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 of 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 receivers are incident upon the splitter surface of the beam splitter at different incident angles. An optical element is provided in a light path of at least the larger quantity of beams of the object-carrying light, among those to be received by the pair of light receivers so as to reduce the quantity of light to be transmitted therethrough.

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 an optical element that is provided between the splitter surface and the pair of light receiving elements, wherein one of the object-carrying light beams having a larger quantity is reduced in the quantity of light to be transmitted through the optical element, to thereby reduce the difference in quantity of light between each object-carrying light beam respectively received by the pair of light receiving elements. 
     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 which splits object-carrying light transmitted through the objective optical system of the telescopic optical system via a splitter surface thereof; 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; an optical element provided between the beam splitter and the auto-focus sensor module in a light path of at least the larger quantity of beams of the object-carrying light, to be received by the optical sensors so as to reduce the quantity of light to be transmitted therethrough. 
     Preferably, the optical element includes an optical element having a higher transmittance disposed in the light path of a smaller quantity of light, and an optical element having a low transmittance disposed in the light path of the larger quantity of light. 
     Preferably, the splitter surface is made of a multi-layered dielectric film. 
     It is possible for the optical element, that reduces the quantity of light transmitted therethrough, to have a transmittance distribution which changes continuously or stepwise from a low transmittance to a high transmittance. 
     Preferably, the optical element to reduce the quantity of light transmitted therethrough has a uniform transmittance distribution. 
     Preferably, the optical element to reduce the quantity of light transmitted therethrough includes an ND filter. 
     Preferably, the splitter surface is defined by a reflection surface of an optical element which constitutes an image erecting optical system provided in the telescopic optical system. 
     Preferably, the optical element which constitutes an image erecting optical system is a Porro prism. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No.10-126231 (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 an ND filter, 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. 6 is an end view of an ND filter, viewed from an arrow L in FIG. 6A; 
     FIG. 7 is a perspective view of a Porro prism shown in FIG.  6 A. 
     FIG. 8 is a diagram illustrating a stepped transmittance distribution of an optical element utilized in the present invention; and 
     FIG. 9 is a diagram illustrating a continuous transmittance distribution of an optical element utilized in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. A telescope  3  of the total station  1  forms an erect image of an object on a focusing plate  5  via an objective optical system which includes an objective lens  8 , a focusing lens  4 , and a Porro prism  6 . An operator can view the object image formed on the focusing plate  5  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  19  which is optically equivalent to the focusing plate  5 . 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) device. 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  20  and a pair of separator lenses  21  are provided in this order optically behind the reference focusing surface  19 . A pair of CCD sensors  15  are provided behind the corresponding separator lenses  21 . The light transmitted through the condenser lens  20  is split by the separator lenses  21  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  15  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  19 , i.e., when the image of a target is formed correctly on the reference focusing surface  19  represented by the principal rays  9  and  10 ; when the image is formed in front of the reference focusing surface  19  represented by the rays  9   f  and  10   f  (front focus); or when the image is formed behind the reference focusing surface  19  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 device to which the output of the CCD sensors  15  are input, amplifies the output by a preamplifier and performs the calculation by a calculation circuit to detect an “in-focus”, “out-of-focus”, “front focus” or “rear focus”. Consequently, the amount of defocus on the reference focusing surface  19  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  23  is attached to a second reflection surface  22   b  of the Porro prism  6 , so that the boundary surface therebetween defines the coated splitter surface  11  of a beam splitter (FIG.  5 ). 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  20  connecting the centers of the splitter surface  11  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 (see FIG.  4 A). 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  10  transmitted through the splitter surface  11 . As shown in FIG. 4A, the beams of light  9  and  10  are incident upon the splitter surface  11  at different incident angles α and γ. 
