A distance-measuring system, which comprises a light source unit for emitting a distance-measuring light, a photodetection optical system having a photodetection optical axis, a projecting optical system having a projecting light optical axis and for projecting the distance-measuring light from the light source unit to an object to be measured and for guiding the distance-measuring light reflected from the object to be measured toward the photodetection optical system, and an internal reference optical system for guiding the distance-measuring light from the light source unit to the photodetection optical system as an internal reference light, wherein the light source unit can emit two distance-measuring lights with different spreading angles, and one of the light source unit and the projection optical system has a decentering member for decentering the distance-measuring light with respect to the projecting light optical axis.

BACKGROUND OF THE INVENTION

The present invention relates to a distance-measuring system for measuring a distance to an object by using a laser beam.

In recent years, a non-prism distance-measuring system, a distance to an object to be measured is measured by directly projecting a laser beam for distance measurement to the object to be measured.

In the non-prism distance-measuring system, a laser beam with a smaller beam diameter is used. By using the laser beam with a smaller beam diameter, the laser beam can be projected to the object to be measured by pinpoint. A measuring position on the object can be clearly defined, and it is possible to measure a crest line or a specific point of the object to be measured.

Because an intensity of the projected laser beam is limited from reasons such as safety, in the non-prism distance measuring system, in which high reflection from the object to be measured cannot be expected, the measured distance is shorter compared with a distance-measuring system using a prism (corner cube).

For this reason, a prism is used as the object to be measured in long-distance measurement. Also, a laser beam having a relatively large beam spreading is used to facilitate collimation and to achieve measurement with high accuracy.

As described above, the beam diameter of the laser beam is small in the non-prism distance-measuring system, and it is difficult to project the laser beam to the prism. Accordingly, this is not suitable for the measurement of long distance by using a prism.

However, it is not very economical to provide a distance-measuring system for long distance using a prism and a non-prism distance-measuring system. In this respect, it is proposed now to have a distance-measuring system, in which it is possible to carry out distance-measurement using a prism and non-prism distance-measurement in a single distance-measuring system.

For instance, a distance-measuring system is proposed as described in JP-A-2000-88566 (FIG. 1, and paragraphs [0029]–[0035]), in which distance-measurement using a prism and distance-measurement on non-prism basis can be carried out by a single distance-measuring system.

Referring toFIG. 6, brief description will be given below.

There are provided a first light source2for emitting a visible laser beam1and a second light source4for emitting an infrared laser beam3. The visible laser beam1and the infrared laser beam3are emitted independently from each other. The visible laser beam1is a laser beam of parallel luminous fluxes with a small beam diameter, and the infrared laser beam3is a divergent laser beam.

The visible laser beam1or the infrared laser beam3is selected depending upon the object to be measured. For instance, in case an object to be measured5is a reflector such as a corner cube, the divergent infrared laser beam3is projected. In case the object to be measured5is a wall surface of a building, etc., the visible laser beam1with a small beam diameter is projected. A reflected light11from the object to be measured5is received by a detector8through an objective lens6and a filter7. Based on a signal from the detector8, a distance to the object to be measured5can be measured by an arithmetic operation unit12.

The filter7transmits only wavelength ranges of the visible laser beam1and the infrared laser beam3. Unnecessary light such as sunlight is cut off to improve detection accuracy of the detector8to detect the reflection light11.

In the conventional system as described above, two light sources are used, and this requires a complicated light emitting unit for controlling the light sources, etc. Also, the visible laser beam1and the infrared laser beam3are used, and the filter7must cope with wavelength ranges of the two laser beams, and this means that higher cost is involved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a distance-measuring system, in which it is possible to carry out distance measurement using a prism and non-prism distance measurement, and which contributes to simple designing of the system.

