Distance measuring apparatus

A distance measuring apparatus in which two, right and left optical systems respectively comprise sensors (5, 6) is disclosed. Sensor output values (35, 36) representing received-light images (31, 32) on the sensors (5, 6) suffer variations due to sensitivity differences between the sensors (5, 6). In order to correct the variations, the minimum values (RD.sub.L, RD.sub.R) of the two sensor output values are normalized to zero level, and thereafter, the smaller maximum value (e.g., S.sub.R) is normalized to the larger maximum value (e.g., S.sub.L). The correlation calculation is made on the basis of the two normalized sensor outputs to calculate the distance to the object to be measured.

BACKGROUND OF THE INVENTION 
The present invention relates to a distance measuring apparatus and method 
for measuring the distance to an object to be measured on the basis of a 
signal obtained by photoelectrically converting an optical image of the 
object to be measured. 
In a conventional distance measuring apparatus, a light-emitting element 
projects a beam spot brought to a focus via a projection lens, light 
reflected by an object is received by a position detection means (such as 
a PSD or the like), and the distance to the object is measured based on 
the principle of trigonometric measurements using the received-light 
output. Also, for example, Japanese Patent Publication No. 5-22843, 
Japanese Patent Application No. 7-40542, or the like has proposed a 
distance measuring apparatus, which can perform so-called skimming for 
discharging (resetting) a predetermined amount of charges, which 
correspond to external light components other than signal components 
obtained upon incidence of a beam spot and are obtained from a 
light-receiving element, by circulating the received-light output obtained 
by receiving the beam spot projected by a light-emitting element and 
brought to a focus via a projection lens, i.e., charges obtained by 
photoelectric conversion, in a charge transfer means such as a CCD, which 
is arranged in a ring shape, so as to integrate the charges. Furthermore, 
based on this apparatus, a distance measuring apparatus, which has two 
light-receiving systems and calculates the distance on the basis of the 
correlation between two received-light images obtained by the two 
light-receiving systems, has been proposed by Japanese Patent Application 
No. 7-263182. Such distance measuring apparatus is used in an AF 
(auto-focusing) mechanism of a camera and the like. 
The distance measuring apparatus proposed by Japanese Patent Application 
No. 7-263182, i.e., the apparatus which can perform skimming, has two 
light-receiving systems, and calculates distance based on the correlation 
between two received-light images obtained by the two light-receiving 
systems, will be briefly described below with reference to FIG. 6. This 
apparatus is the foundation of the present invention. 
Referring to FIG. 6, reference numeral 1 denotes a first light-receiving 
lens for forming the first optical path; 2, a second light-receiving lens 
for forming the second optical path; 3, a projection lens for projecting a 
beam spot onto the object to be measured; and 4, a light-emitting element 
which is turned on/off to project the beam spot. Reference numerals 5 and 
6 denote first and second sensor arrays, each consisting of a linear array 
of a plurality of sensors. Reference numeral 7 denotes a first clear 
portion providing an electronic shutter function of clearing charges 
photoelectrically converted by the sensors of the first sensor array 5 in 
accordance with pulses ICG (Integration Clear Gate). Reference numeral 8 
denotes a second clear portion providing an electronic shutter function 
for clearing charges photoelectrically converted by the sensors of the 
second sensor array 6 in accordance with the pulses ICG as in the first 
electronic shutter portion 7. 
Reference numeral 9 denotes a first charge accumulation portion, which 
includes ON and OFF accumulation portions (not shown) for accumulating 
charges obtained by the first sensor array 5, and accumulates charges in 
units of pixels in accordance with pulses ST (storage) 1 and ST2 which are 
respectively synchronous with the ON and OFF periods of the light-emitting 
element 4. Reference numeral 10 denotes a second charge accumulation 
portion, which accumulates charges obtained by the second sensor array 6 
in units of pixels in accordance with the pulses ST1 and ST2 as in the 
first charge accumulation portion 9. Reference numeral 11 denotes a first 
charge transfer gate for parallelly transferring charges accumulated in 
the first charge accumulation portion 9 to a charge transfer means (e.g., 
a CCD; to be described below) in accordance with pulses SH (shift). 
