Range finder

An apparatus that measures the distance to an object by projecting light, receiving reflected light of the projected light through at least two light receiving devices disposed at different positions, and combining information from the two light receiving device to cancel, the influence of a change in the centroid position of the reflected light at the same distance. The apparatus has a signal processor performs a first processing operation for determining the distance to the object by combining information from the two light receiving devices, and a second processing operation for determining the distance to the object using information from only one of the two light receiving devices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improvement in a range finder that 
measures the distance to an object by projecting light to the object and 
receiving reflected light from the object. 
2. Description of the Related Art 
Conventional active type range finders for cameras that operate by 
projecting light from light projecting means to an object, receiving light 
reflected back from the object, and calculating the distance to the object 
based on the quantity of reflected light received or the light receiving 
position. 
In contrast with passive type range finders, such active type range finders 
advantageously can measure the distance to a low-contrast object and 
measure the distance in the dark without using auxiliary light. 
Disadvantageously, however, active type range finders may make a 
measurement error if a measured object has a certain contrast or if the 
entire projected light beam does not hit the object. This is because the 
centroid position of the reflected light spot of the projected light beam 
changes in accordance with the contrast and the incompleteness of the 
light spot. 
To solve this problem, a method has been proposed in Japanese Patent 
Application No. 287280/1993, that uses two light receiving means 
positioned in asymmetrical positions on a light projecting means in the 
base length direction. A change in light centroid position due to the 
contrast condition of an object is canceled by using outputs from the two 
light receiving means. 
The above-described method is advantageous because it allows an active 
automatic focusing apparatus to be unaffected, in theory, by the contrast 
of the object. According to this method, however, the two light receiving 
means are disposed at different distances from the light projecting means. 
Therefore, the light image received on the light receiving means having 
the larger distance from the light projecting means along the base length 
direction moves out of the sensing area of the light receiving sensor when 
measuring a short distance, resulting in a measurement error. 
SUMMARY OF THE INVENTION 
In view of this problem, an object of the present invention is to provide a 
range finder having a large accurate distance measuring range including 
long and short distances. 
To achieve this object, according to one, the present invention relates to 
a range finder that measures the distance to an object by projecting 
light, to the object receiving light reflected from the object through at 
least two light receiving devices disposed at different positions, and by 
combining information from the two light receiving devices to correct, the 
influence of any change in the centroid position of the reflected light at 
the same distance. The apparatus comprises a signal processor that 
performs first processing that determines the distance to the object by 
combining information from the two light receiving devices, and second 
processing that determines the distance to the object by using information 
from one of the two light receiving devices. 
Other aspects of the invention will become apparent from the following 
detailed description of the preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will be described with 
reference to the accompanying drawings. 
FIG. 1 is a block diagram of the configuration of essential components of a 
range finder that represents a first embodiment of the present invention, 
i.e., an application of the present invention to a camera. 
A light projecting device 1 is an infrared light emitting diode, a xenon 
tube or any other lamp device. A drive circuit 2 drives the light 
projecting device 1. A light projecting lens 3 condenses light (or 
infrared light) projected from the light projecting device 1 and directs 
the light toward an object. The light emitting device 1, the drive circuit 
2 and the light projecting lens 3 constitute light projecting means. Light 
receiving lenses 4 and 5 are disposed so as to have optical axes parallel 
to that of the light projecting lens 3. 
Assuming that the distance between the optical axes of the light projecting 
lens 3 and the light receiving lens 4 is K1 and that the distance between 
the optical axes of the light projecting lens 3 and the light receiving 
lens 5 is K2, the light projecting lens 3 and the light receiving lens 5 
are placed so that K1.noteq.K2. In this embodiment, K1&gt;K2. 
Light receiving sensors 6 and 7 corresponding to the light receiving lens 4 
and 5 are formed of semiconductor position sensing devices (PSD) or the 
like. Light projected toward the object (distance-measured object) is 
reflected by the object, and a part of the reflected light is condensed by 
the light receiving lenses 4 and 5 to form images on the sensors 6 and 7. 
