JPH044891B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a focus detection device for a fundus observation device.

(Background of the Invention) With devices for observing the fundus of the eye, it is very difficult to focus due to insufficient light intensity and poor contrast due to the low reflectance of the fundus. Therefore, the special public service in 1977-
As seen in Publication No. 13294, an apparatus has been proposed that projects an index onto the fundus of the eye and focuses using the index image.

This device divided an index image using a split prism or the like, focused the image on the fundus, observed its lateral shift, and focused the image. Here, the observation optical system is used only to observe the state of this lateral shift, and it is the index projection optical system that focuses directly on the fundus.
This index and the observation image plane (and the photographing image plane, if necessary) are caused to move together by a mechanical interlocking device, and by making the index conjugate to the fundus, the fundus is indirectly focused.

However, with such an indirect structure, mechanical errors in the interlocking device between the index projection optical system and observation optical system, or errors due to differences in each optical system (the index projection optical system has a large NA and Because it is used, aberrations become large, and the observation optical system is N.
(Due to the difference that A. is small, the depth of focus is deep, and aberrations are relatively small).In addition, the interlocking device itself becomes complicated, leading to an increase in the size and cost of the device.

(Object of the Invention) The object of the present invention is to eliminate these drawbacks, simplify the target projection optical system, omit the interlocking device between the target projection optical system and the focus information detection optical system, and observe the focus information. The object of the present invention is to provide a focus detection device that can be obtained directly from an optical system.

(Summary of the Invention) The present invention has the following technical points. The target projection optical system is arranged so that a light source image as small as possible is created near the pupil of the eye to be examined, and a target image is created near the fundus of the eye.

Further, the focus information detection means is provided with a dividing image forming means that divides the light beam from the exit pupil of the observation optical system of the fundus observation device to form a plurality of images of the index to be projected onto the fundus of the eye to be examined. A light receiving means is provided for detecting the image intensity in the dividing direction of the plurality of index images formed by the image means. Then, a calculation means is provided for extracting a phase component by Fourier transform of the image intensity in the division direction of the two index images divided based on the signal from the light receiving means and detecting the in-focus state, and based on the output of the calculation means. The focusing optical system of the fundus observation device is driven by the servo means.

This simplifies the target projection optical system, eliminates the need for a linking device between the target projection optical system and the observation optical system, and allows focus information to be obtained directly from the observation optical system.

(Example) Examples of the present invention will be described below.

FIG. 1 is a diagram showing the overall configuration of a fundus camera having a focus detection device.

Usually, the illumination system includes an observation illumination light source 1, a light source relay lens 2, a reflector 3, and a strobe tube 4 arranged conjugately with the light source 1 by the light source relay lens 2.
It is composed of a ring slit 5 (a plan view is shown in FIG. 3), ring slit relay lenses 6 and 8, a perforated mirror 9, and an objective lens 11. As is well known, the ring slit 5 is operated by the ring slit relay lenses 6, 8 and the objective lens 11, so that the fundus camera and the eye 12 are operated almost conjugately to the cornea of the eye 12 to be examined. The distance is adjusted. The index projection light source 16 passes through the index projection system relay lens 17a and illuminates a slit-formed index plate 18 (the plan view is shown in FIG. 4) from behind. The light source 16 passes through the slit of the index plate 18, forms an image, and passes through the index projection system relay lens 17.
b, a perforated mirror 9 formed by a dichroic prism 7 and a ring slit relay lens 8;
The image is again formed near the pupil of the eye 12 to be examined by the objective lens 11, and the fundus of the eye is illuminated. These rays are depicted in dash-dotted lines in FIG.

In the case of the optical system shown in FIG. 1, the index input light source 16 is an infrared light emitting diode, and the dichroic prism 7 has characteristics of reflecting infrared light and transmitting visible light. Therefore, the dichroic prism 7 transmits the visible light from the observation light source 1 and the strobe tube 4, but reflects the infrared light emitted from these light sources 1 and 4 to the outside of the illumination optical system.
In the end, the fundus of the eye is illuminated only by the infrared light from the index projection light source 16. When the dichroic mirror 14 is made to reflect infrared light and transmit visible light, the light that reaches the array sensor 23 (to be described later) is only the light transmitted through the index plate 18 illuminated by the index projection light source 16, and the index image is (Slit image) has good contrast.

