JP3548282B2 - Optical branching optical system - Google Patents

Description

[0001]
【Technical field】
The present invention relates to a light splitting optical system.
[0002]
[Prior art and its problems]
The light branching optical system is used, for example, in a lightwave distance measuring device. FIG. 7 is a diagram showing a configuration of a conventional example, in which 1 is a light source such as an LED, 2 is a light receiving element, 3 is a half mirror, 4 is an objective lens system, and 5 is a prism for splitting light. A light splitting element, 6 is a prism for image inversion, and 7 is an eyepiece. The objective lens system 4 includes a front group 4a and a rear group 4b, and a light splitting element 5 is located between the two groups.
[0003]
An optical system including the objective lens system 4, the light splitting element 5, the image inverting prism 6, and the eyepiece 7 confirms a distance measuring object (generally, a corner cube) CC with eyes, that is, with visible light. To make up a telescope.
[0004]
The light source 1 emits infrared light as a first light flux. This infrared light has a wavelength of about 850 nm, and has a different wavelength from the visible light as the second light flux. The infrared light emitted from the light source 1 passes through the half mirror 3 and reaches the light splitting element 5 for splitting light. A dichroic coat made of a dielectric multilayer film for reflecting infrared light and transmitting visible light is formed on the division surface 5a of the light dividing element 5. Therefore, the infrared light from the light source 1 is reflected by the dichroic coat, passes through the front group 4a of the objective lens system 4, and then travels to the corner cube CC.
[0005]
The infrared light reflected by the object to be measured returns to the front group 4a of the objective lens system 4, is reflected again by the dichroic coat of the light splitting element 5, is further reflected by the half mirror 3, and enters the light receiving element 2. .
[0006]
As is well known in principle, the lightwave distance measuring device emits infrared light from the light source 1 in a pulsed manner at a predetermined frequency. The time until the light is reflected by the cube CC and enters the light receiving element 2 is detected based on the phase shift between the pulses of the distance measuring light and the reference light, and the distance to the object to be measured is calculated.
[0007]
By the way, the dichroic coat used in the above configuration has a property that its spectral characteristic changes depending on the incident angle of light incident on this coat. FIG. 8 is a diagram illustrating this spectral characteristic, in which the vertical axis represents transmittance (%) and the horizontal axis represents light wavelength. The solid line shows the relationship between the wavelength and the transmittance when the incident angle is 45 °, the broken line shows the relationship at 30 ° and the dashed line shows the relationship at 15 °. The angle of incidence is 0 ° for normal incidence. From this figure, it can be seen that when the incident angle is 15 °, almost 100% of the light is transmitted up to a wavelength of about 750 nm, which is the limit of visible light, and almost 100% of the longer wavelength is reflected. Next, when the incident angle is 30 °, the transmittance starts to decrease from around 700 nm, which is near the upper limit of visible light, but it can be transmitted by about 50% up to about 750 nm, and it can be said that it has almost the same characteristics as the target.
[0008]
However, when the incident angle becomes 45 °, the transmittance from around 600 nm in the visible light region drops to 50% or less, and the transmittance becomes almost 0% between 700 nm and 750 nm. The red component of the image will be significantly reduced. In addition, a slight shift of the observer's eye in a direction orthogonal to the optical axis of the eyepiece 7 changes the color tone of the object being observed.
[0009]
On the other hand, the light incident on the dichroic coat is divergent light from the light source 1 or convergent light that is light returned from the measurement target, and thus has a wide angle at which the light enters the dichroic coat. For example, assuming that the angle α between the reflection film 5a and the optical axis O of the objective lens 4 is 60 °, the incident angle β of the principal ray (the incident ray overlapping with the optical axis) is 30 °. However, the incident angles of the upper ray and the lower ray have a width of about ± 15 ° and extend from 15 ° to 45 °, depending on the F-number of the objective lens. As described above, the spectral characteristic is within the allowable limit up to an incident angle of about 30 °, but when the incident angle is 45 °, the spectral characteristic deteriorates, and there is a problem that the light use efficiency is greatly reduced.
[0010]
[Object of the invention]
The present invention has been made to solve the above-described problem, and has as its object to provide an optical branching optical system in which a change in the incident angle of a light beam incident on a light splitting element is small and the adverse effect of spectral characteristics is small. I have.
[0011]
Summary of the Invention
The light splitting optical system according to the present invention includes a light splitting element that transmits light of a specific wavelength and reflects light in another wavelength range in a light beam condensed by the light collecting lens of the optical system having the light collecting lens. Further, a mirror that reflects light reflected by the light splitting element and guides the light to the outside of the light beam condensed by the light collecting element is disposed in an optical path including the light collecting element and the light splitting element.