In this embodiment, 2-split silicon photo cells (SPC) or the like may be 
used as the light receiving sensors. From opposite ends of the respective 
sensors 6 and 7, a signal current F corresponding to a side of the sensor 
closer to the light projecting means and a signal current N more remote 
from the light projecting means are output, and the ratio of the signal 
outputs F and N changes according to the centroid position of the 
received-light image. Signals F1 and N1 are output from the light 
receiving sensor 6 while signals F2 and N2 are output from the light 
receiving sensor 7. In FIG. 1, a side view of each of the light receiving 
sensors 6 and 7 is illustrated. "F1+N1" and "F2+N2" are hereinafter 
referred to as a first received-light signal and a second received-light 
signal, respectively. 
Signal processing circuits 8 to 11 convert the outputs from the light 
receiving sensors 6 and 7 into voltages, amplify alternating current 
signals of the converted voltages and integrate the amplified signals. A/D 
converters 12 to 15 convert the analog signals from the signal processing 
circuits 8 to 11 into digital signals. A control circuit 16 formed of a 
microcomputer or the like controls the above-mentioned light projecting 
means and processes the digitized received-light signals to obtain 
distance measurement information. 
The distance measurement principle of the range finder of FIG. 1 will now 
be described with reference to FIG. 2. In FIG. 2, the same components and 
quantities as those shown in FIG. 1 are indicated by the same reference 
characters. 
An object 17 of distance measurement is at a distance L from the camera. 
The light receiving lenses 4 and 5 have focal lengths fj. The distance 
between the optical axes of the light receiving lenses 4 and 5 in the base 
length direction is K. The distance between the optical centroid points of 
the received-light images when the distance to the object is infinitely 
great is P0. The distance between the optical centroid points of the 
received-light images when the distance to the object is L is P1. 
A triangle having an apex corresponding to the centroid of the 
projected-light image on the object and having a base corresponding to P1 
is similar to a triangle also having an apex corresponding to the centroid 
of the projected-light image on the object and having a base corresponding 
to the distance K along the base length direction of the light receiving 
lenses 4 and 5. Therefore, 
EQU (L/K)=(L+fj)/P1 (1) 
Equation (1) is transformed into 
EQU L=(fj.times.K)/(P1-K) (2) 
In equation (2), fj and K are known. Therefore, the distance can be 
determined by measuring P1. 
According to this method, no measuring error occurs because the 
relationship of the equations (1) and (2) is not changed even if the 
projected-light image covers only a part of the object or if the portion 
of the object on which the projected-light image is formed has a contrast 
such that the optical centroid of the image changes to laterally shift the 
apexes of the above-mentioned rectangles. 
If the amounts of movement of the received-light images on the light 
receiving sensors 6 and 7 corresponding to the change in the distance to 
the object between the infinite value and L are S1 and S2, respectively, 
then 
EQU P1-K=S1+S2 (3) 
The ratio of the amount of movement of the received-light image on each 
light receiving sensor to the entire length of the light receiving sensor 
and the ratio of the outputs are equal to each other as expressed below. 
EQU S1/LPSD=N1/(F1+N1) (4) 
EQU S2/LPSD=N2/(F2+N2) (5) 
From equations (4) and (5), 
EQU S1+S2=LPSD {N1/(F1+N1)+N2/(F2+N2)} (6) 
When the entire received-light images are on the light receiving sensors 6 
and 7, 
EQU F1+N1+F2+N2 
since light receiving sensors 6 and 7 are equal in entire length LPSD, and 
the light receiving lenses 4 and 5 are equal in focal length fj and in 
effective diameter. 
Then, using F1+N1=FN, F2+N2=FN, equation (6) is changed into 
EQU S1+S2=LPSD {(N1+N2)/FN} (7) 
From equation (3) and equation (7), 
EQU P1-K=LPSD {(N1+N2)/FN} (8) 
Equation (8) is substituted in equation (2) to obtain 
EQU L={(fj.times.K)/LPSD}.multidot.{FN/(N1+N2)} (9) 
Since fj, K and LPSD are known, "FN/(N+N2)" is measured and calculated to 
determine the distance. 
