JPH08304694A - Focus detector - Google Patents

Abstract

PURPOSE: To provide a focus detector capable of performing stable focusing operation over a wide range in the height direction of an examinee in addition to the accurate decision of focusing and coping with the examinee having various shapes. CONSTITUTION: This focus detector is constituted of a focus detecting means provided with photodetectors 28 and 29 and a signal processing system 31 for bringing a surface to be measured 25 into focus by using reflected light from the surface 25 by irradiating the surface 25 with measuring light from a laser projection means 20 through an optical system including an image-formation lens 23 and an objective lens 24; and a position detecting means provided with a photodetector 38, a threshold voltage generator 35 and a comparator 34 for detecting that the surface 25 is separated by a specified amount from a focusing position based on the quantity of the reflected light from the surface 25 through the optical system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detecting device used for a microscope, an optical measuring instrument or the like for focusing on a subject.

[0002]

2. Description of the Related Art Focus detection for irradiating a measurement surface (hereinafter referred to as a measurement surface) of a subject with a probe light through an objective lens and performing focus detection for the subject based on light reflected from the measurement surface. Devices are known in the art. Although several configurations are known as this type of focus detection device,
Representative devices among these will be described below.

FIG. 3 is a diagram showing the structure of a conventional focus detection device. As shown in this figure, first, the laser beam emitted from the semiconductor laser 1 is transmitted through the polarization beam splitter 2
Incident on. The light reflected by the polarization beam splitter 2 is collimated by the imaging lens 4 via the quarter-wave plate 3 and is condensed on the surface 6 to be measured via the objective lens 5.
Then, the reflected light reflected by the measured surface 6 is again
It enters the polarization beam splitter 2 through the objective lens 5, the imaging lens 4 and the quarter-wave plate 3. At this time, when the incident light is transmitted through the quarter wavelength plate 3, the polarization direction thereof is shifted by 90 degrees. Therefore, this incident light passes through the polarization beam splitter 2 and is split into two directions by the beam splitter 7. And this one ray is
The first light receiving element 9 is irradiated with light through the first diaphragm 8 which is arranged in front of the focusing point P of the imaging lens 4 by a distance L,
Further, the other light ray has a distance L from the condensing point P of the imaging lens 4.
The second light receiving element 11 is irradiated with light through the second diaphragm 10 arranged only at the rear.

First light receiving element 9 and second light receiving element 11
Has a function of generating an electric signal corresponding to the amount of the reflected light from the measured surface 6 which is respectively received, and sending the electric signal to the signal processing system 12. Further, the signal processing system 12 has a function of performing a predetermined calculation on each input signal and outputting a displacement signal according to the displacement of the surface 6 to be measured. Now, from the first light receiving element 9 and the second light receiving element 10 to the signal processing system 1
Suppose that the electric signals B and A having the characteristics shown in FIG.
As a signal for detecting the displacement of the measured surface 6 by (A-
B) / (A + B) is calculated, and a displacement signal that becomes 0 at the focal point F as shown in FIG. 4B is obtained.
Then, this displacement signal becomes 0, that is, {(A−B)
Focusing operation is performed by moving the measured surface 6 itself or the focus detection device so that the measured surface 6 comes to a position that satisfies / (A + B)} = 0.

Further, as is clear from the graph shown in FIG. 4A, when the measured surface 6 is located at a position away from the focal point F, the output levels of the generated signals A and B are Since it becomes low, it is easily affected by electrical and optical noises. As a result, even when the surface 6 to be measured is located at a position other than the true focal point F, a determination result of {(A−B) / (A + B)} = 0 may occur. (Hereafter referred to as pseudo focus).
Therefore, in order to prevent the occurrence of such pseudo focusing, a threshold value T is set for the signal (A + B) as shown in FIG. In such a case, that is, when the surface 6 to be measured is separated from the focus F by a certain amount or more, the focus determination is not performed and the focus operation range is limited.

[0006]

However, in the conventional focus detection device, an error occurs in the determination of the in-focus point F due to the reflected light noise at the optical member of the device, the electrical noise, the adjustment degree of the optical member, or the like. There are cases. This focusing error is measured in the height direction of the subject by measuring the position of the objective lens or the height of the subject when the focus detection device is focused on the surface to be measured. When used in an optical measuring device, it causes a problem in measurement accuracy.

In order to make a more accurate focus determination in a realistic scene taking such noise into consideration, a large inclination of the displacement signal in the vicinity of the focus point F is set as described later (see FIG. 5). ), The displacement of the measured surface can be controlled to be small with respect to the same amount of noise. As a device configuration for executing this, if the magnification of the optical system provided in the focus detection device, that is, the magnification by the objective lens 5 and the imaging lens 4 in the conventional example shown in FIG. Good. As a result, the amount of movement of the condensing point P of the imaging lens 4 on the side of the light receiving element 11 with respect to the displacement of the surface 6 to be measured increases, and the inclination of the displacement signal near the focal point F can be increased. .