To attain the above object, the present invention provides a distance-measuring system, which comprises a light source unit for emitting a distance-measuring light, a photodetection optical system having a photodetection optical axis, a projecting optical system having a projecting light optical axis and for projecting the distance-measuring light from the light source unit to an object to be measured and for guiding the distance-measuring light reflected from the object to be measured toward the photodetection optical system, and an internal reference optical system for guiding the distance-measuring light from the light source unit to the photodetection optical system as an internal reference light, wherein the light source unit can emit two distance-measuring lights with different spreading angles, and one of the light source unit and the projection optical system has a decentering member for decentering the distance-measuring light with respect to the projecting light optical axis. Also, the present invention provides the distance-measuring system as described above, wherein the photodetection optical system can receive the reflected distance-measuring light decentered in a direction opposite to the projected distance-measuring light. Further, the present invention provides the distance-measuring system as described above, wherein the projecting optical system comprises an optical path deflecting member for deflecting the distance-measuring light toward the direction of the object to be measured and for decentering the distance-measuring light with respect to the projecting light optical axis, wherein the projecting optical system guides the reflected distance-measuring light entering with decentered in the direction opposite to the projected distance-measuring light. Also, the present invention provides the distance-measuring system as described above, wherein the deflecting member is a mask having a hole decentered with respect to the optical axis and decenters the distance-measuring light from the optical axis by allowing the distance-measuring light to pass through the hole. Further, the present invention provides the distance-measuring system as described above, wherein the mask is provided on a distance-measuring optical path with a larger spreading angle. Also, the present invention provides the distance-measuring system as described above, wherein the mask is provided on a common optical path for two distance-measuring lights. Further, the present invention provides the distance-measuring system as described above, wherein the light source unit has a first optical path and a second optical path, the distance-measuring light is projected via the first optical path and the second optical path, the distance-measuring light is projected with a smaller spreading angle via the first optical path, and the distance-measuring light is projected with a larger spreading angle via the second optical path. Also, the present invention provides the distance-measuring system as described above, wherein the light source unit has an optical path switching means, and the optical path switching means selectively guides the distance-measuring light emitted from a light source either to the first optical path or to the second optical path. Further, the present invention provides the distance-measuring system as described above, wherein the light source unit has two light sources each emitting the distance-measuring light, the distance-measuring light emitted from one of the light sources is projected via the first optical path, and the distance-measuring light emitted from the other light source is projected via the second optical path. Also, the present invention provides the distance-measuring system as described above, wherein an optical fiber is arranged on the second optical path, and an end surface of the optical fiber acts as a light source. Further, the present invention provides the distance-measuring system as described above, wherein the optical path switching means is a rhombic prism which is provided so as to span between the first optical path and the second optical path, and optical paths can be switched over by inserting or removing the rhombic prism to or from the first optical path and the second optical path. Also, the present invention provides the distance-measuring system as described above, wherein the two light sources are selectively turned on. Further, the present invention provides the distance-measuring system as described above, wherein the photodetection optical system comprises a doughnut lens for converging the reflected distance-measuring light deviated from the photodetection optical axis to the photodetection optical axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be given below on embodiments of the invention referring to the drawings.

FIG. 1is a schematical block diagram of a first embodiment according to the present invention. In the figure, reference numeral15denotes a light source unit,16is a projecting optical system,17is an internal reference optical system,18is a photodetection optical system, and19is an ocular optical system (telescope).

First, description will be given on the light source unit15.

A laser light source21emits an infrared distance-measuring light with a wavelength of 780 nm, for instance. On an optical axis20of the laser light source21, there are provided a first collimator lens22, a mixing means23, and an optical path switching means24.

As the mixing means23as described above, means disclosed in JP-A-2002-196076 is used, for instance.

The mixing means23described in JP-A-2002-196076 has a pair of gradient index lenses provided on the optical axis and a phase plate placed between the gradient index lenses to shut off the optical path so that the phase plate can be rotated. The phase plate has projections and recesses formed on a checkerboard pattern. The projections and recesses are designed in such manner that there occurs phase difference of π/2 of the wavelength of the laser beam.

The optical path switching means24can select a first optical path25or a second optical path26. It is designed that the first optical path25and the second optical path26are aligned with a projecting light optical axis27by the optical switching means24.

