JPS5810709A - Optical system for automatic focus detecting device - Google Patents

Abstract

PURPOSE:To make the photometric range constant independently of the variation of the length of the optical path, by arranging a telecentric optical system between an object lens and an optical path length changing element to make main rays of the luminous flux going toward a photoelectric element parallel with the optical axis. CONSTITUTION:An optical path length changing element 1 consisting of plural parallel plane plates different in thickness is provided rotatably and exchangeably between an object lens 6 and a photoelectric element 7, and a lens 8 is provided between the object lens 6 and the optical path length changing element 1. The lens 8 is so arranged that its object-side focus coincides with the position of the exit pupil of the object lens 6 to constitute a telecentric optical system, and therefore, main rays emitted from an object point (a) at the outside of the axis and transmitted through a center Q of the exit pupil of the object lens 6 is refracted by the lens 8 so as to be parallel with the optical axis. Consequently, since the photometric visual field is constant even if the optical path length changing element 1 is exchanged, the focus position is detected with a high precision on a basis of the output signal of the photoelectric element 7.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention focuses a spatial image formed by an objective lens of an optical device such as a microscope on a photoelectric element, and analyzes a video signal obtained from the image to determine the focal position. The present invention relates to an optical system of an automatic focus detection device.

Conventionally, the method of automatically detecting the focal position using only one photoelectric element involves moving the stage or objective lens on which the object is placed in the optical axis direction, and moving the image to the front and back of the light receiving element. After that, the one that detects the position with the highest contrast, and the one that vibrates part or all of the objective lens in the optical axis direction, determines the direction with higher contrast, and moves the lens to that higher direction to find the in-focus position. is known to detect
ing. In the former case, it is necessary to scan a long distance before and after the combined heating position, which takes time, and when an object to be examined with a large height difference is continuously observed while moving the stage or objective lens, It is impossible to automatically adjust the focus by following the focus shift.

Furthermore, in the latter case, since the lens is vibrated back and forth, there is a risk that the entire optical device may vibrate. To solve these drawbacks, an automatic focusing system with almost no vibration and rapid focus adjustment was developed using multiple optical path length changing elements (parallel By rotating and exchanging the flat plate), the optical path length of the optical system can be changed quickly and periodically.
The applicant has already filed a patent application (Japanese Patent Application No. 55-91918) regarding an automatic focus adjustment device that determines the in-focus state by moving an image back and forth on a photoelectric element. However, in this focus adjustment device, the photometry range (photometering field of view) is expanded or reduced as the optical path length expands or contracts, so the measurement range is not constant and there is a risk that the accuracy of focus adjustment may be reduced.

An object of the present invention is to provide a focus detection optical system in which the photometry range does not change regardless of changes in the optical path length in an automatic focus detection apparatus having the optical path length changing means as described above.

Hereinafter, the present invention will be described in detail with reference to examples.

1 and 2 are explanatory diagrams of an automatic focusing device for which a patent application has been filed by the present applicant. In FIG. 1, the optical path length changing elements are five parallel plane plates 1 (two of which are shown in the figure) of different thickness made of transparent plastic or glass, and are circular. Hole 3 of plate holder 2
Further, this holder has a hollow hole (not shown) without the parallel plane plate arranged around the rotation axis 50 of the motor 40 together with the parallel plane plate. These five parallel plane plates 1 and the hollow holes are sequentially placed on the optical axis of the microscope objective lens 6 by driving the motor, so the optical path length between the objective lens 6 and the photoelectric element 7 changes each time, and the motor Each rotation of the rotation axis 501, object images are formed at six different positions on the optical axis near the photoelectric element 7. In this case, although the imaging position changes, the image simply moves in parallel, and the size of the image does not change. In Figure 2,
The image of object point 0 on the optical axis formed by the objective lens 6 is at OL when there is no parallel plane plate 1, and at OL when there is plane parallel plate 1.
Assuming that each image is formed at tC, the chief ray emanating from object point a and passing through the center Q of objective lens B is refracted at points A and C on both sides of the plane-parallel plate, and then returns to image point A! pass through. In addition, if there is no parallel plane plate 1, the object moves straight and passes through the image point A1. Both image points A and 8! The height X from the optical axis does not change as stated above. Now, assuming that the width of the photoelectric element from the optical axis is equal to the height X, the image point O! When the photoelectric element is located at the position o, the photoelectric element will not receive light from an object point outside the object Oa that is conjugate to A2. In addition, there is no parallel plane plate, and the photoelectric element has an image point of 0. The same is true when it is in .

