Abstract:
This invention allows photography even when distance measurement is impossible, reduces the number of out-of-focus photographs, and reduces the cost of an apparatus. In this distance measurement apparatus, an object is illuminated with an auxiliary light-emitting unit, and light reflected by the object and transmitted through a pair of optical units spaced apart at a predetermined interval is received by a light-receiving unit. The light-receiving unit outputs two image signals to an arithmetic unit. In the first distance measurement mode, the arithmetic unit and a control unit calculate object distance on the basis of the relative image distance between the two image signals. Also, in the second distance measurement mode, the arithmetic unit and the control unit calculate object distance on the basis of the difference between the light amounts from the object in the illuminated and non-illuminated states with the auxiliary light-emitting unit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a distance measurement apparatus for automatically performing focus control of a camera in accordance with the distance to an object to be photographed. More particularly, the present invention relates to a method that can cope when distance measurement is impossible in a distance measurement apparatus for detecting object distance by obtaining the relative image distance between the first and second object images formed by a pair of lenses spaced apart at a predetermined interval. 
     In a conventional camera using a passive AF (Auto Focus) function, when distance measurement is impossible, the shutter is locked (kept inoperative) or the focusing lens is extended to a predetermined position regardless of the object distance, and then the shutter is operated (actuated). 
     As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-84231, there is also known a method of extending the focusing lens to infinity for a bright object not requiring use of an electronic flash, and to the position to which light from the electronic flash can reach for a dark object requiring use of the electronic flash. 
     Jpn. Pat. Appln. KOKAI Publication Nos. 64-78205 and 64-78206 disclose techniques for measuring the distance to a low-luminance, low-contrast object by phase difference detection using CCD output data of only auxiliary light. 
     The conventional techniques except lock processing in all the prior arts described above extend the focusing lens regardless of the object distance and photograph, thereby resulting in a large number of out-of-focus photographs. These prior arts were primarily devised to eliminate the confusion and dissatisfaction experienced when the user cannot take photographs. A large number of out-of-focus photographs, however, do not mean that this objective has satisfactorily been achieved. How to reduce this large number of out-of-focus photographs is still left unsolved. 
     The techniques disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 64-78205 and 64-78206 take a long time to acquire (integrate) data enough to operate correlation for CCD sensor elements in the ON state of auxiliary light. In addition, the complexity of distance measurement operation requires a high-speed arithmetic means (CPU), resulting in a high-cost apparatus. 
     BRIEF SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a distance measurement apparatus capable of photography even when distance measurement cannot be done, reducing the number of out-of-focus photographs, and preventing an increase in apparatus cost. 
     It is the first object of the present invention is to provide a distance measurement apparatus comprising: 
     auxiliary light means for illuminating an object to be photographed; 
     a pair of optical means placed at a predetermined interval; 
     light-receiving means for receiving light from the object through the optical means and outputting two image signals; 
     first distance measurement means for calculating an object distance on the basis of a relative image distance between the two image signals; and 
     second distance measurement means for calculating the object distance on the basis of a difference between light amounts from the object in illuminated and non-illuminated states with the auxiliary light means. 
     It is the second object of the present invention to provide a distance measurement apparatus comprising: 
     an auxiliary light unit for illuminating an object to be photographed; 
     a pair of optical units placed at a predetermined interval; 
     a light-receiving unit for receiving light from the object through the optical units and outputting two image signals; 
     a first distance measurement unit for calculating an object distance on the basis of a relative image distance between the two image signals; 
     a second distance measurement unit for calculating the object distance on the basis of a difference between light amounts from the object in illuminated and non-illuminated states with the auxiliary light unit; and 
     a control unit for determining reliability of the object distance calculated by the first distance measurement unit, and operating the second distance measurement unit upon determining that the object distance calculated by the first distance measurement unit is not reliable. 