FIGS. 5 (a) and 5 (b) show the effect of the magnification change of the optical system and its displacement signal in the above-mentioned conventional example. However, in the conventional device configuration, when the magnification of the optical system of the focus detection device is increased, the photodetector output also changes as shown in FIG. 5A, and the output is compressed into a narrow region near the in-focus point F. become. Further, the above-mentioned signal (A + B) also changes similarly. Therefore, the range in which the focusing operation can be substantially performed is limited to a narrow region.

As described above, in the conventional focus detection device,
It has been difficult to realize accurate focus determination and focus operation over a wider area at the same time. This has been a problem in that the high accuracy of measurement and the wide range of the measurable height direction are not compatible with each other particularly when the focus detection device is applied to the optical measurement device.

Therefore, the present invention has been made in view of the above problems of the prior art, and covers a wide area along the height direction of the subject while ensuring the accuracy of focus determination. An object of the present invention is to provide a focus detection device which can perform a focusing operation and can deal with objects having various shapes.

[0011]

In order to achieve the above object, a focus detection apparatus according to the present invention is a focus detection apparatus which irradiates a subject with measurement light through an optical system and focuses the subject on the reflected light. And means for detecting that the subject is within a predetermined distance range from the in-focus position based on the amount of light reflected from the subject through the optical system. Further, the focus detection means of the focus detection apparatus according to the present invention comprises a light emitting means, an optical system for irradiating the subject with the measurement light from the light emitting means and condensing the reflected light from the subject, and the reflected light. A light splitting means for splitting, first and second light detecting means respectively arranged on the front side and the rear side of the condensing position of the reflected light, and a signal processing system. And Further, the position detecting means of the device according to the present invention comprises a light detecting means arranged at an image forming position of the optical system,
And a threshold value setting means for setting a threshold value to the output from the light detection means.

[0012]

In the focus detecting apparatus of the present invention, the position detecting means is provided separately from the focus detecting means, and this position detecting means detects that the measured surface of the subject is away from the in-focus point by a predetermined amount. be able to. Therefore, by setting the detectable range of the position detecting means to a wider range including the focus detectable range of the focus detecting means, the surface to be measured is focused by the focus detecting means based on the detection result of the position detecting means. It becomes possible to drive into the operable range, and the focusable range is substantially expanded.

[0013]

The present invention will be described in detail below with reference to the illustrated embodiments.

FIG. 1 is a diagram showing the structure of a focus detection device according to the present invention. In the apparatus of the present invention, as shown in FIG. 1, the laser beam emitted from the laser emission means (light source) 20 is reflected by the polarization beam splitter 21,
The light enters the imaging lens 23 via the four-wave plate 22. Then, this incident light is made into a parallel light flux by the imaging lens 23,
It is condensed on the measured surface 25 via the objective lens 24. Next, the light reflected on the surface 25 to be measured again passes through the objective lens 24, the imaging lens 23, and the quarter-wave plate 22, and then passes through the polarization beam splitter 21 and is guided to the beam splitter 36. , Here, the optical path is divided into two optical paths. Of the light split in the two directions,
One of the light beams passes through the beam splitter 36 and is then guided to the beam splitter 26 where it is split into two optical paths again. Then, one of the lights is transmitted to the beam splitter 2
After passing through 6, the first light receiving element 28 is passed through the first diaphragm 27 arranged in front of the condensing point Q of the imaging lens 23.
Incident on. Further, the light reflected by the beam splitter 26 is configured to be incident on the second light receiving element 30 via the second diaphragm 29 arranged behind the converging point Q of the imaging lens 23. There is.

The first light receiving element 28 and the second light receiving element 3
Reference numeral 0 denotes a photoelectric conversion element, which is a surface to be measured 25 that receives light.
It has a function of generating electric signals B and A corresponding to the reflected light from and outputting the electric signals to the signal processing system 31. Then, the signal processing system 31 performs an operation of (A−B) / (A + B) on the input signals A and B,
It has a function of generating a displacement signal corresponding to the displacement of the measured surface 25.

On the other hand, the light reflected by the beam splitter 36 is guided to the third light receiving element 38 via the third diaphragm 37 arranged at the position of the condensing point Q of the imaging lens 23. This third light receiving element 38 is also the first light receiving element 28.
And a photoelectric conversion element similar to the second light receiving element 30.
The signal C output from the third light receiving element 38 is sent to the comparator 34, where it is compared with the threshold voltage T ₁ sent from the threshold voltage generator 35 to determine the range of the focusing operation. The specified signal is output.

