Distance Measurement System

The present invention relates to a distance measurement system for use with cameras or other optical devices.

As conventional distance measurement systems which determine the distance from an object to a camera or the like by detecting light reflected from the object by using photoelectric elements, a space frequency detection system (contrast detection system), a double image overlapping system (trigonometrical measurement system) and a time-lag measurement system are known. In the systems of the space frequency detection type, those that detect an extreme value of space frequency are in general use. In this type of space frequency detection system, a dip effect of a photoelectric element, that is, the phenomenon that the electrical resistance of the photoelectric element exhibits an the extreme value when the image of the object is in focus, is utilized. In the double image overlapping system, the image of the object is detected by a pair of photoelectric elements and the distance to the object is measured by changing the viewing angle of one photoelectric element while performing mirror-scanning to detect the point at which the two images completely overlap. This system is disclosed in Japanese Patent Publication No. 46-028,500 and U.S. Pat. No. 3,274,914. Honeywell type distance measurement system belongs to that type. A representative example of the time-lag measurement system, which measures the distance between the object and a predetermined point by measuring the time required for ultrasonic sound to travel from that point to the object and to return from the object to the point, is disclosed in Japanese patent publication No. 47-048,408.

The photoelectric elements for space frequency detection system are expensive, and that system involves a complicated signal processing procedure. The double image overlapping system requires some mechanical elements for mirror-scanning and has a limitation to the space obtained for the ground line. Moreover, the time-lag measurement system requires a ultrasonic sound generator which is rather large in size and expensive.

It is therefore an object of the invention to provide a distance measurement system for measuring the distance from the object to a predetermined point by measuring the intensity of light reflected from the object. Such a distance measurement system is simple in mechanism, does not require any mechanical operation, and is advantageous with respect to the space required and cost.

Another object of the invention is to provide a distance measurement system capable of distinguishing the light reflected from the object and external light, and measuring the distance from the object to a predetermined point, regardless of the intensity of external light.

FIG. 1 is a diagram for explaining the principles of the invention.

FIG. 2 is a circuit diagram for use in one embodiment of a distance measurement system according to the invention.

FIG. 3 is an operational timing chart for the circuit diagram in FIG. 2.

FIG. 4 is a circuit diagram for use in another embodiment of the invention.

FIG. 5 is part of a circuit diagram for use in a further embodiment of the invention.

Referring to FIG. 1, light from a first light source 1 reaches an object 2, and the light is reflected from the object 2. Thus, the object 2 becomes may be considered a second light source 3. The quantity B of light from the second light source 3 to a light receiving element 4 is inversely proportional to the square of the distance M between the first light source 1 (and light receiving element 4) and the object 2. Therefore, when the reflection ratio of the object 2 is .alpha. and the quantity of light projected from the first light source 1 is A, the following relationship holds:

Therefore, distance measurement can be performed on the basis that the intensity of the light reflected from the object 2 is reduced at rate inversely proportional to the square of the distance M. In this case, the reflection ratio .alpha. may vary depending upon each object. However, when high accuracy is not required for the distance measurement, the above-mentioned principle can be utilized in a simple auto-focusing device for a camera with two or three focusing zones.

FIG. 2 shows an embodiment of a distance measurement system based on the above-mentioned principle according to the invention. FIG. 3 is a timing chart of the operation of the embodiment.

A pulse generated from an oscillator OSC is inverted by an inverter N1 and is then applied, via a resistor R1, to a projector LED1 comprising a light emitting diode or the like. The projector LED1 is disposed in an appropriate portion of the camera, together with a light receiving element PS, and intermittently emits light or other suitable radiation with a predetermined intensity to an object OB. The light receiving element PS is constructed of photodiodes or the like, and light reflected from the object OB enters the light receiving element PS. The light receiving element PS, a diode D1 and an operational amplifier OP1 constitute a light receiving portion S. The output from the light receiving element PS is subjected to logarithmic compression by the diode D1 and is output from the light receiving portion S. When the projector LED1 is off and does not emit light, the output of the light receiving portion S is the output obtained by detecting external or background light, that is, the light excluding the light from the projector LED1 which enters the light receiving portion after being reflected from the object OB. That output from the light receiving portion S is stored in a capacitor C1 through an analog switch S1 in accordance with the output from the oscillator OSC. Analog switches S2 and S3 are off when the output of the oscillator OSC is applied thereto through an inverter N2. When the projector LED1 projects light, the analog switch S1 is off, while the analog switchs S2 and S3 are on, so that the output from the light receiving portion S is subjected to voltage division by a variable resistor VR1 and the divided voltage is compared, by a comparator COM1, with the voltage stored in the capacitor C1. When the object is positioned at a long distance from the distance measurement system and light from the projector LED1 decreases greatly so that the light from the projector LED1 scarcely enters the light receiving element PS and cannot be distinguished from the external light, the output from the comparator COM1 remains zero. On the other hand, when the object is located close at hand, the output from the comparator COM1 is at a high level. The output of the comparator COM1 is applied to a display device LED2 comprising a light emitting diode through the analog switch S2 and a resistor R2 and, at the same time, to a display device LED3 comprising a light emitting diode through an inverter N3, the analog switch S3 and a resistor R3. Therefore, when the object is close at hand, the display device LED2 is lit. On the other hand, when the object is at a long distance, the display device LED3 is lit. In this case, a display is given to indicate whether the object is closer to the distance measurement system than a target distance or at a longer distance beyond the target distance. The target, or threshold, distance can be varied by the variable resistor VR1.