     It is the third object of the present invention to provide a distance measurement apparatus comprising: 
     auxiliary light means for illuminating an object to be photographed; 
     a pair of optical means placed at a predetermined interval; 
     light-receiving means for receiving light from the object through the optical means and outputting two image signals; 
     first distance measurement mode arithmetic means for calculating a first distance measurement mode, the first distance measurement mode arithmetic means including first arithmetic means for obtaining a relative image distance of the two image signals on the basis of outputs from the optical means and second arithmetic means for calculating the object distance on the basis of the relative image distance; 
     second distance measurement mode arithmetic means for calculating a second distance measurement mode, the second distance measurement mode arithmetic means including third arithmetic means for obtaining a light amount from the object, when only the auxiliary light means is used as an illumination light source for the object, in accordance with a difference of light amounts from the object in illuminated and non-illuminated states with the auxiliary light means, and fourth arithmetic means for obtaining the object distance on the basis of the light amount obtained by the third arithmetic means; and 
     control means for performing distance measurement in the first distance measurement mode, and performing distance measurement in the second distance measurement mode when the first distance measurement mode is incapable of distance measurement. 
     It is the fourth object of the present invention to provide a distance measurement apparatus comprising: 
     first distance measurement means for obtaining an object distance by a phase difference detection method; 
     second distance measurement means for obtaining an object distance upon detection of an auxiliary light amount; and 
     control means for actuating the second distance measurement means when the first distance measurement means is incapable of distance measurement. 
     It is the fifth object of the present invention to provide a distance measurement apparatus comprising: 
     auxiliary light means for illuminating an object to be photographed; 
     a pair of optical means placed at a predetermined interval; 
     light-receiving means for receiving light from the object through the optical means and outputting two image signals; 
     first distance measurement means for calculating an object distance on the basis of a relative image distance between the two image signals; 
     second distance measurement means for calculating the object distance on the basis of a reflected light amount from the object upon illumination by the auxiliary light means; and 
     control means for determining reliability of the object distance calculated by the first distance measurement means, and operating the second distance measurement means upon determining that the object distance calculated by the first distance measurement means is not reliable. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments give below, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram showing the concept of a distance measurement apparatus of the present invention; 
     FIG. 2 is a flow chart for explaining the operation of a camera to which the distance measurement apparatus is applied according to the first embodiment of the present invention; 
     FIG. 3 is a flow chart for explaining the operation of the second distance measurement mode in detail; 
     FIG. 4 is a graph of the integrated characteristics of the photocurrent from the brightest portion of an object, and shows the relationship between integration times t 1  and t 2 ; 
     FIG. 5 is a flow chart for explaining the operation of the second distance measurement mode according to the second embodiment of the present invention; and 
     FIG. 6 is a graph of the integrated characteristics of the photocurrent from the brightest portion of an object, and shows the relationship between integrals v 1  and v 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention will be described in detail with reference to the accompanying drawing. 
     FIG. 1 is a block diagram showing the concept of a distance measurement apparatus according to the present invention. 
     Referring to FIG. 1, this distance measurement apparatus comprises an auxiliary light-emitting unit  14  for emitting auxiliary light to an object  12 , an optical unit  16  composed of a pair of lenses having a predetermined interval therebetween, a light-receiving unit  18 , an arithmetic unit  20 , a control unit  22 , and a memory  24 . 
     The auxiliary light-emitting unit  14  includes an LED, a lamp, and an electronic flash. The light-receiving unit  18  comprises a plurality of photoelectric conversion elements. The light-receiving unit  18  receives an object image focused by each lens of the optical unit  16  and outputs an electrical signal corresponding to the light amount of the object image. The arithmetic unit  20  has at least a function of calculating object distance on the basis of the relative image distance between two image signals obtained from the light-receiving unit  18 , a function of calculating an object luminance on the basis of the light-receiving unit  18  outputs in the illuminated and non-illuminated states of auxiliary light from the auxiliary light-emitting unit  14 , and a function of calculating the object distance from the resultant object luminance. 
     The control unit  22  controls the operations of at least the auxiliary light-emitting unit  14 , the arithmetic unit  20 , and the memory  24 . The control unit  22  includes an object luminance determination means for determining on the basis of the calculated object luminance whether the auxiliary light-emitting unit  14  is to be operated. The control unit  22  may also control predetermined camera operations except the distance measurement operation. The memory  24  stores image signals obtained from the light-receiving unit  18 . Although the optical unit  16  and the light-receiving unit  18  are illustrated as separate blocks in FIG. 1, an AF module (not shown) may be integrated with the optical unit  16  and the light-receiving unit  18 . 