However, the photoelectric element has an image point of 0. , and since the parallel plane plate is in the optical path, the image is at point 0. Assuming that the image is formed at the position, the chief ray a b c Al passing through point A: passes through point d, which intersects o, A. Therefore, the photoelectric element located at the position 0IAI can receive an extra amount of light from a point outside the object Oa by an amount corresponding to the width dA. Conversely, if the optical path length is increased by replacing the parallel plane plate, the photometric range (photometric field of view) of the photoelectric element is reduced. Therefore, since the photometry range is not constant, differences occur in the video signals of images formed at different positions with respect to the photoelectric element, resulting in a decrease in focus detection accuracy. This change in the photometry range is due to the fact that the chief ray of the light beam focused on the off-axis image point is inclined with respect to the optical axis, as shown in FIG. In the present invention, in order to eliminate the above-mentioned drawbacks, a telesend link optical system is formed between the objective lens and the parallel plane plate (so that the photometry range is constant regardless of changes in the optical path length).゜ Fig. 3 is a detailed explanatory diagram of the present invention, in which 1 is the aforementioned optical path length changing element, which, like Fig. 1, is composed of five parallel plane plates of different thicknesses, and a separately provided hollow plate. A lens 8 is provided along with the hole so as to be rotatably exchangeable in the optical path between the objective lens 6 and the photoelectric element 7, and furthermore, a lens 8 is provided between the objective lens 6 and the optical path length changing element 1. The side focal point is arranged to match the position of the exit pupil of the objective lens 6, forming an h1 telecentric optical system.Therefore, an off-axis object point a is emitted and the main focus passing through the center of the exit pupil of the objective lens is emitted. The ray C is refracted by the lens 8 so that it is parallel to the optical axis.The image points 0 and A1 are the same as the image points 0 and A1 when there is no plane parallel plate, that is, when the hollow hole is on the optical axis E.
, correspond to object points O and a when the parallel plane plate is on the optical axis. Now, assuming that the thickness of the parallel plane plate is t1 and the refractive index is n, the difference d in optical path length is expressed by the following equation.

d in t□ 7°L Therefore, the thickness of the parallel plane plate may be changed to move the image point, or the refractive index or both the thickness and the refractive index may be changed. In this case, the height of the image points A1 and A2 corresponding to the object point a and the distance #1) from the optical axis does not change because the chief ray is parallel to the optical axis. 9 is the aperture. Its diameter is determined according to its position so as not to distort the luminous flux. Aperture 9 collects stray light 1-1st that has already occurred due to internal reflection of the lens barrel, etc.
, and also has the function of reducing scattered light since it cuts the incident light rays by the side surface of the optical path length changing element, and as a result, an image with good contrast can be obtained.

FIG. 5 shows an embodiment in which the optical system of the present invention is installed in a microscope with an imaging device, in which a solid-state image sensor 7 is used as a photoelectric element,
Parallel plane glass plates 1 of different thicknesses are provided as optical path length changing elements, and are configured to be rotated and replaced by a motor as in FIG. 1. Reference numeral 6 denotes a microscope objective lens, 8 a lens for a telescopic link optical system, and 9 an aperture diaphragm. There is. Replacing the parallel plane glass plate 1 (Thus, the periodically changing output video signal of the image sensor 7 is analyzed by the signal processing device [112, and a signal for focusing is sent to the stage drive device 1.
3 (feeds, the stage 14 moves below F, and focusing is completed.

As described above, in the present invention, a telesend link optical system is arranged between the objective lens and the optical path length changing element 0, so that the principal ray of the light beam facing the photoelectric cable t becomes parallel to the optical axis.
- Because of this structure, even if the imaging position moves, the height of the image (the distance between the image point corresponding to the same object point and the optical axis) does not change. Therefore, even if the optical path length changing element is replaced, the photometric range (photometric field of view)
Since is constant, the focus position can be detected with high accuracy from the output signal of the photoelectric element.

[Explanation of symbols of main parts]

1... Optical path length changing element 6... Objective lens 7... Photoelectric element 8... Lens for telescopic link optical system 9... Aperture Applicant Nippon Kogaku Kogyo Co., Ltd. Agent Takashi Watanabe, 3 years old, 4 years old

Claims (1)

[Scope of Claims] (1) A telecentric optical system is constructed by arranging a lens in an optical path between an objective lens and a photoelectric element, and an optical path between the lens for the telecentric optical system and the photoelectric element 1. An optical system for an automatic focus detection device, characterized in that a plurality of optical path length changing elements are replaceably provided therein. 2. In the optical system according to claim 1, the lens for the telescopic link optical system has an aperture larger than the effective light receiving surface of the photoelectric element, and the object side focal point of the 1/lens is An optical system for an automatic focus detection device, characterized in that the optical system is placed at an exit pupil position of the objective lens. (3) In the optical system according to claim 1, each of the plurality of optical path length changing elements is a parallel plane plate made of plastic or glass and having a different thickness, and is larger than the light receiving surface of the photoelectric element. An optical system for an automatic focus detection device characterized by having a light-transmitting surface. 1. An optical system for an automatic focus detection device, characterized in that an aperture plate is provided between a lens and the optical path length changing element to block diffusely reflected light. The lenses are a microscope objective lens and an imaging device, respectively, and the photometric optical system including the telesend link optical system lens and the imaging device is provided on an optical path divided from the main optical axis of the microscope. Optical system of automatic focus detection device.