Since the above-mentioned embodiment does not require the movement of any mechanical devices such as a mirror scanning device, it is inexpensive. Moreover, continuous distance measurement can be performed by the variable resistor VR1. Further, since the output of the light receiving element PS is compressed by the diode D1, it will be difficult to distinguish the reflected light of the projector from external light when the external light is far more intensive than the light of the projector reflected from the object. For elimination of such a short-coming, a filter for abstracting the reflected light, which is disposed in the output side portion .circle.A of the light receiving portion, is effective. For that purpose, a differential circuit disposed in an output side portion of the light receiving section, abstracting the reflected light, is also effective when a strobo flash is employed as the light projecting source. The following embodiment is also useful for detecting only such reflected light.

FIG. 4 shows that embodiment of the present invention. When the projector LED1 is off, the pulse generated from the oscillator OSC is applied to an analog switch S4 through inverters N1, N4, so that the analog switch S4 is on. As a result, the output of the light receiving portion S is stored in the capacitor C2 through an operational amplifier OP2 and the analog switch S4. The voltage stored in the capacitor C2 is applied to the output side of an operational amplifier OP3, so that current i.sub.2 flows from the light receiving element PS to the operational amplifier OP3 through the diode D2. When the projector LED1 is on, the analog switch S4 is off, and the voltage of the capacitor C2 becomes constant. However, when light reflected from the object is detected, the current which flows through the light receiving element PS increases in proportion to the intensity of the reflected light. However, current i.sub.2 is the same as that when the projector LED1 is off, because of the voltage of the capacitor C2. In other words, at that time, the current i.sub.2 is that generated by the external light. The remainder current i.sub.1, which is proportional to the intensity of the reflected light, flows through the diode D1, so that the reflected light can be detected independently of the external light. The output of the light receiving portion S is compared with the reference voltage ref1 of a comparator COM2 and with the reference voltage ref2 of a comparator COM3. When the object is close at hand, the output voltage of the light receiving portion S is smaller than the reference voltage ref1 and ref2, so that the output of the comparator COM2 and that of the comparator COM3 both increase to a high level, and when light is projected, the output of a NAND circuit NA1 decreases to a low level. As a result, a light emitting diode LED4 is lit. When the object is an intermediate zone, the output of the light receiving portion S is greater than the reference voltage ref1 and smaller than the reference voltage ref2, so that the output of the comparator COM2 is at a low level, while the output of the comparator COM3 is at a high level. Therefore, when light is projected, the output of the NAND circuit NA2 decreases to a low level, so that the light emitting diode LED5 is lit. When the object is at a a long distance, the output of the light receiving portion S is greater than the reference voltage ref2, so that the output of the comparator COM2 and that of the comparator COM3 both decrease to a low level, and when light is projected, the output of the NAND circuit NA3 decreases to a low level and a light emitting diode LED6 is lit.

The light emitting diodes LED4 to LED6 are respectively connected between the NAND circuits NA1 to NA3 and a D.C. voltage source via respective resistors. The diode D1 can be a resistor or can be combined with a resistor to form a combination circuit. As shown in FIG. 5, the distance measurement signals can be output continuously by use of NAND circuits NA4 to NA6 and latch circuits FF1 to FF3, in place of the NAND circuits NA1 to NA3. In this case, the output of any of the NAND circuits NA4 to NA6 decreases to a low level, depending upon the location of the object, that is, in the near zone, in the intermediate zone or in the far zone. The output of any of the NAND circuits NA4 to NA6 is then latched by the latch circuits FF1 to FF3 in accordance with the output pulse of the inverter N1. As a result, any of the light emitting diodes LED4 to LED6 is lit.