Reference numeral 13 denotes a first charge transfer means, which 
partially or entirely has a ring-shaped arrangement, and independently 
adds charges accumulated in the first charge accumulation portion 9 during 
the ON and OFF periods by circulating them. A portion that forms the 
circulating portion will be referred to as a ring CCD hereinafter, and a 
portion other than the circulating portion will be referred to as a linear 
CCD hereinafter. Reference numeral 12 denotes a second charge transfer 
gate, which is the same as the first charge transfer gate 11. Reference 
numeral 14 denotes a second charge transfer means, which is the same as 
the first charge transfer means. 
Reference numeral 15 denotes a first initialization means for performing 
initialization by resetting charges in the first charge transfer means 13 
in response to pulses CCDCLR (clear). Reference numeral 17 denotes a first 
skim means for resetting a predetermined amount of charges. Reference 
numeral 18 denotes a second skim means similar to the first skim means 17. 
Reference numeral 19 denotes a first output means for outputting a signal 
SKOS1 used for discriminating whether or not a predetermined amount of 
charges are to be reset. The first output means 19 reads out the charge 
amount present in the first charge transfer means 13 in a non-destructive 
manner while leaving them as charges. Reference numeral 20 denotes a 
second output means for similarly outputting a signal SKOS2. Reference 
numeral 21 denotes an output means for sequentially reading out charges in 
the first charge transfer means 13, and outputting them as signals OS1. 
Similarly, reference numeral 22 denotes an output means for outputting 
signals OS2 based on charges in the second charge transfer means 14. 
Reference numeral 23 denotes a first comparator for discriminating based 
on the signal SKOS1 if skimming is to be performed. Reference numeral 24 
denotes a second comparator for performing discrimination based on the 
signal SKOS2 as in the first comparator 23. Reference numeral 25 denotes a 
control unit including a microcomputer for controlling the entire 
apparatus and performing calculations. 
Skimming in the above-mentioned distance measuring apparatus will be 
explained below. 
FIGS. 7A and 7B show received-light images 33 and 34 as the signal 
waveforms of the output signals OS1 and OS2 from the sensors, which 
respectively correspond to received-light images 31 and 32 on the first 
(left) and second (right) sensor arrays 5 and 6. In the coordinate system 
in FIGS. 7A, and 7B the ordinate plots the magnitude of the output signals 
OS1 and OS1, and the abscissa plots the position, x, on the sensor. The 
signal levels of pixels in a portion other than the received-light images 
33 and 34 of the sensor output signals equal a reset level RD of the CCD. 
In this apparatus, the distance is calculated by calculating the 
correlation between the two images. 
However, even when the received light images on the sensor arrays 5 and 6 
are the same, as shown in FIGS. 7A, and 7B different levels RD (RD.sub.L 
and RD.sub.R) due to different reset levels of the two sensors, and 
different received-light outputs (S.sub.L and S.sub.R) due to differences 
in sensitivity of the sensor, gain of the output means 21 and 22, 
brightness of the-optical systems, and the like are produced in practice, 
as shown in FIGS. 8A and 8B. As a consequence, two received-light images 
35 and 36 have different shapes. The differences between the L (left) and 
R (right) images are determined by the individual differences of the 
apparatus. When correlation is calculated for the two images shown in 
FIGS. 8A and 8B, respectively correlation reliability is impaired as 
compared to the correlation result of ideal sensor outputs that form two 
images of the same shape, as shown in FIGS. 7A and 7B, respectively thus 
lowering the distance measurement precision. 
SUMMARY OF THE INVENTION 
The present invention has been made to solve the above-mentioned problems, 
and has as its object to provide a distance measuring apparatus and 
method, which can improve reliability of the correlation calculation 
result. 
More specifically, a distance measuring apparatus, which comprises light 
projection means for projecting light toward an object to be measured, 
first and second photoelectric conversion means each consisting of an 
array of a plurality of photoelectric conversion elements for receiving 
reflected light of the light projected to the object to be measured by the 
light projection means, and calculation means for calculating a distance 
to the object to be measured by performing a correlation calculation on 
the basis of first and second output signals output from the first and 
second photoelectric conversion means, comprises: 
normalization means for normalizing minimum and maximum values in the first 
and second output signals, 
wherein the calculation means calculates the distance to the object to be 
measured on the basis of the first and second output signals normalized by 
the normalization means. 
Preferably, when at least some light components of the reflected light fall 
outside a predetermined photoelectric conversion region and/or regions of 
the first and/or second photoelectric conversion means or are present in 
the vicinity of an end portion of a predetermined photoelectric conversion 
region and/or regions of the first and/or second photoelectric conversion 
means, or when an absolute value of a difference between the maximum 
values of the first and second output signals is more than a predetermined 
value, the normalization by the normalization means is inhibited. 