Furthermore, 
##EQU1## 
Then, from equation (9), 
EQU L=(fj.times.K)/(2.times.LPSD.multidot.{F1+N1+F2+N2)/(N1+N2)}(10) 
is obtained. Thus, the distance can be determined by measuring and 
calculating "(F1+N1+F2+N2)/(N1+N2)". 
If the outputs of the two light receiving sensors are accurately measured, 
F1+N1.apprxeq.F2+N2 due to a variation in the characteristics of the 
sensitivity processing circuit and so on. Therefore, the distance 
measuring accuracy is higher when using equation (10) than when using 
equation (9). 
FIGS. 3(a) and 3(b) are diagrams showing the relationship between the 
distance to the object and the received-light image. Components 1 and 3 to 
7 shown in FIGS. 3(a) and 3(b) are the same as those shown in FIG. 1. FIG. 
3(a) shows side views of light receiving sensors 6 and 7 while FIG. 3(b) 
shows front views of the same. 
Three distances to respective objects, i.e., a long distance L1, a first 
short distance L2 and a second short distance L3 (smaller than the first 
short distance L2), are illustrated. A received-light image 19 is formed 
by the light receiving lens 4 when the object is at the long distance. A 
received-light image 20 is formed by the light receiving lens 5 when the 
object is at the long distance. A received-light image 21 is formed by the 
light receiving lens 4 when the object is at the first short distance. A 
received-light image 22 is formed by the light receiving lens 5 when the 
object is at the first short distance. A received-light image 23 is formed 
by the light receiving lens 4 when the object is at the second short 
distance. A received-light image 24 is formed by the light receiving lens 
5 when the object is at the second short distance. 
When the object is positioned in the range from the long distance to the 
first short distance, the received-light images are on the light receiving 
sensor 6 and 7, the above-described equations (4) and (5) are established 
and the distance to the object can be determined by the above-described 
equation (10). In this case, the distance can be measured without being 
affected by the contrast of the object or incompleteness of the projected 
light spot. 
On the other hand, when the object is positioned in the range from the 
first short distance to the second short distance, the received-light 
image 24 is on the light receiving sensor 7 but the received-light image 
23 is out of the area of the light receiving sensor 6, so that the 
above-described equation (4) is not established. In this case, if the 
distance is calculated by the above-described equation (10), the result of 
the calculation does not coincide with the actual distance. 
In this embodiment, therefore, the distance is measured only through the 
second light receiving means, i.e., the light receiving sensor 7 when the 
object is located in the range from the first short distance to the second 
short distance. 
From the principle of distance measurement based on triangulation, 
EQU L=(fj.times.K2)/S2 (11) 
From the above-described equation (5) and equation (11), 
EQU L=(fj.times.K2)/LPSD.multidot.{(F2+N2)/N2} (12) 
Since fj, K2 and LPSD are known, the distance can be determined from the 
above-described equation (12). 
The overall operation will be described with reference to the flowchart of 
processing of the control circuit 16 shown in FIG. 4. 
Step 101! A light projecting signal is output to the drive circuit 2 by 
the control means 16, thereby emitting light from the light projecting 
device 1. The light emitted passes through the light projecting lens 3 and 
travels toward the object. 
The light projected to the object is reflected by the object, and a part of 
the reflected light is received by the light receiving lenses 4 and 5 to 
form images on the light receiving sensors 6 and 7. The light receiving 
sensors 6 and 7 convert signals of the received light into electrical 
signals and output the same to the signal processing circuits 8 to 11 in 
the subsequent stage. 
In the signal processing circuits 8 to 11, the input signals are amplified 
and integrated. The amplified and integrated signals are converted into 
digital signals by the A/D converters 12 to 15 in the subsequent stage. 
The converted signals are input to the control circuit 16. 
