Rangefinder apparatus

Details Rangefinder apparatus Rangefinder apparatus

1999-10-07

2001-01-30

Perkey, W. B. (Department: 2851)

Photography

With exposure objective focusing means, focusing aid, or...

Having auxiliary illumination

C396S120000, C356S003040

Reexamination Certificate

active

06181877

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rangefinder apparatus for measuring the distance to an object to be measured; and, in particular, to an active type rangefinder apparatus suitably used in a camera or the like.
2. Related Background Art
In active type rangefinder apparatus used in cameras and the like, an infrared light-emitting diode (IRED) projects a luminous flux toward an object to be measured, the reflected light of thus projected luminous flux is received by a position sensitive detector (PSD), a signal outputted from the PSD is arithmetically processed by a signal processing circuit and an arithmetic circuit and then is outputted as distance information, and the distance to the object is detected by a central processing unit (CPU). In general, since errors may occur when the distance is measured upon a single light-projecting operation, light is projected a plurality of times so as to obtain a plurality of distance information items, which are then integrated by an integrating circuit and averaged.
As such an active type rangefinder apparatus, one shown in
FIG. 1
has conventionally been known.
FIG. 1
is a configurational view of a rangefinder apparatus in accordance with first prior art.
In the rangefinder apparatus shown in this drawing, a CPU
1
controls individual elements according to a program stored in an electrically erasable and programmable read-only memory (EEPROM)
2
and, under the control of the CPU
1
, a driver
3
drives an IRED
4
so as to make it output infrared light, which is then projected through a projection lens
101
to an object to be measured. The infrared light reflected by the object is collected by a PSD
5
by way of a light-receiving lens
102
, and the PSD
5
outputs two signals I
1
and I
2
according to the position at which the reflected light of the infrared light is received. A first signal processing circuit
11
eliminates a steady-state light component contained in the signal I
1
which becomes a noise, whereas a second signal processing circuit
12
eliminates a steady-state light component contained in the signal I
2
which becomes a noise.
According to the signals I
1
and I
2
from which the steady-state light components have been eliminated, an arithmetic circuit
14
determines an output ratio (I
1
/(I
1
+I
2
)) by an arithmetic operation, and outputs an output ratio signal corresponding to the distance to the object. An integrating circuit
15
integrates at an integrating capacitor
6
the output ratio signals thus outputted from the arithmetic circuit
14
a plurality of times, thereby improving the S/N ratio. The signal outputted from this integrating circuit
15
(hereinafter referred to as “AF signal”) corresponds to the distance to the object. Then, according to the AF signal outputted from the integrating circuit
15
, the CPU
1
determines a distance signal by carrying out a predetermined arithmetic operation, and controls a lens driving circuit
7
according to this distance signal, so as to move a lens
8
to an in-focus position.
FIG. 2
is a graph showing the relationship between the AF signal outputted from the integrating circuit
15
in this first prior art and the distance to the object. In this graph, the abscissa indicates the reciprocal (
1
/L) of the distance L to the object, whereas the ordinate indicates the output ratio (I
1
/(I
1
+I
2
)), i.e., AF signal. As shown in this graph, the output ratio has substantially a linear relationship with respect to the reciprocal (
1
/L) of the distance L at a certain distance L
4
or less, such that the output ratio decreases as the distance L is longer (
1
/L is smaller). At the distance L
4
or greater, by contrast, the influence of the steady-state light component increases as the distance L is greater. Letting I
n
(I
n
≧0) be the steady-state light component, the output ratio is (I
1
+I
n
)/(I
1
+I
n
+I
2
+I
n
) whereby the output ratio would shift so as to increase at the distance L
4
or greater. Also, since I
n
occurs randomly, it becomes unstable depending on the distance measuring condition. It is due to the fact that, as the distance L increases, the intensity of reflected light received by the PSD
5
decreases, whereby the steady-state light component I
n
becomes relatively greater. If such a phenomenon occurs, the distance to the object L cannot be determined uniquely from the output ratio.
Therefore, as a rangefinder apparatus overcoming such a problem, one which will be explained in the following has been known.
FIG. 3
is a configurational view of the rangefinder apparatus in accordance with second prior art. In the rangefinder apparatus in accordance with the second prior art, a clamping circuit
13
is disposed between the second signal processing circuit
12
and arithmetic circuit
14
in the rangefinder apparatus in accordance with the first prior art. The clamping circuit
13
inputs therein the signal I
2
outputted from the second signal processing circuit
12
, compares a clamp signal I
c
having a certain constant level and the signal I
2
with each other in terms of magnitude of their levels, and outputs the clamp signal I
c
if the former is greater and outputs the signal I
2
as it is if not. Namely, the arithmetic circuit
14
outputs the output ratios (I
1
/(I
1
+I
2
) and (I
1
/(I
1
+I
c
) when the distance L to the object is shorter and longer, respectively.
FIG. 4
is a graph showing the relationship between the AF signal outputted from the integrating circuit
15
in this second prior art and the distance to the object. In this graph, the abscissa indicates the reciprocal (
1
/L) of the distance L to the object, whereas the ordinate indicates the output ratio, i.e., AF signal. As shown in this graph, in each of the respective ranges smaller and greater than a certain distance L
4
, the output ratio has substantially a linear relationship with respect to the reciprocal (
1
/L) of the distance L, such that the output ratio decreases as the distance L is greater(
1
/L is smaller). As a consequence, the distance L to the object can be determined uniquely from the output ratio.
SUMMARY OF THE INVENTION
In the rangefinder apparatus in accordance with the second prior art (FIG.
3
), however, when switching the level of the clamp signal I
c
in the clamping circuit
13
, if the level is once initialized and then a new level is to be set, it will take time to switch these levels. Namely, the time for initializing the level of the clamp signal I
c
and the time, subsequent thereto, for setting a new level for the clamp signal I
c
are necessary.
This problem of clamp level switching time would be serious in particular when switching the levels of clamp signal during a continuous distance measurement in which a plurality of distance measuring operations are carried out in series. Examples thereof include the cases where the levels of clamp signal in distance measuring operations are switched according to results of their preceding distance measuring operations, where errors in distance measurements are determined from individual measured distance values obtained by distance measuring operations with respective values of level of the clamp signal, and the like. In such a continuous distance measurement, if it takes time to switch the level of the clamp signal I
c
upon the second or later distance measuring operation, then the time difference required for the period from the release operation until the exposure becomes longer. As a consequence, when a moving object (object to be measured) is to be photographed, for example, a photograph with a desirable composition may not be obtained.
In order to overcome the problems mentioned above, it is an object of the present invention to provide a rangefinder apparatus which takes only a short time to switch levels of a clamp signal.
The rangefinder apparatus in accordance with the present invention comprises: (1) light-projecting means for projecting a luminous flux toward an object to be

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