Patent Publication Number: US-6339499-B1

Title: Beam splitter for automatic focusing device

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 which splits object-carrying light transmitted through the objective optical system from the telescopic optical system via a splitter surface; and 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 The splitter surface of the beam splitter is coated with a thin metal coating. As the angle-dependency of a metal coating is low, even if the incident angle of the object-carrying light beam on the splitter surface differs, the difference in the amount of light to be received by the AF sensor is minimal. 
     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 via the objective optical system can be viewed; a beam splitter which splits object-carrying light transmitted through the objective optical system from the telescopic optical system via a splitter surface; 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. The splitter surface of the beam splitter is coated with a thin metal coating. 
     Preferably, the thin metal coating includes aluminum, silver, gold, nickel or chrome. The thickness of the metal coating is determined according to the desired amount of the transmittance and the type of the metal used. 
     Preferably, the splitter surface is defined by one of a plurality of reflection surfaces 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. 10-126232 (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. 4 is an enlarged front view of a main portion of a total station according to a first embodiment of the present invention; 
     FIG. 5 is a perspective view of a Porro prism shown in FIG. 4; 
     FIG. 6 is a top view of a main portion of a second embodiment of a total station according to the present invention; and 
     FIG. 7 is a perspective view of a Porro prism shown in FIG.  6 . 
    
    
     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. 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  5  through an eyepiece (ocular lens) which constitutes a viewing optical system. 
     FIG. 4 shows 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 a 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  16  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 autofocus 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  17  and a pair of separator lenses  18  are provided behind the reference focusing surface  16 . A pair of CCD sensors  15  are provided behind the corresponding separator lenses  18 . The light transmitted through the condenser lens  17  is split by the separator lenses  18  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  16 , i.e., when the image of a target is formed correctly on the reference focusing surface  16  represented by the principal rays  9  and  10 ; when the image is formed in front of the reference focusing surface  16  represented by the rays  9   f  and  10   f  (front focus); or when the image is formed behind the reference focusing surface  16  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  15  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  16  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. 4 which is an enlarged front view of a main part of a total station  1 , the auto-focus sensor module  7  is located below the Porro prism  6 , and a prism  20  is closely attached to a second reflection surface  19   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 . 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  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 the related art, 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 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  and are received by the CCD sensors  15 . To solve this problem, the splitter surface  11  is coated with a thin metal coating which has less angle-dependency so as to reduce the difference in the transmittance depending on the incident angle, so that the difference in the quantity of light received by the pair of CCD sensors  15  can be minimized. The metal coating for the splitter surface  11  can be selected from, for example, aluminum, silver, gold, nickel or chrome. The metal coating used in this embodiment is a chrome surface having a thickness of about 20 nm with a transmittance-reflectance ratio of 3:7. 
     Namely, in this embodiment, if beams of light are incident upon the splitter surface at different incident angles, the difference in the quantity of light to be received by the pair of CCD sensors  15  is reduced since the splitter surface has less angle-dependency. Thus, a precise focusing operation can be achieved. 
     The present invention can be equally applied to an alternative arrangement in which the splitter surf ace is defined by the first reflection surface  19   a , the third reflection surface  19   c , or the fourth reflection surface  19   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  17  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 reflectance 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   a  is coated with a thin metal coating having a low angle-dependency. Consequently, the difference in the reflectance depending on the incident angle is minimized, so that the difference in the quantity of light received by the CCD sensors  15  can also be minimized. Likewise with the first embodiment, the metal coating used in this embodiment is also a chrome surface having a thickness of about 20 nm with a transmittance-reflectance ratio of 3:7. 
     Namely, in the second embodiment, the splitter surface  11   a  is defined by a thin metal coating having a low angle-dependency, the difference in the quantity of light of the beams  12  and  13  reflected by the splitter surface  11  and reaching the auto-focus sensor module  7  is minimized, thus resulting in a precise focusing operation. 
     As can be understood from the above discussion, since the splitter surface of the beam splitting optical system which splits the object-carrying light in the direction toward the focus detection optical system is coated with a thin metal coating having a small angle-dependency, 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 a 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.