Step 102! From the input digital signals, the control circuit 16 makes a 
determination with respect to the outputs of the first and second light 
receiving means (light receiving sensors 6 and 7). When the distance to 
the object is smaller than the above-mentioned first short distance, the 
received-light image on the light receiving sensor 6 moves out of the area 
of this sensor, and only the light on the sensor 6 in the received light 
is converted into electrical signal, so that "(first received light 
signal)&lt;(second received-light signal)". A threshold value .alpha. is 
provided to prevent determination errors due to a variation in the 
characteristics of the circuits for processing the first and second 
received-light signals, a variation in the characteristics of the optical 
systems, noise in the circuits and the like. 
Then, the control circuit 16 compares "first received-light signal 
(F1+N1)+.alpha." and "second received-light signal (F2+N2)". 
If "(F1+N1)+.alpha."&lt;"(F2+N2)" as a result of this comparison, the process 
advances to Step 104. If "(F1+N1) +.alpha.".gtoreq."(F2+N2), the process 
advances to Step 103. 
Step 103! Distance measurement free from an error due to contrast or the 
like is performed by processing the first and second received-light 
signals by the above-described equation (10). 
Step 104! The distance between the first short distance and the second 
short distance is measured by processing the second received-light signal 
by the above-described equation (12). 
In FIGS. 5 to 8, the abscissa represents the distance from the infinite 
distance corresponding to the origin to the second short distance. 
FIG. 6 is a graph in which the ordinate represents the ratio of the first 
received-light signal to the second received-light signal in this 
embodiment. When the distance is smaller than L2, the received-light image 
on the light receiving sensor 6 moves out of the area of this sensor, so 
that "(first received-light signal)/(second received-light signal)" 
becomes closer to zero. 
FIGS. 5, 7 and 8 show results (along the ordinate) of measurements of the 
distance (along the abscissa). FIG. 5 shows an example of measurement by 
the conventional method. In the conventional method, as described above, 
"(F1+N1+F2+N2)/(N1+N2)" is measured through the two received-light outputs 
to determine the distance from the long distance to the second short 
distance. In the range from L2 to L3, therefore, a non-linearity of the 
result of distance measurement, i.e., a measurement error, occurs. 
FIGS. 7 and 8 show results of measurement in this embodiment. In the range 
from L1 to L2 in which "(first received light signal)/(second received 
light signal).apprxeq.1", the distance is determined by measuring and 
calculating "(F1 +N1+F2+N2)/(N1+N2)". In the range from L2 to L3 in which 
"(first received light signal)&lt;(second received light signal)", the 
distance is determined by measuring and calculating "(F2+N2)/(N2)". Thus, 
the distance can be measured through the range from the long distance to 
the second short distance. 
FIG. 9 is a flowchart of processing of control circuit 16 in accordance 
with a second embodiment of the present invention. The second embodiment 
is the same as the first embodiment in circuit configuration and so on. 
Therefore, only the operation of the second embodiment will be described 
with reference to the flowchart of FIG. 9, in which the same steps as 
those of the first embodiment (FIG. 4) are represented by the same step 
numbers. 
In Step 101, the same operation is performed in the same manner as the 
first embodiment, that is, light is projected toward the object, reflected 
light from the object is received and received-light signals are input to 
the control circuit 16 through the A/D converters. 
In Step 104, a distance-measuring calculation is performed by using the 
second received-light signal. The distance in the range from the long 
distance to the second short distance can be measured. 
In Step 105, a determination is made as to whether the distance determined 
(measurement result) in Step 104 is smaller than a predetermined distance 
(L2 in the second embodiment). If the measured distance is smaller than 
the predetermined distance, the obtained distance measurement data is 
directly used and the operation is terminated. If the measured distance is 
equal to or larger than the predetermined distance, the process moves to 
Step 103 to perform a distance-measuring calculation by using the 
received-light signals from the two sensors. 