In the optical path switching means24, a rhombic prism28is rotatably supported, for instance, when the first optical path25is selected, a distance-measuring light from the laser light source21passes through the mixing means23and enters the rhombic prism28. Then, the distance-measuring light is reflected twice by the rhombic prism28and is aligned with the projecting light optical axis27while it is running in parallel to the optical axis20.

The second optical path26is aligned with an extension of the optical axis20. On the second optical path26, there are provided a second collimator lens29and an optical fiber31. A third collimator lens32is arranged at an exit end of the optical fiber31, and an optical axis of the third collimator lens32is aligned with the projecting light optical axis27. A mask30is arranged between the rhombic prism28and the third collimator lens32on the projecting light optical axis27.

As seen inFIG. 2, the mask30has a transmission hole30′ decentered with respect to the projecting light optical axis27so that a part of the distance-measuring light projected from the optical fiber31is intercepted. InFIG. 2, the transmission hole30′ is designed in circular shape, while the transmission hole30′ may be in rectangular shape or in elliptical shape or in circular shape with a partially lacking portion, etc. It may be designed in such manner that a graphic gravity center of the transmission hole30′ is decentered with respect to the projecting light optical axis27and that a luminous flux of the distance-measuring light on the projecting light optical axis27can pass through.

Under the condition that the optical path switching means24selects the second optical path26, the rhombic prism28is deviated from the optical axis20. The distance-measuring light from the mixing means23is converged by the second collimator lens29. Then, the distance-measuring light enters the optical fiber31through an incident end of the optical fiber31. The distance-measuring light projected from the optical fiber31is turned to a parallel luminous flux by the third collimator lens32, which are then projected along the projecting light optical axis27.

The light source unit15has a pointer light source33. An LD is used as the pointer light source33. It emits a visible pointer laser beam, and the pointer laser beam is turned to a parallel luminous flux by a fourth collimator lens34. The pointer laser beam is projected toward an object to be measured (not shown) by the projecting optical system16.

Now, description will be given on the projecting optical system16.

On the projecting light optical axis27, there are provided a beam splitter35, a concave lens36, a first optical path deflection member37, a second optical path deflection member38, and an objective lens39. A projecting light amount adjusting means41is arranged between the beam splitter35and the concave lens36. The second optical path deflection member38has its graphic gravity center decentered with respect to the projecting light optical axis27. The second optical path deflection member38has a size large enough to reflect the distance-measuring light, which enters via the mask30.

The second optical path deflection member38is decentered upward with respect to the projecting light optical axis27as shown inFIG. 1, for instance. The luminous flux of the distance-measuring light reflected by the second optical path deflection member38is also decentered upward with respect to the projecting light optical axis27.

The projecting light amount adjusting means41comprises a light amount adjusting plate43, which has transmission light amount continuously changing in a circumferential direction, and which is rotated by a light amount adjusting motor42such as a stepping motor or the like having a positioning function. The light amount adjusting plate43is provided to intercept the projecting light optical axis27.

The concave lens36is arranged in such manner that a focusing position of the concave lens36is aligned with a focusing position of the objective lens39. Together with the objective lens39, it makes up a beam expander so that the parallel luminous flux guided to the concave lens36are spread and projected. In this respect, influences by optical elements such as the beam splitter35, the light amount adjusting plate43, etc. can be minimized. Compared with a structure where the laser light source21is arranged at the focusing position of the objective lens39, light projecting efficiency is improved.

The beam splitter35transmits nearly all of the distance-measuring light (infrared light) from the laser light source21and reflects a part of the light. The beam splitter35also totally reflects the pointer laser beam (visible light) from the pointer light source33. The first optical path deflection member37is a mirror, etc., and the second optical path deflection member38is a mirror or a dichroic mirror, etc. to reflect the distance-measuring light. The second optical path deflection member38totally reflects the distance-measuring light. The second optical path deflection member38partially reflects the pointer laser beam (visible light) and partially transmits the pointer laser beam.

Now, description will be given on the internal reference optical system17.

The internal reference optical system17is provided between the light source unit15and the photodetection optical system18as to be described later. The internal reference optical system17has an internal reference optical axis44aligned with a transmission light optical axis of the beam splitter35. On the internal reference optical axis44, there are provided a condenser lens45, a density filter46, and a dichroic prism47.

