drawings in which: 
     FIG. 1 is a partially sectioned side view of a surveying instrument total station according to an embodiment of the present invention; 
     FIG. 2 is a front elevational view of a total station shown in FIG. 1; 
     FIG. 3 is a conceptual view of the principle of an auto-focus system; 
     FIG. 4A is an enlarged front view of a main portion of a total station according to a first embodiment of the present invention; and FIG. 4B is an end view of a coated splitter surface, viewed from an arrow L in FIG. 4A; 
     FIG. 5 is a perspective view of a Porro prism shown in FIG. 4A; 
     FIG. 6A is a top view of a main portion of a second embodiment of the present invention; and FIG. 6B is an end view of a coated splitter surface, viewed from an arrow L in FIG. 6A; 
     FIG. 7 is a perspective view of a Porro prism shown in FIG. 6A; 
     FIG. 8 is a graph showing transmittance and reflectance for right-side light incident at an incident angle of α; and 
     FIG. 9 is a graph showing transmittance and reflectance for left-side light incident at an incident angle of γ. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention will be discussed below. 
     FIGS. 1 and 2 are side and front views of a surveying instrument total station, respectively. An image of an object placed within the collimation field of a telescope  3  of the total station  1  is formed as an erect image on a focusing plate  5  on a predetermined focusing surface by an objective lens  8  which constitutes an objective optical system, a focusing lens  4 , and a Porro prism  6 . An operator can view the object image formed on the focusing plate through an eyepiece (ocular lens) which constitutes a viewing optical system. 
     FIGS. 4A and 4B show the positional relationship between the Porro prism  6  and an auto-focus sensor module  7 . The auto-focus sensor module  7  is provided on the light path of the focus detection optical system which is split from the light path of the objective optical system by a coated splitter surface  11  of the Porro prism  6  to detect the focus state (amount of defocus) on a reference focusing surface  18   18  which is optically equivalent to the focusing plate  5 . Namely, object-carrying light transmitted through the objective lens  8  is split into light which is made incident upon the focusing plate  5  and focus detection light which is made incident upon the auto-focus sensor module  7 . The auto-focus sensor module  7  receives the object-carrying light via a pair of CCD sensors and sends electric signals to a focus state calculation (defocus calculation) portion (not shown). The auto-focus sensor module  7  is well known in the art. 
     FIG. 3 shows the main concept of the focus detection of the auto-focus sensor module  7  by way of example. In FIG. 3, a condenser lens  19  and a pair of separator lenses are provided behind the reference focusing surface  18 . A pair of CCD sensors  15  are provided behind the corresponding separator lenses  20 . The light transmitted through the condenser lens  19  is split by the separator lenses and the split beams are received by the respective CCD sensors  15  to form object images. Principal rays of the beams which form the object images on the sensors are indicated by the numerals  9  and  10 . 
     The image formation position of the CCD sensors  15  at which the object images are formed varies depending on the position of the image on the reference focusing surface  18 , i.e., when the image of a target is formed correctly on the reference focusing surface  18  represented by the principal rays  9  and  10 ; when the image is formed in front of the reference focusing plane  19  represented by the rays  9   f  and  10   f  (front focus); or when the image is formed behind the reference focusing surface  18  represented by the rays  9   r  and  10   r  (rear focus), as shown in FIG.  3 . The deviation from the focus position is detected based on the distance between the object images formed on the CCD sensors  15 . The focus state calculation portion to which the output of the CCD sensors  15  are input, amplifies the output by a preamplifier (not shown) and performs the calculation by a calculation circuit (not shown) to detect an “in-focus”, “out-of-focus”, “front focus” or “rear focus”. Consequently, the amount of defocus on the reference focusing surface  18  and the displacement of the focusing lens  4  necessary to move the same to the focal position are determined. 
     In the first embodiment, as shown in FIG. 4A which is an enlarged front view of the Porro prism  6 , the auto-focus sensor module  7  is located below the Porro prism  6 , and a prism  22  is attached to a second reflection surface  21   b  of the Porro prism  6 , so that the boundary surface therebetween defines the coated splitter surface  11  of a beam splitter (FIG.  5 ). Namely, in this embodiment, light incident upon the Porro prism  6  is split by the splitter surface  11  into reflected light and transmitted light. Thereafter, the reflected light forms an erect image on the focusing plate  5 , and the transmitted light reaches the auto-focus sensor module  7  and forms object images on the pair of CCD sensors  15 . The CCD sensors  15  are disposed in a plane normal to the optical axis of the condenser lens  17  connecting the centers of the splitter surface  11  and the auto-focus sensor module  7  and are juxtaposed in the lateral direction in FIG. 4, i.e., in the lateral direction in the collimation field. This arrangement is particularly advantageous when an object, for which the surveying instrument is to be collimated, is a vertically elongated member such as a pole. The CCD sensors  15  receive the beams  9  and transmitted through the splitter surface  11 . As shown in FIG. 4A, the beams of light  9  and are incident upon the splitter surface  11  at different incident angles α and γ. 
     In general, the coated splitter surface  11  is defined by a multi-layered dielectric film coated thereon and having less absorption of light, the transmittance and reflectance thereof being determined based on the incident angle (45°) of light incident upon the center portion thereof. The transmittance of the dielectric film varies depending on the incident angle. Therefore, there is no difference in the quantity of light between the beams  9  and  10  before the splitter surface  11 , but there is a difference in the quantity of light between the beams  9  and  10  that are transmitted through the splitter surface  11 . To prevent this problem, in this embodiment, the splitter surface  11  coated with the multi-layered dielectric film is divided into two halves, i.e., right and left coated areas  16  and  17  with respect to the center line thereof. The optical properties of the coated areas  16  and  17  are designed in accordance with the incident angles α and γ of the right and left beams of light  9  and  10 . 
     Numerical data of the multi-layered dielectric film used in the first embodiment is shown in Tables 1 and 2 below. Tables 1 and 2 show the structure of the dielectric film which meets the incident angles α (47.5°) and γ (42.5°) of the right-side light and left-side light. In Tables 1 and 2, the materials A, B, C have different refractive indexes and form layers having thicknesses as indicated in the Tables between the BK7 (optical glass), i.e., the prism  22  and an adhesive layer. The change in the transmittance and reflectance depending on the wavelength of the incident light at the incident angles α and γ is shown in FIGS. 8 and 9, respectively. The film structure is an example, and the invention is not limited thereto. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                 Refractive 
                   