As described above, the distance-measuring calculation is performed by 
using the output (second received-light signal) from light receiving 
sensor 7, which is capable of measuring the distance from the second short 
distance to the long distance. If the distance thereby calculated is in 
the range from the long distance, to the first short distance in which 
range the distance-measuring calculation using the received-light signals 
from the two sensors is linear, a distance-measuring calculation using the 
received-light signals from the two sensors is performed. By using this 
method, it is possible to provide a range finder free from measurement 
error even if the distance measurement object has a certain contrast. 
FIG. 10 is a diagram of the configuration of an optical system of a range 
finder that represents a third embodiment of the present invention. In 
FIG. 10, the same components and quantities as those shown in FIG. 3 are 
indicated by the same reference characters. 
This embodiment differs from the first embodiment in that the distances K1 
and K2 of the optical axes of the light receiving lenses 4 and 5 from the 
optical axis of the light projecting lens 3 are the same, and that the two 
light receiving sensors have different lengths LPSD1 and LPSD2. 
The shorter light receiving sensor (light receiving sensor 6) is used as 
first light receiving means while the longer light receiving sensor (light 
receiving sensor 7) is used as second light receiving means. Distance 
measurement can be performed by the same control as in the first or second 
embodiment. 
According to each of the above-described embodiments, when the object is 
located in the range from the long distance to the first short distance 
(ordinary photographing distance), accurate distance measurement can be 
performed without being affected by the contrast of the object, 
incompleteness of the projected light spot or the like. 
When the object is located in the range from the first short distance to 
the second short distance (macrophotographing distance or the like), the 
probability of a reduction in the completeness of the projected light spot 
is low and the influence of the contrast of the object upon measurement 
through the projected light image is small because the object is so close 
to the range finder that the object is sufficiently larger than the 
projected light image. Therefore, the distance in the range from the first 
short distance to the second short distance can be measured by using only 
the second received-light signal. 
Therefore, it is possible to provide an accurate range finder having a wide 
distance measuring range from a long distance to a short distance. 
Other applications of the present invention based on the above-described 
embodiments are possible in which two or more light projecting means and 
three or more light receiving means are provided. 
Further applications of the present invention based on the above-described 
embodiments are possible in which two light receiving means may have any 
distance measuring ranges between a long distance and a short distance. 
For example, the distance to an object in a short-distance range is 
measured by combining information from two light receiving means while the 
distance to the object in a long-distance range is measured by using 
information from one of the two light receiving means. 
Still further applications of the present invention based on the 
above-described embodiments are possible in which any positional 
relationship may be set between two light receiving means provided in 
association with a light projecting means as long as reflected light is 
received at different positions. 
Still further applications of the present invention based on the 
above-described embodiments are possible in which processing for 
determining the distance to an object by combining information from two 
light receiving means and processing for determining the distance to the 
object by using information from one of the two light receiving means are 
performed in parallel with each other. 
The present invention can also be applied to arrangements for projecting 
and receiving a signal in the form of a sound wave, an electric wave and 
the like, i.e., waves other than light. 
While the present invention has been described with respect to what is 
presently considered to be the preferred embodiments, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
On the contrary, the invention is intended to cover various modifications 
and equivalent arrangements included within the spirit and scope of the 
appended claims. The scope of the following claims is to be accorded the 
broadest interpretation so as to encompass all such modifications and 
equivalent structures and functions. 
The individual components shown in schematic or block form in the drawings 
are well-known in the camera arts and their specific construction and 
operation are not critical to the operation or best mode for carrying out 
the invention. 
According to the present invention, the above-described embodiments or 
their technical elements may be combined as desired. 
According to the present invention, the entire or a part of each 
arrangement according to the appended claims may form one unit, may be 
combined with a different unit or may be used as an element constituting a 
unit. 
The present invention can also be applied to various types of cameras, such 
as single-lens reflex cameras, leaf-shutter cameras and video cameras, 
various optical apparatuses other than cameras, other kinds of 
apparatuses, and units or elements constituting such cameras, optical 
apparatuses and other kinds of apparatuses.