A chopper means48is provided so as to span between the projecting light optical axis27and the internal reference optical axis44. The chopper means48has a chopper plate49for intercepting the projecting optical axis27and the internal reference optical axis44and a chopper motor51, which can rotate the chopper plate49and determine a position of the chopper plate49. Under the condition that the chopper plate49intercepts the projecting light optical axis27, the internal reference optical axis44is open to pass through. Under the condition that the chopper plate49intercepts the internal reference optical axis44, the projecting light optical axis27is open to pass through.

By rotating the chopper plate49, it can be alternatively selected whether the distance-measuring light from the light source unit15is projected along the projecting light optical axis27or it is projected to the internal reference optical axis44as an internal reference light.

The density filter46is to adjust light intensity of the internal reference light so that light intensity of the distance-measuring light reflected from the object to be measured would be approximately equal to the light intensity of the internal reference light.

The photodetection optical system18has a photodetection optical axis52, which is aligned with an extension of the internal reference optical axis44. On the photodetection optical axis52, there are provided the dichroic prism47, a doughnut lens53, a photodetection fiber54, a fifth collimator lens55, an interference filter56, a condenser lens57, and a photodetection element58. As the photodetection element58, an avalanche photodiode (APD) is used, for instance. The interference filter56has such a characteristic as to transmit a light of narrow band, e.g. a light in wavelength range near 800 nm. When the photodetection element58receives a reflected distance-measuring light, a photodetection signal is sent to an arithmetic operation unit65. At the arithmetic operation unit65, a distance to the object to be measured is calculated based on the photodetection signal.

Description will be given now on the ocular optical system19.

The ocular optical system19has an ocular optical axis60. The ocular optical axis60is aligned with an extension of the optical axis of the objective lens39, which passes through the dichroic prism47. On the ocular optical axis60, there are provided a focusing lens61movably arranged along the ocular optical axis60, an erect prism62for converting an image to an erect image, a collimation plate63with a collimation line such as a cross, and an ocular lens64.

Now, description will be given on operation.

First, the pointer light source33is turned on, and a laser beam for pointer is emitted. The pointer laser beam is reflected by the beam splitter35. Then, the pointer laser beam is projected via the first optical path deflection member37and the second optical path deflection member38toward the object to be measured through the ocular lens39. The pointer laser beam is projected coaxially with the projecting light optical axis27, and the pointer laser beam is accurately projected to a measuring point. A projecting point of the pointer laser beam is observed by the ocular optical system19, and a measuring point is determined. When the measuring point is determined, the pointer light source33is turned off.

The pointer light source33is turned on only when necessary. As a result, chances are reduced to project the laser light to eyes of an operator or a passer-by at a working place. This eliminates the possibility to give uncomfortableness to the operator or the passer-by or to make them feel dizziness.

When non-prism distance measurement is performed by using a wall surface of a building as the object to be measured, non-prism measurement is selected.

When the non-prism measurement is selected, the rhombic prism28is positioned to intercept the second optical path26and the projecting light optical axis27. The distance-measuring light emitted from the laser light source21is mixed by the mixing means23. After being mixed, light amount speckles is eliminated, and measurement accuracy is increased. By the rhombic prism28, the optical path is deflected toward the first optical path25. The distance-measuring light passes through the beam splitter35and is projected to the object to be measured by the projecting optical system16.

A beam diameter and a spreading angle of the projected distance-measuring light depend upon the size of the light source. A light emitting point of the laser light source21is about 3 μm in diameter in case of a semiconductor laser (LD), and a distance-measuring light with a small diameter is projected.

The distance-measuring light is projected from the projecting optical system16to the object to be measured. The distance-measuring light reflected by the object to be measured has generally a reflection surface, which is not in form of a mirror surface or a surface similar to mirror surface, and the light is diffused. The reflected distance-measuring light passes through the projecting light optical axis27and enters the objective lens39. Then, the reflected distance-measuring light is converged by the objective lens39, enters the dichroic prism47and is further reflected by the dichroic prism47. The luminous flux of the reflected distance-measuring light entering the dichroic prism47is sufficiently larger than the second optical path deflection member38.