               
               
                   
                 BK7 (Optical 
                 Index 
                 Optical 
               
               
                   
                 Glass) 
                 1.5181 
                 thickness (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 A 
                 2.3470 
                 231.256 
               
               
                   
                 2 
                 B 
                 1.4693 
                 261.927 
               
               
                   
                 3 
                 A 
                 2.3470 
                 175.5971 
               
               
                   
                 4 
                 B 
                 1.4693 
                 281.899 
               
               
                   
                 5 
                 A 
                 2.3470 
                 176.4881 
               
               
                   
                 6 
                 B 
                 1.4693 
                 228.8399 
               
               
                   
                 7 
                 A 
                 2.3470 
                 177.634 
               
               
                   
                 8 
                 B 
                 1.4693 
                 285.815 
               
               
                   
                 9 
                 A 
                 2.3470 
                 151.225 
               
               
                   
                 10 
                 B 
                 1.4693 
                 200.995 
               
               
                   
                 11 
                 A 
                 2.3470 
                 124.105 
               
               
                   
                 12 
                 B 
                 1.4693 
                 186.2864 
               
               
                   
                 13 
                 A 
                 2.3470 
                 125.8359 
               
               
                   
                 14 
                 B 
                 1.4693 
                 197.7924 
               
               
                   
                 15 
                 A 
                 2.3470 
                 134.431 
               
               
                   
                 16 
                 C 
                 1.6400 
                 325.351 
               
             
          
           
               
                   
                 Adhesive Layer 
                 1.4900 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                 Refractive 
                   
               
               
                   
                 BK7 (Optical 
                 Index 
                 Optical 
               
               
                   
                 Glass) 
                 1.5181 
                 thickness (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 A 
                 2.3470 
                 253.369 
               
               
                   
                 2 
                 B 
                 1.4693 
                 191.139 
               
               
                   
                 3 
                 A 
                 2.3470 
                 249.773 
               
               
                   
                 4 
                 B 
                 1.4693 
                 167.027 
               
               
                   
                 5 
                 A 
                 2.3470 
                 171.428 
               
               
                   
                 6 
                 B 
                 1.4693 
                 304.890 
               
               
                   
                 7 
                 A 
                 2.3470 
                 136.285 
               
               
                   