     The arithmetic unit  20  and the control unit  22  can be implemented by one CPU. 
     The operation of a camera to which the distance measurement apparatus according to the first embodiment of the present invention is applied will be described with reference to the flow chart in FIG.  2 . 
     When the user presses the release button of the camera, the distance measurement operation is started. The optical unit  16  is initialized in step S 1 . This initialization includes power supply to the optical unit  16  and setting of sensor sensitivity, and the like. 
     The control unit  22  determines in steps S 2  to S 5  whether object illumination by the auxiliary light-emitting unit  14  is required. The object is illuminated with the auxiliary light-emitting unit  14 , as needed. 
     More specifically, when image signal acquisition is started in the light-receiving unit  18  in step S 2 , the luminance of the object  12  is determined in step S 3 . If it is determined in step S 3  that the object is bright, the flow advances to step S 6 ; otherwise, the flow advances to step S 4 . 
     In step S 4 , the object  12  is illuminated with auxiliary light from the auxiliary light-emitting unit  14 . In step S 5 , reset and restart for acquiring image signals in the illuminated state with the auxiliary light are performed in the light-receiving unit  18  through the optical unit  16 . 
     In this embodiment, whether object illumination is required is determined using the output from the distance measurement light-receiving unit  18 . However, this determination can be made using an output from the photometric means (not shown) for exposure of the camera. 
     In step S 6 , the image signals from the light-receiving unit  18  are completely acquired, so that the first and second image signals of the object  12  are obtained. The image signals quantized by the light-receiving unit  18  are stored in the memory  24 . 
     In step S 7 , the first arithmetic operation is performed. The first arithmetic operation calculates the relative image distance (or phase difference) between the first and second image signals of the object  12 . This arithmetic operation is a known technique generally called a correlation means. 
     In step S 8 , information such as low contrast of the object  12  or a low degree of coincidence between the first and second image signals is detected from the first and second image signals of the object  12 . The arithmetic operation result of the first arithmetic operation is evaluated. If it is determined by this evaluation result that the arithmetic operation result of the first arithmetic operation is not reliable, distance measurement is determined impossible. 
     In multi AF or the like having a plurality of distance measurement points, distance measurement is determined impossible when all the arithmetic operation results of all the distance measurement points are unreliable. Whether distance measurement is possible is determined by the control unit  22 . The control unit  22  outputs a signal indicating whether distance measurement was successfully done. 
     When it is determined in step S 8  that the arithmetic operation results of the first arithmetic operation are highly reliable and the distance measurement has been normally performed, the flow advances to step S 9 . In step S 9 , the relative image distance between the first and second image signals of the object  12  is converted into object distance information by the second arithmetic operation. 
     It is determined in step S 10  whether auxiliary light is present/absent. When the object  12  is illuminated with the auxiliary light-emitting unit  14 , the flow advances to step S 11  to end illumination and complete distance measurement. The execution timings of steps S 10  and S 11  are not specifically limited as far as these steps are performed after step S 6 . 
     When it is determined in step S 8  that distance measurement is impossible in the first distance measurement mode, the flow advances to step S 12  to determine whether auxiliary light is present/absent. If the object is not illuminated with the auxiliary light-emitting unit  14 , the flow advances to step S 13  to start illuminating the object. When the auxiliary light-emitting unit  14  is built by a lamp or the like which requires a long time to stabilize the emission amount upon the start of emission, an emission stabilization time is required. 
     In steps S 14  and S 15 , the first and second image signals of the object  12  during illumination with the auxiliary light-emitting unit  14  are acquired. In steps S 16  to S 18 , the first and second image signals of the object  12  after the illumination with the auxiliary light-emitting unit  14  is complete and illumination is not done are acquired. 
     The third arithmetic operation is performed in step S 19 . That is, the auxiliary light amount is calculated in accordance with the difference between the light amounts from the object  12  in the illuminated and non-illuminated states of the object  12  with the auxiliary light-emitting unit  14 . The fourth arithmetic operation is then performed in step S 20  to convert the auxiliary light amount into the distance information of the object  12 , thereby completing the distance measurement. 