For example, the normalization means preferably comprises: 
minimum value normalization means for normalizing the minimum values of the 
first and second output signals; and 
maximum value normalization means for normalizing a smaller one of the 
maximum values of the first and second output signals to a larger maximum 
value after the normalization by the minimum value normalization means. 
More specifically, distance measurement method, which comprises the 
photoelectric conversion step of projecting light toward an object to be 
measured, and photoelectrically converting, by first and second 
photoelectric conversion element groups, reflected light of the projected 
light to the object to be measured, and the calculation step of 
calculating a distance to the object to be measured by performing a 
correlation calculation on the basis of first and second output signals 
output from the first and second photoelectric conversion element groups, 
comprises: 
the normalization step of normalizing minimum and maximum values in the 
first and second output signals, 
wherein the distance to the object to be measured is calculated in the 
calculation step on the basis of the first and second output signals 
normalized in the normalization step. 
The normalization step preferably includes: 
the minimum value normalization step of normalizing the minimum values of 
the first and second output signals; and 
the maximum value normalization step of normalizing a smaller one of the 
maximum values of the first and second output signals to a larger maximum 
value after the normalization in the minimum value normalization step. 
Other features and advantages of the present invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings, in which like reference characters designate the same or similar 
parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
&lt;First Embodiment&gt; 
A distance measuring apparatus according to the present invention will be 
described hereinafter. The present invention corrects the sensitivity 
difference of light-receiving systems prior to correlation calculations so 
as to improve the reliability of the correlation calculations. 
Accordingly, assume that the hardware arrangement of a distance measuring 
apparatus according to each embodiment of the present invention is the 
same as that shown in FIG. 6, and a detailed description thereof will be 
omitted. Also, the coordinate system in each of FIGS. 1A and 1B, 2A and 
2B, etc., to 5A and 5B, respectively is the same as that in FIGS. 7A and 
7B, respectively and a detailed description thereof will be omitted. 
The actual sensor outputs from the apparatus shown in FIG. 6 form 
received-light images 36 and 35 as L and R images with different shapes, 
which have different levels RD (RD.sub.L and RD.sub.R) due to different 
reset levels of two sensor arrays, and different received-light outputs 
(S.sub.L and S.sub.R) due to differences in sensitivity of the sensor 
arrays, gain of the output means 21 and 22, brightness of the optical 
systems, and the like, as has been described above with reference to FIGS. 
8A and 8B. 
The output portions (hatched portions) from pixels on which no 
received-light images are formed in FIGS. 8A and 8B i.e., portions 
corresponding to levels RD (RD.sub.L and RD.sub.R) as DC (direct current) 
components of the sensor output signals, have no influence on the 
correlation result of known correlation calculations performed after the 
correction processing of the present invention. Accordingly, offset 
adjustment is performed as Min normalization so that Min.sub.L and 
Min.sub.R respectively become 0, as shown in FIGS. 1A and 1B. This 
adjustment is attained by subtracting the level RD.sub.L from the 
respective pixel outputs of the sensor output signals for the L image, and 
subtracting RD.sub.R for the R image. 
More specifically, only the outputs of pixels on which the two 
received-light images are formed are generated, but the received-light 
outputs (S.sub.L and S.sub.R) still remain different. Such different 
outputs are generated due to the sensitivity difference between the two 
light-receiving systems including the sensor arrays and optical systems. 
The method of correcting the sensitivity difference between the 
light-receiving systems will be explained below. 
FIGS. 2A and 2B shows the correction method according to the first 
embodiment. 
In FIGS. 2A and 2B, as Max normalization, the Max values of the two images 
are normalized to the higher one (S.sub.L in this case) in FIGS. 1A and 
1B. In this processing, the respective pixel outputs of the sensor output 
signals on the S.sub.R side are multiplied by a coefficient given by: 
EQU K=S.sub.L /S.sub.R 
That is, when the outputs of pixels on which no received-light image is 
formed are multiplied by K, they yield 0, and the outputs of only pixels 
on which the received-light image is formed on the sensor array with lower 
sensitivity are subjected to sensitivity correction so that S.sub.L and 
S.sub.R in FIGS. 1A and 1B are corrected to satisfy S.sub.L =S.sub.R, as 
shown in FIGS. 2A and 2B. 