                 8 
                 B 
                 1.4693 
                 157.466 
               
               
                   
                 9 
                 A 
                 2.3470 
                 183.828 
               
               
                   
                 10 
                 B 
                 1.4693 
                 76.190 
               
               
                   
                 11 
                 A 
                 2.3470 
                 192.053 
               
               
                   
                 12 
                 B 
                 1.4693 
                 124.447 
               
               
                   
                 13 
                 A 
                 2.3470 
                 121.583 
               
               
                   
                 14 
                 C 
                 1.6400 
                 643.913 
               
             
          
           
               
                   
                 Adhesive Layer 
                 1.4900 
               
               
                   
                   
               
             
          
         
       
     
     With the divided areas of the coated splitter surface, the difference in the quantity of light between the beams  9  and  10  received by the CCD sensors  15  is reduced, so that the output difference of the auto-focus sensor module  7  can be reduced. 
     Namely, in the first embodiment, the difference in the quantity of light between the beams  9  and  10  is minimized when the beams pass through the corresponding divided areas of the splitter surface coated with the multi-layered dielectric film and thereafter, the beams reach the auto-focus sensor module  7 . Thus, a precise focusing operation can be achieved. 
     The present invention can be equally applied to an alternative arrangement in which the splitter surface  11  is defined by the first reflection surface  21   a,  the third reflection surface  21   c,  or the fourth reflection surface  21   d  of the Porro prism  6 . 
     In the second embodiment, the beam splitter  14  is provided on the front side of the Porro prism  6 , i.e., on the objective lens side (see FIG.  7 ), as can be seen in FIG.  6  which shows a top view of the Porro prism  6  and its surroundings. A pair of CCD sensors  15  are disposed in a plane normal to the optical axis of the condenser lens  19  connecting the centers of the splitter surface  11   a  and the auto-focus sensor module  7  and are juxtaposed in the lateral direction; i.e., in the lateral direction with respect to the field of view. In this embodiment, the beams  12  and  13  are incident upon the coated splitter surface  11   a  at different incident angles α and γ, so that the reflectances of the splitter surface  11   a  at the different incident points are different, due to the design, or the angle-dependency of, the multi-layered dielectric film. Consequently, there is a difference in the quantity of light between the beams  12  and  13  received by the CCD sensors  15 . 
     To prevent this problem, the splitter surface  11  coated with the multi-layered dielectric film is divided into two halves, i.e., right and left areas  16   a  and  17   a  separated from one another with respect to the center line thereof. The optical properties of the coated areas  16   a  and  17   a  are designed in accordance with the incident angles α and γ of the right and left beams  9  and  10 . The structure of the dielectric film in the second embodiment is the same as that in the first embodiment mentioned above, but is not limited thereto. Consequently, the difference in the quantity of light received by the CCD sensors  15  within the auto-focus sensor module  7  is minimized. 
     Namely, in the second embodiment, the light entering the optical system through the objective lens  8  is transmitted through the focusing lens  4  and is split by the beam splitter  14  located in front of the Porro prism  6  into transmitted light and reflected light. The transmitted light is incident upon the Porro prism to form an erect image on the focusing plate  5 . The beams of the reflected light are reflected by the corresponding right and left areas  16   a  and  17   a  of the splitter surface. Consequently, the difference in the quantity of light between the beams  12  and  13  is reduced and thereafter, the beams reach the auto-focus sensor module  7  to carry out a precise focusing operation. 
     The invention can be generally applied to an arrangement in which the beam splitter  14  and the auto-focus sensor module  7  are arranged so that the beams of light  12  and  13  to be received by a pair of CCD sensors  15  are incident upon the splitter surface  11   a  at different incident angles. 
     As can be understood from the above discussion, since the divided areas of the splitter surface is coated with a multi-layered dielectric film corresponding to the incident angle of each incident beam to be received by a pair of light receiving elements within the focus detection optical system. Therefore, there is little or no difference in the quantity of light to be received by the light receiving elements even in a conventional layout of the optical components in which the difference in the quantity of light could be otherwise produced. Consequently, the freedom of layout of the components can be enhanced, thus resulting in miniaturization, reduction of weight, and improvement of operability. 
     Moreover, in comparison with the correction of the difference in the quantity of light, using the correction coefficient, the optical system is less influenced by electrical noise. Thus, a precise automatic focusing operation can be achieved.