Due to the arrangement of the projecting optical system16, such as the positioning of the second optical path deflection member38on the projecting light optical axis27, the reflected distance-measuring light guided to the photodetection optical system18is a luminous flux, which lacks the central portion. For this reason, when the object to be measured is at near distance, it happens sometimes that the lacking portion of the luminous flux of the reflected distance-measuring light agree with an incident end surface of the photodetection fiber54and the reflected distance-measuring light does not enter the photodetection optical system18. The doughnut lens53is used to refract the luminous flux in the periphery of the reflected distance-measuring light and to make it enter the photodetection fiber54. As a result, regardless of whether the measuring distance is far or near, the reflected distance-measuring light is guided toward the photodetection optical system18.

When the reflected distance-measuring light enters the photodetection fiber54and is guided to the fifth collimator lens55by the photodetection fiber54, it is turned to a parallel luminous flux by the fifth collimator lens55. Disturbance light is cut off by the interference filter56, and the luminous flux is converged to the photodetection element58by the condenser lens57. The photodetection element58receives a distance-measuring light with a higher S/N ratio.

The light amount adjusting motor42rotates the light amount adjusting plate43according to the distance measurement and adjusts the intensity of the projected distance-measuring light by the light amount adjusting plate43. Regardless of the distance to the object to be measured, intensity of the reflected distance-measuring light received by the photodetection element58is adjusted to a constant value. The chopper means48switches whether the distance-measuring light should be projected to the object to be measured or to the photodetection optical system18as the internal reference light. The density filter46adjusts light intensity of the internal reference light so that the intensity of the internal reference light is approximately equal to the light intensity of the reflected distance-measuring light.

The photodetection element58transmits photodetection signals of the reflected distance-measuring light and the internal reference light to the arithmetic operation unit65, and the arithmetic operation unit65calculates a distance to the object to be measured according to the photodetection signals from the photodetection element58. As described above, disturbance light except the light of wavelength range of the reflected distance-measuring light is removed by the interference filter56. As a result, the reflected distance-measuring light received by the photodetection element58has a higher S/N ratio, and this makes it possible to measure the distance with high accuracy.

In the prism measurement, for the purpose of reducing the error caused by deviation of a visual axis of a telescope from the distance-measuring optical axis, a luminous flux with a larger spreading angle is projected.

When the prism measurement is selected, the rhombic prism28is placed at a position deviated from the second optical path26and the projecting light optical axis27. The distance-measuring light emitted from the laser light source21is mixed by the mixing means23. By the mixing, light amount speckles is eliminated, and measurement accuracy is improved.

The distance-measuring light is converged to and enters an incident end surface of the optical fiber31by the second collimator lens29. An exit end surface of the optical fiber31is positioned on the projecting light optical axis27. The distance-measuring light projected from the optical fiber31is converged by the third collimator lens32, passes through the mask30and the beam splitter35and is projected to the object to be measured (a retroreflection prism such as a corner cube) by the projecting optical system16. After passing though the mask30, a part of the distance-measuring light is shut off, and the luminous flux of the distance-measuring light is decentered with respect to the projecting light optical axis27(decentered upward inFIG. 1).

As described above, a beam diameter and a spreading angle of the projected distance-measuring light depend on the size of the light source. In the prism measurement, the exit end surface of the optical fiber31acts as a secondary light source. The end surface of the optical fiber31is 300 μm in diameter. This is sufficiently larger than the diameter 3 μm of the semiconductor laser (LD) in the non-prism measurement as described above. Thus, a distance-measuring light with a larger spreading angle is projected.

In the prism measurement, the conditions for the measurement with high accuracy include a larger spreading angle and an uniform distance-measuring light. The projected distance-measuring light is mixed by the mixing means23and is turned to multi-mode by multiple reflections when it passes through the optical fiber31. Thus, speckles caused by coherence of the laser beam can be prevented and the light amount speckles can be eliminated.