     As described above, according to the outline of the first embodiment, a distance measurement is performed in the first distance measurement mode. If this distance measurement is determined as normal, the distance measurement is complete. The distance measurement result in the first distance measurement mode is defined as object distance information. However, if the measurement result in the first distance measurement mode is determined as abnormal, another distance measurement is performed in the second distance measurement mode. The distance measurement result in the second distance measurement mode is defined as object distance information. 
     The operation in the second distance measurement mode will be described in detail with reference to the flow chart in FIG.  3 . 
     In step S 21 , the auxiliary light-emitting unit  14  starts illuminating the object. Image signal acquisition (to be referred to as integration hereinafter) in the light-receiving unit  18  is started in step S 22 . Note that an auxiliary light emission stabilization time may be required between steps S 21  and S 22 . 
     Time counting is started in step S 23 . When this integration is started, the brightest portion of the object  12  is detected in step S 24 , and an integral corresponding to the brightest portion is monitored. Step S 25  is a limiter for the integration time. When the integration time is long, the flow advances to step S 42  to execute infinity processing. 
     When the integration is completed within a predetermined time in step S 25 , the flow advances to step S 26  to determine whether the integral has reached a predetermined amount. If NO in step S 26 , the flow returns to step S 24 ; otherwise, the flow advances to steps S 27  to S 29  to complete time counting, acquire a time counting result t 1 , and complete integration in the order named. 
     The auxiliary light-emitting unit  14  stops illuminating the object in step S 30 . A time t 2  required until the integral of the brightest portion of the object  12  reaches the predetermined amount is measured in steps S 31  to S 38 . More specifically, the integration is started in the light-receiving unit  18  in step S 31 , and time counting is started again in step S 32 . The brightest portion of the object is detected in step S 33 , and an integral corresponding to the brightest portion is monitored. Step S 34  is a limiter for the integration time. When the integration time is long, the flow advances to step S 40  to perform dark state processing. 
     When the integration is completed within the predetermined time in step S 34 , the flow advances to step S 35  to determine whether the integral has reached the predetermined amount. If NO in step S 35 , the flow returns to step S 33 ; otherwise, time counting is completed, a time counting result t 2  is acquired, and integration is completed in steps S 36  to S 38  in the order named. 
     Note that an auxiliary light complete turn-off time may be required between steps S 30  and S 31 . 
     The integration time t 1  upon illumination of the object and the integration time t 2  without illumination of the object are obtained in steps S 21  to S 38 . 
     In step S 39 , an integration time t corresponding to the auxiliary light amount is obtained. 
     FIG. 4 is a graph of integration characteristics of the photocurrent from the brightest portion of the object  12 , and shows the relationship between the integration times t 1  and t 2 . 
     When a line “only ordinary light” is subtracted from a line “ordinary light +auxiliary light” in FIG. 4, a line “only auxiliary light” is obtained. In this relationship, the integration time t is given by: 
     
       
           t =t 1 ×t 2 /(t 2−t   1 )  (1) 
       
     
     The denominator in equation (1) may become zero or less due to a cause such as a high luminance, a measurement error, infinity, or a long distance. In this case, the denominator is determined to be zero, and the object distance is determined as infinity or a long distance. When the auxiliary light amount is obtained, the object distance can be obtained because the auxiliary light amount is in inverse proportion to the square of the object distance. 
     The dark state processing is performed in step S 40 . This dark state processing is executed when integration progresses little under the condition that the auxiliary light-emitting unit  14  does not illuminate the object  12 , i.e., the object  12  is in the dark. The dark state processing is performed for t 2 =0 and t=t 1 . 
     The object distance is obtained in step S 41 . If an auxiliary light amount a at 1 m is stored in the memory  24  as an adjustment value, the object distance can be obtained using this adjustment value as a reference using equation (2) below. More specifically, the amount a is the auxiliary light amount measured when a standard reflecting plate (gray: reflectance of 18%) is placed 1 m apart from the auxiliary light-emitting unit  14 . The light amount distance measurement is performed using this as the reference. 