By arranging the light splitting element and the mirror in this way, the angle of incidence of the light beam on the light splitting element can be kept within a certain small range, so that the spectral characteristics are not adversely affected.
[0012]
When the mirror is provided near the convergence point of the light beam reflected by the light splitting element, the size of the mirror can be reduced most. Further, if the mirror is fixed to the condenser lens, a support member for the mirror is not required, so that the amount of light blocking can be reduced.
The light splitting element has a plane perpendicular to the optical axis of the condensing lens, and is constituted by a parallel flat plate having one surface provided with a dichroic coat, or similarly, a condensing lens having a dichroic coat provided on one surface thereof. And a curved plate having a convex or concave surface. If the division surface is a convex surface or a concave surface, the degree of freedom of the position of the mirror is increased, and particularly in the case of a convex surface, the angle of incidence can be reduced.
[0013]
The light beam reflected by the light dividing element is, for example, infrared light for distance measurement, and the condenser lens and the light dividing element are provided in a telescope of the distance measuring optical system.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of the present invention. In the drawing, reference numeral 11 denotes a condenser lens located in the collimating telescope of the lightwave distance measuring apparatus, and in this embodiment, F = 1.8. Reference numeral 12 denotes a light splitting element, which has a dichroic coat on one surface (a split surface 12a) of the plane-parallel plate glass. The light splitting element 12 is arranged such that the split surface 12a is located at a position that divides the distance between the condenser lens 11 and the image forming surface 14 into two. Reference numeral 13 denotes a micromirror on the condenser lens 11 and on the optical axis, and reflects all light.
[0015]
The infrared light emitted as a distance measuring light beam from the light source 1 is an LED light having a wavelength of 850 nm, transmits through the half mirror 3, further transmits through the imaging lens 8, and is focused on the mirror 13. The light is reflected and now reaches the light splitting element 12 while diverging. This infrared light is then reflected by the dichroic-coated division surface 12 a of the light division element 12, diverges like a light beam emitted from the focal point O on the imaging surface 14, and reaches the condenser lens 11. When the infrared light further passes through the condenser lens 11, it becomes a parallel light flux and is applied to an object to be measured such as a corner cube. The condenser lens 11 forms a part of a telescope in a distance measuring optical system, and the telescope is used to collimate a distance measuring object such as a corner cube with visible light.
[0016]
The distance measuring light beam (infrared light) reflected from the object to be measured is superimposed on the visible light as the second light beam, enters the condenser lens 11, and converges toward the image plane 14 of the telescope. Proceed to. When the light reaches the light splitting element 12, the infrared light is reflected by the splitting surface 12a. On the other hand, visible light passes through the light splitting element 12 and forms an image on the image forming surface 14.
[0017]
The reflected infrared light converges on a mirror 13 provided on a condenser lens 11, enters the imaging lens 8 while being reflected and diverges, and then passes through the half mirror 3 and enters the light receiving element 2. Form an image. The phase difference is obtained from the infrared light received by the light receiving element 2, and the distance to the object to be measured is calculated.
[0018]
In the above configuration, since the light splitting element 12 is inserted at right angles to the optical axis of the condenser lens 11 and the splitting surface 12a is a plane, the principal ray of the light beam incident on the light splitting element 12 ( The light ray overlapping the optical axis) enters and reflects perpendicularly to the division plane. On the other hand, the upper ray and the lower ray are symmetrical with respect to the principal ray, and their respective incident angles i _U and i _L become the maximum. However, even when the F-number of the condenser lens described above is 1.8, these rays are not scattered. The value falls within about 15 °. That is, if the F-number of the condenser lens is larger than 1.8, the incident angle is always approximately 15 ° or less. FIG. 6 shows spectral characteristics when the incident angle of the dichroic coat 5a in FIG. 7 is from 0 ° to 15 °. As is clear from this figure, in this range, the change in the characteristics is extremely small, and the above-mentioned problem does not occur.
[0019]
FIG. 2 shows a second embodiment of the present invention. In this embodiment, the light splitting element 12 is tilted by a small angle α from a position perpendicular to the optical axis. Angle of incidence i _U of the upper beam is increased when this configuration, the smaller the angle of incidence i _L of lower ray. In addition, the position of the mirror 13 is also slightly away from the optical axis. When the F-number of the condenser lens is 3 and α is 5 °, the incident angle i _U of the upper ray is 14.46 ° and the incident angle i _L of the lower ray is 4.46 °, all of which are set to 15 ° or less. be able to.