After being reflected by the object to be measured, the luminous flux of the distance-measuring light is decentered downward inFIG. 1. Then, the distance-measuring light enters the objective lens39via the projecting light optical axis27, and the distance-measuring light is converged by the objective lens39.

After passing through the objective lens39, a part of the reflected distance-measuring light is intercepted by the second optical path deflection member38. The luminous flux of the distance-measuring light is decentered downward, and, further, the second optical path deflection member38is decentered upward. As a result, the part of the reflected distance-measuring light not intercepted by the second optical path deflection member38enters the dichroic prism47.

After being reflected by the dichroic prism47, the reflected distance-measuring light enters the photodetection fiber54. By the photodetection fiber54, the reflected distance-measuring light is guided to the fifth collimator lens55. Then, it is turned to a parallel luminous flux by the fifth collimator lens55. Disturbance light is cut off by the interference filter56. The luminous flux is converged by the condenser lens57and is received by the photodetection element58.

Also, in the distance measurement by using the prism measurement, disturbance light is cut off by the interference filter56, and an S/N ratio is increased. The reflected distance-measuring light entering the interference filter56is turned to normal incident light by the fifth collimator lens55, and reduction of the light amount of the reflected distance-measuring light due to the interference filter56is prevented. These are the same as in the non-prism measurement.

Referring toFIG. 3, description will be given on the optical path switching means24.

The rhombic prism28is held by a prism holder66. A rotation shaft67is protruded from the prism holder66, and the rhombic prism28is rotatably supported via the rotation shaft67. A motor (not shown) and an actuator (not shown) such as, solenoid, etc. are connected with the rotation shaft67, and the rhombic prism28is rotated at a predetermined angle by the actuator so as to be insertable to and removable from the second optical path26or the projection light optical axis27.

FIG. 4shows another type of optical path switching means24.

InFIG. 4, the same component as inFIG. 1is referred by the same symbol.

A first half-mirror68is arranged as a beam splitter on the second optical path26, and a second half-mirror69is provided as a beam splitter on the projecting light optical axis27. The first half-mirror68and the second half-mirror69are arranged at opposed positions and in parallel to each other and are mechanically fixed on a housing or the like of the distance-measuring system. By setting the second optical path26and the projecting light optical axis27to parallel to each other, the first optical path25is formed between the first half-mirror68and the second half-mirror69. The distance-measuring light reflected by the first half-mirror68and reflected by the second half-mirror69after passing through the first optical path25runs along the projecting light optical axis27and is projected by the projecting optical system16. A luminous flux switcher71is spanned between the second optical path26and the first optical path25. The luminous flux switcher71comprises a luminous flux switching plate72having a transmission hole (not shown) and a motor73for rotating the luminous flux switching plate72. The luminous flux switching plate72intercepts the first optical path25when the second optical path26is opened, and it intercepts the second optical path26when the first optical path25is opened.

The luminous flux switcher71guides the distance-measuring light passing through the first half-mirror68to the optical fiber31. The luminous flux switcher71also guides the distance-measuring light reflected by the first half-mirror68toward the projecting optical system16via the second half-mirror69.

The mask30may be provided on the projecting light optical axis27. In this case, a part of the distance-measuring light is shut off not only in the prism measurement but also in the non-prism measurement. In case of the non-prism measurement, the reflected distance-measuring light is a diffused light. Thus, even when a part of the reflected distance-measuring light is shut off, sufficient light amount can be obtained, and no trouble occurs in the measurement.

FIG. 5shows a second embodiment of the present invention.

In the second embodiment, such a case is shown that a light source for the prism measurement and a light source for the non-prism measurement are provided separately from each other.FIG. 5shows a light source unit15, and the other arrangement is the same as shown inFIG. 1, and detailed description is not given here.

On an optical axis20, there are provided a laser light source21as a light source for non-prism measurement, a first collimator lens22, a mixing means23, and a beam splitter74. On a second optical path26perpendicularly crossing the optical axis20at the beam splitter74, there are provided an auxiliary laser light source75, an auxiliary first collimator lens76, an auxiliary mixing means77, a second collimator lens29, an optical fiber31, and a third collimator lens32. It is designed in such manner that an auxiliary distance-measuring light emitted from the auxiliary laser light source75enters the beam splitter74.