Next, the operation of the focus detection device of the present invention will be described with reference to FIG. FIG. 6A is a graph for explaining a method of setting a focusing operation range by a conventional method,
FIG. 3B is a graph for explaining the method of setting the focus operation range by the device of the present invention.

First, as shown in FIG. 2 (a), a threshold value T ₁ for avoiding pseudo focusing is set for a signal (A + B) forming a substantially mountain-shaped curve, and a signal which does not fall below this value is set. The range of the level of (A + B) is the displacement signal {(A−B) /
(A + B)} is set as the effective range, and the focusing operation is performed in this range by the same method as the conventional method.

If the signal (A + B) is smaller than the threshold value T ₁ , the comparator 34 outputs the signal C from the third light receiving element 38 to the threshold voltage generator 35. By judging the level based on the threshold value T ₂ ,
It is possible to identify the range including the focal point F and moving away from the focal point F by a predetermined amount. Regarding the focusing operation, by moving the surface to be measured 25 or the focus detection device within the range including the in-focus point F to bring it into the conventional focusing operation range,
You can do it. For example, as shown in FIG.
If the surface to be measured or the focus detection device is once moved in the + direction and it is identified by the level judgment by the threshold value T ₂ that the predetermined distance from the in-focus point F is detected, the conventional focusing operation can be performed by reversing the moving direction. It can be put in the range, displacement signal (A-B) /
By (A + B), focusing can be achieved by a method similar to the conventional method. In this case, the curve of the signal C is the third aperture 37
The displacement signal {(A-
B) / (A + B)} can be enlarged or reduced without affecting, and a wider focusing operation range than before can be arbitrarily set together with the threshold value T ₂ .

[0020] Thus, the apparatus of the present invention provides that the use of the signal C and the threshold value T _2, FIG. 2 (a), the
As shown in (b), it becomes possible to arbitrarily set a focusing operation range exceeding the focusing operation range by the conventional method without affecting the displacement signal. Therefore, a wide focusing operation range can be secured even if the optical system magnification is increased to secure focusing accuracy, and accurate focusing determination can be realized with a wide focusing operation range. Note that the focus detection means is not limited to the method described in the embodiment, and other known methods such as the pupil division method and the astigmatism method can be applied.

[0021]

As described above, the focus detection device of the present invention can perform the focusing operation over a wider area while ensuring the accuracy of the focusing determination, and the object having various shapes can be obtained. It has the advantage that it is also possible to deal with.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a focus detection device according to the present invention.

2A and 2B are graphs for explaining the operation of the focus detection apparatus according to the present invention, in which FIG. 2A is a graph showing a focus operation range in which a conventional method is applied, and FIG. 2B is enlarged according to the present invention. It is a graph which shows the focus operation possible range.

FIG. 3 is a diagram showing a configuration of a conventional focus detection device.

4A is a graph showing characteristics of an output signal by a light receiving element provided in the device shown in FIG. 3, and FIG. 4B is output by a signal processing system based on the signal shown in FIG. FIG. 6C is a graph showing a displacement signal for focus determination, and FIG. 7C is a graph for explaining a method of determining a focus operation range based on the signal shown in FIG.

5A is a graph showing a relationship between a magnification change of an optical system of a conventional focus detection device and its photodetector output, and FIG. 5B is a displacement signal for focusing determination in the case of FIG. 5A. 3 is a graph showing the state of change of.

[Explanation of symbols]

 20 laser emitting means 21 polarization beam splitter 22 quarter wave plate 23 imaging lens 24 objective lens 25 surface to be measured 26, 36 beam splitter 27 first diaphragm 28 first light receiving element 29 second diaphragm 30 second Light receiving element 31 Signal processing system 34 Comparator 35 Threshold voltage generator 37 Third diaphragm 38 Third light receiving element P, Q Focusing point

Claims (3)

[Claims] 1. A focus detection unit that irradiates a subject with measurement light through an optical system and focuses the subject on the reflected light, and from the amount of light reflected from the subject through the optical system. A focus detection apparatus comprising: a position detection unit that detects that the subject is within a predetermined distance range from the in-focus position. 2. The focus detecting means divides the reflected light, a light emitting means, an optical system for irradiating the subject with the measurement light from the light emitting means and condensing the reflected light from the subject. A light splitting means, first and second light detecting means respectively arranged on the front side and the rear side with respect to the condensing position of the reflected light, and a signal processing system. The focus detection device according to claim 1. 3. The position detecting means comprises a light detecting means arranged at an image forming position of the optical system, and a threshold value setting means for setting a threshold value to an output from the light detecting means. The focus detection device according to claim 1, wherein