[0020]
FIG. 3 shows a third embodiment of the present invention. In this embodiment, the dichroic coat surface of the light splitting element 12 is a spherical surface convex toward the condenser lens 11. Therefore, the position where the light splitting element 12 is arranged is also closer to the image plane 14 than the midpoint between the condenser lens 11 and the image plane 14. With such a configuration, the incident angles i _U and i _L can be reduced even if the F number of the condenser lens 11 is reduced. Further, there is an advantage that the position of the light splitting element can be freely determined by appropriately selecting the radius of curvature of the light splitting element. Although only the convex surface is shown in FIG. 3, it goes without saying that a concave surface can be used depending on the situation.
[0021]
FIG. 4 shows a fourth embodiment of the present invention. In this embodiment, the condenser lens 11 includes a first lens 11a and a second lens 11b, and is disposed between the two lenses 11a and 11b on the surface of the lens 11a on the micromirror 13. Other configurations are basically the same as the embodiment of FIG.
[0022]
FIG. 5 shows a fifth embodiment of the present invention. In this embodiment, a micromirror 13 is provided on the opposite side of the light splitting element 12 of the condenser lens 11 and the mirror 13 is arranged so that a light beam incident or reflected on the mirror does not interfere with the condenser lens 11. Is inclined. The fourth and fifth embodiments are different from the first to third embodiments in that the condenser lens 11 (11b) is located between the mirror 13 and the light splitting element 12. The configuration is such that the reflected light from the light splitting element 12 is focused on the mirror 13 in consideration of the power of the condenser lens 11 (11b). Therefore, the same operation as the first to third embodiments can be obtained.
[0023]
The reflected light flux from the light splitting element 12 does not need to be strictly focused on the mirror 13. However, when the light flux is focused on the mirror 13, the size of the mirror can be reduced, and the cut amount of the amount of light passing through the condenser lens can be reduced. If the position of the light splitting element 12 and the size of the mirror 13 are appropriately determined, an arrangement that does not require a condenser lens is also possible.
[0024]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a light splitting optical system in which the incident angle of a light beam incident on a light splitting element is small, is not adversely affected by spectral characteristics, and has high light use efficiency. .
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a first embodiment of a light splitting optical system according to the present invention.
FIG. 2 is a diagram showing a configuration of a second exemplary embodiment of the present invention.
FIG. 3 is a diagram showing a configuration of a third embodiment of the present invention.
FIG. 4 is a diagram showing a configuration of a fourth embodiment of the present invention.
FIG. 5 is a diagram showing a configuration of a fifth embodiment of the present invention.
FIG. 6 is a diagram showing an example of spectral characteristics when an incident angle on a dichroic coat is in a range of 0 ° to 15 °.
FIG. 7 is a diagram showing a configuration of a conventional light splitting optical system.
FIG. 8 is a diagram showing spectral characteristics when an angle of incidence on a dichroic coat is in a range of 15 ° to 45 °.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light source 11 Condensing lens 12 Light splitting element 13 Mirror 14

Claims (8)

An optical system having a condenser lens;
A light splitting element that transmits light of a specific wavelength and reflects light of another wavelength range, which is disposed in a light beam condensed by a light condensing lens of the optical system; and
A mirror disposed in an optical path including the condensing lens and the light splitting element, for reflecting the light reflected by the light splitting element and guiding the light out of the light condensed by the condensing lens;
A light splitting optical system comprising: 2. The optical branching optical system according to claim 1, wherein the mirror is provided between a light splitting element and a condenser lens. 2. The light splitting optical system according to claim 1, wherein at least one condenser lens is located between the mirror and the light splitting element. 4. The optical branching optical system according to claim 1, wherein the mirror is provided near a convergence point of a light beam reflected by the light splitting element. The light splitting optical system according to any one of claims 1 to 4, wherein the light splitting element has a plane perpendicular to the optical axis of the condenser lens, and a plane parallel plate having a dichroic coat surface on one surface thereof. . 5. The optical splitting optical system according to claim 1, wherein the light splitting element has a curved surface provided with a dichroic coat. 7. The optical branching optical system according to claim 1, wherein the mirror is located on an optical axis near the condenser lens. 7. The optical branching optical system according to claim 1, wherein the mirror is fixed to the condenser lens.

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