On the other hand, as shown by the dotted line in FIG.
The slit is imaged in the vicinity of the dichroic prism 7 by the relay lens 17b for the index projection system, and then imaged at the rear focal position of the objective lens 11 by the relay lens 8 and the perforated mirror 9. Therefore, the light transmitted through the slit after passing through the objective lens 11 becomes almost parallel light and enters the eye 12 to be examined.
If the eye 2 is emmetropic, the slit will be imaged on the fundus. However, depending on the diopter of the eye 12 to be examined, the slit image may be out of focus. However, the light source that actually illuminates the index plate 18 is the eye 12 to be examined.
Since a conjugate image is formed near the pupil and the size of the light source image is small, the numerical aperture (NA) of the light beam exiting the slit becomes small and the depth of focus becomes deep. Therefore, in the practical eye range (approximately ±10 diopters), the blurring of the image due to defocus is not very large, and contrast sufficient for focus detection is guaranteed.

Next, as shown by the solid line in FIG. 2, the slit image projected onto the fundus of the eye becomes a secondary light source and is focused once by the objective lens 11. Passes through aperture 10, relay lens 1
3, the light is reflected by a dichroic mirror 14 that reflects infrared light and transmits visible light, and then forms an image near the field stop 19. This field stop 19 is made conjugate with the array sensor 23 by the re-imaging lens 21, and furthermore, the array sensor 23 is made conjugate with the imaging surface (observation image surface not shown) 15. Therefore, the slit image from the fundus is re-imaging lens 21.
The image is formed near the array sensor 23 by the pupil-splitting roof prism 22 (a three-dimensional view is shown in Fig. 5).In Figs. (However, this is for the convenience of expression; in reality, the beam is rotated 90 degrees around the optical axis so that the edge is parallel to the surface.) The light beam is divided into two parts, and each The luminous flux is aligned with the direction of the ridge of the array sensor 23 (prism 22
The cells of the array sensor 23 are arranged so that the direction in which they are arranged is perpendicular to the plane of the paper. ) are focused on different parts of the image.

Here, as shown by the dotted line in FIG. 2, the prism edge of the pupil splitting prism 22 is almost conjugate with the aperture stop 10 by the relay lens 13 and the field lens 20 behind the field stop 19. Since this aperture stop 10 corresponds to the exit pupil of the focusing optical system, the image on the array sensor 23 is formed by the light beams from two different parts of the pupil of this optical system. As is well known, the two images obtained are laterally shifted from each other due to the front and rear focusing. Therefore, the in-focus state can be detected by measuring the distance between two images on the array sensor 23. In other words, as shown by the solid line in Figure 6, if the distance between the two images is L during focusing, when the in-focus point is behind the field stop 19 (dotted line in Figure 6), the image interval will be as shown in Figure 6. As shown by the dotted line, the image interval becomes narrower than L, and when the focal point is in front of the field stop 19, the image interval becomes wider than L.

When measuring the distance between the images, if the images themselves have good contrast and are in a good imaging state, the distance between the two image positions can be easily measured. However, in reality, images are often formed on the array sensor 23 in a defocused state, and a good image is not necessarily obtained.

Therefore, in order to accurately extract the lateral displacement of the image even under such bad conditions, we developed a method for
Use the method proposed in Publication No. 68667, etc. This is a method in which the image on the array sensor 23 is Fourier transformed, its phase part is extracted, and the lateral shift is determined using the transition theorem of Fourier transformation.

If the intensity of the image on the array sensor 23 is a function f(x) of _the position I can do it.

F ₁ (S)=∫ ^∞ _-∞ f(X)e ^-2 〓 ^ixs dx ……(1) When this image is laterally displaced by α, the Fourier transform F ₂ is as shown in (2) according to the transition theorem. Become.

F ₂ (S) = e ^-2 〓 ^ias F ₁ (S) ...(2) Comparing equations (1) and (2), the magnitude of the Fourier component does not change due to displacement, but only the phase shifts by 2πas Therefore, if the shifted phase is Δθ, the displacement a can be found as shown in (3) by knowing Δθ.

a=Δθ/2πs (3) To specifically express this, discrete Fourier transform (DFT) is performed with the spatial frequency as the reciprocal (1/l) of the sample length l on the array sensor 23.

That is, as shown in FIG. 7, for one of the two slit images on the array sensor 23, Ii is the electrical signal corresponding to the amount of light incident on each cell on the array sensor 23, and the number of samples of cells is Ii. N, the length of the array sensor 23 of the sampled portion is set as l, and the following electric quantities I _X and I _Y are determined.

_{I _} ^_ _{_ _ _} ^_ _{_ _ _} 23, the real part (I _X ) and imaginary part (I _Y ) of the Fourier component with a spatial frequency of 1/l are multiplied by N/l.

Therefore, the phase deviation angle θ of the Fourier component is determined from I _X and I _Y as shown in (6).

θ=arctanI _Y / _I I can do it.