     The difference between the actual reflectance of the object  12  and the reflectance of the standard reflecting plate results in a distance measurement error. However, design is done to minimize any distance measurement error in the actual reflectance distribution of the object  12  by selecting the standard reflecting plate serving as a reference for determining the camera exposure and a lamp having relatively flat spectral emission characteristics for an adjustment chart and the auxiliary light-emitting unit  14 , respectively.              L   =         t   a                       (     a   =   Const     )               (   2   )                                
     The infinity processing in step S 42  is executed when integration progresses little even by illumination of the object  12  with the auxiliary light-emitting unit  14 , i.e., when the object  12  is at infinity in the dark. No arithmetic operation is performed, and the object distance is infinity or a long distance. 
     As described above, according to the first embodiment, the object distance can be measured even if distance measurement by the phase difference scheme is impossible. 
     The second embodiment of the present invention will be described below. 
     FIG. 5 is a flow chart for explaining the operation in the second distance mode according to the second embodiment of the present invention. The remaining parts of the second embodiment are substantially the same as those of the first embodiment, and its detailed description will be omitted. 
     In steps S 51  to S 59 , integration is performed while an auxiliary light-emitting unit  14  illuminates an object. Note that steps S 51  to S 54  are the same as steps S 21  to S 24  in the flow chart of FIG. 3, and its detailed description will be omitted. 
     Step S 55  is a limiter for the integration time. When the integration time has not yet reached a predetermined time, the flow returns to step S 54 ; otherwise the flow advances to step S 56  to determine whether the integral has reached a predetermined amount. 
     If NO in step S 56 , i.e., if an integral v 1  is the predetermined amount or less, the object is determined to be located at infinity in the dark. The flow then advances to step S 72 . If YES in step S 56 , the flow advances to steps S 57  to S 59  to complete integration, acquire the integral v 1 , and complete time counting in the order named. In this manner, the integral v 1  for the brightest portion of the object  12  a predetermined time after the start of integration is measured. 
     Integration for the object  12  in the non-illustrated state is performed in steps S 60  to S 68 . Note that steps S 60  to S 63  are the same as steps S 30  to S 33  in the flow chart of FIG. 3, and its detailed description will be omitted. 
     Step S 64  is a limiter for the integration time. When the integration time has not reached a predetermined time, the flow returns to step S 63 ; otherwise, the flow advances to step S 65  to determine whether the integral reaches the predetermined amount. 
     When it is determined in step S 65  that the integral has not reached the predetermined amount, i.e., when an integral v 2  is the predetermined amount or less, the object is determined to be in the dark, and the flow advances to step S 70 . If it is determined in step S 65  that the integral has reached the predetermined amount, the flow advances to steps S 66  to S 68  to complete integration, acquire the integral v 2 , and complete time counting in the order named. The integral v 2  of the brightest portion of the object a predetermined time after the start of integration is measured. 
     In step S 69 , an integral v corresponding to the auxiliary light amount is obtained from the integrals v 1  and v 2  by equation (3) below. Note that the method of determining an adjustment value b is the same as that of determining the adjustment value a described above. 
     
       
           v =v 1 −v 2   (3) 
       
     
     Dark state processing is performed in step S 70 . This dark state processing is performed for the integration values v 2 =0 and v=v 1 . An object distance L is given using the integration value v by:              L   =         b   v                       (     b   =   Const     )               (   4   )                                
     Infinity processing is performed in step S 72 . This infinity processing is executed to determine infinity or long distance as the object distance L without performing any arithmetic operation. 
     FIG. 6 is a graph of integration characteristics of the photocurrent from the brightest portion of the object, and shows the relationship between the integration values v 1 , v 2 , and v. 
     As is apparent from FIG. 6, when a line “only ordinary light” is subtracted from a line “ordinary light+auxiliary light”, a line “only auxiliary light” is obtained. 
     As described above, according to the second embodiment, the object distance can be measured even if distance measurement by the phase difference scheme is impossible. 
     In each embodiment described above, an auxiliary light amount is calculated using first and second image signals in the second distance measurement mode. However, the auxiliary light amount may be obtained using one of the first and second image signals. 
     As has been described above, according to the present invention, there can be provided a simple, low-cost distance measurement apparatus capable of accurate focusing even if distance measurement by the phase difference scheme cannot be made for an object such as an unintended object. 
     According to the present invention, since it suffices to monitor the integration value of the brightest portion of an object, the arithmetic operation time can be shortened. 
     Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.