In this way, when the Min and Max normalizations are performed, the 
sensitivity correction can be attained without preparing any correction 
coefficient in advance, and the two images can have nearly the same 
shapes, thus improving the reliability of the correction result. 
&lt;Second Embodiment&gt; 
The second embodiment will be described below. 
As described in the first embodiment, since the sensitivity correction can 
be attained and the two images can have nearly the same shapes by 
performing the Min and Max normalizations, the reliability of the 
correlation result can be improved. By adding other normalization 
conditions, Max normalization errors can be prevented, and the reliability 
of the correlation result can be further improved. Since other 
arrangements are the same as those in the first embodiment, a detailed 
description thereof will be omitted. 
The conditions will be explained below. 
When the object to be measured is located at the nearest-distance position, 
incident light rays make large angles with the optical axis like the 
paraxial light ray in FIG. 6. For this reason, as shown in FIGS. 3A and 
4A, one received-light image 35 falls outside the sensor array area. In 
this case, Min normalization can be made. However, according to one 
additional condition, when the Max value on one sensor array falls outside 
the sensor array area and cannot be detected, as shown in FIG. 3A, i.e., 
when the Max value falls outside a predetermined range of the sensor 
output coordinate system, the Max normalization is added inhibited. 
According to another additional condition, as shown in FIG. 4A, when the 
Max value can be recognized on the sensor array, but is located near the 
end of the sensor array (received-light image 31 in FIG. 4A), such image 
is readily influenced by disturbance light and noise components, and the 
reliability of the Max value is low. In practice, this image may be one 
shown in FIG. 3A. For this reason, in the case of FIG. 4A as well, the Max 
normalization is added inhibited. Such case can be easily determined by 
comparing the sensor output state with the predetermined range on the 
coordinate system. 
&lt;Third Embodiment&gt; 
The third embodiment will be explained below. 
When one light-receiving system alone receives regularly reflected light 
from an object such as a glossy metal surface that can regularly reflect 
light (note that regular reflection means reflection with little diffusion 
of the light incident on the object), or when an obstacle locally or 
entirely blocks one light-receiving route, one received-light output 
becomes considerably low, as shown in FIG. 5B. More specifically, if the 
Max normalization is performed even though the received-light outputs are 
obviously abnormal, the reliability of the correlation result often 
apparently improves as if a correct distance result were obtained. In view 
of this problem, in the third embodiment, the absolute value of the 
difference between the Max values of the two images is calculated, and if 
the absolute value is equal to or smaller than a predetermined value L, 
the Max normalization is inhibited. 
EQU .vertline.S.sub.L -S.sub.R .vertline..ltoreq.L 
where L is a value that is confirmed by experiments that it does not give 
rise to any distance measurement errors even when the Max normalization is 
made. 
Note that the correction in the first, second, and third embodiments 
described above is performed by the control unit 25 shown in FIG. 6. 
According to the present invention, since received-light images are known 
in advance, the sensitivity difference is corrected by normalizing 
Max.sub.L, Max.sub.R, Min.sub.L, and Min.sub.R in FIGS. 8A and 8B thereby 
improving the reliability of the correlation result. 
In the above description, a so-called active distance measurement method 
that projects a beam spot onto the object to be measured has been 
exemplified. Also, in the case of a so-called passive distance measurement 
method that does not project any beam spot, a coefficient used for 
correcting the sensitivity difference between the two light-receiving 
systems may be pre-stored in a storage means, and the correction may be 
made using the stored correction coefficient. Since other arrangements are 
the same as those in the first and second embodiments, a detailed 
description thereof will be omitted. 
As described above, according to the present invention, the correction 
value for the sensitivity difference between the two sensor arrays can be 
relatively easily and ideally obtained, the reliability of the correlation 
calculations performed later can be improved. Also, since the correction 
can be attained without obtaining and storing any correction value for the 
sensitivity difference between the two sensor arrays in advance in, e.g., 
a memory, the time required for the process of calculating the correction 
value on the production line upon mass-production can be shortened, and a 
memory for storing the correction value can be omitted, thus reducing the 
manufacturing cost. 
As many apparently widely different embodiments of the present invention 
can be made without departing from the spirit and scope thereof, it is to 
be understood that the invention is not limited to the specific 
embodiments thereof except as defined in the appended claims.