In the second embodiment, a mask30is provided between the third collimator lens32and the beam splitter74on the second optical path26. As the auxiliary laser light source75, an LD with the same specification as that of the laser light source21is used. The mask30may be arranged on the projecting light optical axis27.

In the laser light source21and the auxiliary laser light source75, light emission, flashing, etc. are controlled by a light source control unit78.

When the non-prism measurement is carried out, the laser light source21is turned on, and the auxiliary laser light source75is turned off. A distance-measuring light from the laser light source21is converged by the first collimator lens22and is mixed by the mixing means23. Then, the distance-measuring light passes through the beam splitter74and is projected along the projecting light optical axis27. Or, the optical path is switched over by the chopper means48, and the distance-measuring light is guided to the photodetection optical system18(not shown) via the internal reference optical axis44(SeeFIG. 1). As described above, a diameter of a light emitting point of the laser light source21is small, and a distance-measuring light suitable for the non-prism measurement can be obtained.

When the prism measurement is carried out, the auxiliary laser light source75is turned on, and the laser light source21is turned off. An auxiliary distance-measuring light is converged by the auxiliary first collimator lens76and is mixed by the auxiliary mixing means77. Then, the light is converged to an incident end surface of the optical fiber31via the second collimator lens29. After passing through the optical fiber31, the auxiliary distance-measuring light is turned to a parallel luminous flux by the third collimator lens32. Then, the luminous flux is reflected by the beam splitter74and is projected via the projecting light optical axis27. Or, the optical path is switched over by the chopper means48, and the auxiliary distance-measuring light is guided to the photodetection optical system18(not shown) via the internal reference optical axis44(SeeFIG. 1).

In case of the prism measurement, an exit end surface of the optical fiber31acts as a secondary light source. Because the exit end surface of the optical fiber31has a diameter of 300 μm, an auxiliary distance-measuring light with a large spreading angle suitable for the prism measurement can be obtained. The auxiliary distance-measuring light is turned to multi-mode by the optical fiber31. As a result, an uniform auxiliary distance-measuring light without light amount speckles is projected. The effect of the mask30is the same as in the first embodiment.

Regarding reflectivity and transmissivity of the beam splitter74, the light amount may be lesser in the prism measurement than in the non-prism measurement. Thus, transmissivity of the distance-measuring light may be set higher, and reflectivity of the auxiliary distance-measuring light may be set lower.

In the second embodiment, the auxiliary laser light source75and the laser light source21can be switched over by the light source control unit78, and the optical path switching means24as shown in the first embodiment may not be used. There is an individual difference between the laser light source21and the auxiliary laser light source75at the time of production, and it is not possible to emit distance-measuring lights of completely the same wavelength. However, the difference is an error of such order as included in transmission wavelength range of the interference filter56(SeeFIG. 1). There is practically no trouble, and a distance-measuring light having substantially the same wavelength can be emitted. An S/N ratio as high as that of the first embodiment can be obtained, and measurement accuracy can be maintained at high level.

In the second embodiment, the mixing means23may not be used.

The present invention provides a distance-measuring system, which comprises a light source unit for emitting a distance-measuring light, a photodetection optical system having a photodetection optical axis, a projecting optical system having a projecting light optical axis and for projecting the distance-measuring light from the light source unit to an object to be measured and for guiding the distance-measuring light reflected from the object to be measured toward the photodetection optical system, and an internal reference optical system for guiding the distance-measuring light from the light source unit to the photodetection optical system as an internal reference light, wherein the light source unit can emit two distance-measuring lights with different spreading angles, and one of the light source unit and the projection optical system has a decentering member for decentering the distance-measuring light with respect to the projecting light optical axis. As a result, even when the object to be measured is at near distance or at remote distance, and regardless of whether it is non-prism measurement or prism measurement, distance measurement can be carried out, and this facilitates the designing of a system with simple structure.