     In general, the coated splitter surface  11  is made of a multi-layered dielectric film having less absorption of light, the transmittance and reflectance thereof being determined (designed) based on the reference incident angle (45° ) of light incident upon the center portion thereof. However, the transmittance of the dielectric film varies depending on the incident angle. Therefore, if there is no difference in the quantity of light between the beams  9  and  10  before the splitter surface  11 , after the beams  9  and  10  are transmitted through the splitter surface  11 , a difference in the quantity of light between the beams  9  and  10  occurs. To prevent this problem, in the illustrated embodiment, two ND filters  17  and  18  having different transmittances are inserted in the light paths of the beams  9  and  10 . The light having a higher quantity level passes through the ND filter having a lower transmittance and the light having a lower quantity level passes through the ND filter having a higher transmittance (FIG.  4 A). Consequently, the difference in the quantity of light between the beams  9  and  10  incident upon the CCD sensors  15  is reduced, so that the output difference of the auto-focus sensor module can be minimized to thereby eliminate the above-mentioned problem. 
     In other words, the difference in the quantity of light between the beams  9  and  10  transmitted through the splitter surface  11  is reduced when the beams pass through the ND filters  17  and  18 , and thereafter the beams  9  and  10  are received by the auto-focus sensor module  7 . Thus, a precise focusing operation can be achieved. 
     Although the two ND filters are provided in the first embodiment, it is possible to provide a single ND filter which is provided with two transparent portions having different transmittances. 
     Each ND filter  17  and  18  has a uniform transmittance distribution. In practice, it is sufficient for the transmittance distribution to be uniform. However, to enhance the precision, the ND filters can be provided with a non-uniform transmittance distribution which changes stepwise as shown in FIG. 8, or continuously as shown in FIG. 9; in accordance with the incident position and incident angle. Moreover, it is also possible to provide an ND filter only for the light having a larger quantity. 
     Although the second reflection surface  22   b  of the Porro prism  6  defines the splitter surface in the first embodiment, it is possible to define the splitter surface by the first reflection surface  22   a , the third reflection surface  22   c , or the fourth reflection surface  22   d  thereof. 
     FIGS. 6A,  6 B and  7  show the second embodiment of the present invention. The beam splitter  14  is provided on the front side of the Porro prism  6 , i.e., on the objective lens side as clearly shown in FIG. 7, so that the light is split by the reflection at the coated splitter surface  11   a  in the right direction in FIG. 6. A pair of CCD sensors  15  are disposed in a plane normal to the optical axis of the condenser lens  20  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 (see FIG.  6 A). 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 reflectance of the splitter surface at the different incident points are different, due to 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, two ND filters  17  and  18  having different transmittances are inserted in the light paths of the beams  12  and  13  in such a way that the larger quantity of light passes through the ND filter having a lower transmittance and the smaller quantity of light passes through the ND filter having a higher transmittance. Consequently, the difference in the quantity of light transmitted through the ND filters is made small to thereby eliminate the above-mentioned problem. 
     In other words, 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  6  to form an erect image on the focusing plate  5 . The reflected light passes through the ND filters  17  and  18  by which the difference in the quantity of light between the beams  12  and  13  is reduced and reaches the auto-focus sensor module  7  to carry out a precise focusing operation. 
     As in the first embodiment, each of the ND filters  17  and  18  are provided to have a uniform transmittance distribution depending on the incident angle of each beam. 
     In practice, it is sufficient for the transmittance distribution to be uniform. However, to enhance the precision, the ND filters can be provided with a non-uniform transmittance distribution which changes stepwise or continuously, in accordance with the incident position and incident angle. Moreover, it is also possible to provide an ND filter only for the light having a larger quantity. 
     Although in the first and second embodiments, the auto-focus sensor module  7  and the splitter surface  11  or  11   a  a are located at specific positions, the arrangement is not limited thereto. The invention can be generally applied to an arrangement in which the beams of light to be received by a pair of CCD sensors are incident upon the splitter surface  11  or  11   a  at different incident angles. 
     Although only one focus detection optical system is provided in the first or second embodiment, the present invention can be applied to a multi-point AF system having a plurality of focus detection optical systems. 
     As can be understood from the above discussion, since optical elements such as ND filters having different transmittances, are inserted in the light paths of beams to be received by a pair of light receiving elements provided within the focus detection optical system, there is little or no difference in the quantity of light to be received 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 a correction coefficient, the optical system is less influenced by electrical noise. Thus, a precise automatic focusing operation can be achieved.