Therefore, the next distance d is defined by θ as shown in equation (7).

d=lθ/2π...(7) This distance d is determined for each of the two slit images on the array sensor 23, and the interval between the two slit images is calculated from this, and it is compared with the interval at the time of focusing. By this, focusing information can be obtained. As shown in Figure 1, array sensor 2
3, each cell is sequentially driven by a signal from the driving means 24, and a photoelectric conversion signal corresponding to each cell is sequentially output. From this output, the calculation means 25
The above calculation is performed by , and based on the result, the motor M is driven by the motor driving means 26, and the focusing relay lens 13 is moved to focus.

The electrical processing system that performs the above processing is shown in the block diagram of Figure 8, and its operation is shown in Figures 9 and 1.
This will be explained using Figure 0. In the block diagram of FIG. 8, the same members as in FIG. 1 are given the same symbols, but
The array sensor 23 is, for example, what is called a charge-coupled device, and each cell is sequentially activated by a clock pulse (FIG. 9c) following a start signal (FIG. 9b) from a well-known drive means 24. Driven. That is, after the start signal is generated, the array sensor 23 outputs an electric signal corresponding to the amount of light incident on the first cell _c1 of the array sensor 23 (FIG. _9a ) at the first pulse P1. Further, for example, at the seventh pulse _P7, an electric signal corresponding to the amount of light incident on the seventh cell _c7 is output. The signal outputted from the array sensor 23 in this way passes through an amplifier 250 and the like, and then is sent to the A/D converter 2.
51 into a digital signal. This digital signal is input to the microcomputer 252, and the microcomputer 252 processes the signal according to the flowchart shown in FIG.
That is, the microcomputer 252 controls the A/D converter 25 in synchronization with the scanning of the array sensor 23.
1's output I _i is read and stored in built-in memories M ₁ and M ₂ in correspondence with each cell. Here, as shown in FIG. 9a, the memory M ₁ stores signals from cells located approximately on the left half of the array sensor 23, and the memory M ₂ similarly stores signals from cells located approximately on the right half of the array sensor 23. Memorize the signal from. When a predetermined number of clock pulses are input after the start pulse is generated, the microcomputer 252 reads data from the memory M1 to the memory _M1 .
Switch memory to M ₂ . The above is done in step 27. Next, the _{microcomputer} 252 uses equations (4), (5), (6), and
Perform the calculations in (7) sequentially and store the calculated distance d ₁ in memory.
The distance d 2 is stored in M ₃ (step 28), and the calculations of equations (4), (5), (6), and (7) are performed sequentially using the stored value in memory M _{2 .} Store in memory _M4 (step 29). Next, the microcomputer 25
2 performs an operation (l ₁ - _{d 1} + d ₂ ) between the stored values d ₁ and d 2 of the memories M ₃ and M ₄ and the distance l ₁ (step
30). Then, in step 31, the value (l ₁ - _{d 1} + d ₂ ) obtained in step 30, which corresponds to the interval between two additional slits, is compared with the interval at the time of focusing,
The difference is determined (step 31), and the difference determined in step 31 is output as focusing information (step 32).

The motor driving means 26 inputs pulse signals of different frequencies to the analog motor M according to the focusing information output from the microcomputer 252. At that time, the frequency of the pulse signal input to the analog motor M is
When the lens 13 is far out of focus, the pulse signal is large and the motor M rotates rapidly, and when the focusing relay lens 13 approaches the focus position, the pulse signal is small and the motor M rotates at a low speed. The motor M is controlled to stop promptly at the in-focus position.

This method of determining the displacement by Fourier transforming an optical image and extracting the phase component does not rely on the absolute value of the light amount of the optical image in principle, so it is difficult to obtain images with poor contrast or slightly complex images. Displacement can also be accurately determined using a pattern image.

By the way, if the index 18 is a slit plate with slits formed thereon as shown in FIG. 4 and is arranged so that the longitudinal direction of the slit is perpendicular to the direction in which the light receiving cells on the array sensor 23 are lined up, By simplifying the pattern of the index image and forming the slit image near the center of the sample window (length l) on the array sensor 23, the accuracy of detecting the displacement of the optical image increases. This is because when performing Fourier transform, discrete Fourier transform (DFT) is performed only in a region of sample length l, so if an image is present at the edge of the sample window, an error will occur.

In addition, it is convenient to set the longitudinal direction of the slit in the plane of the paper in FIG. 1 and to make the direction of the array sensor 23 perpendicular thereto (perpendicular to the plane of the paper). (However, as already mentioned, for convenience of explanation, the direction of the light-receiving cells of the array sensor 23 is shown in the paper in FIG. Since the slit image is formed at an eccentric position, the slit image moves up and down within the plane of the paper in Figure 1 due to the defocus, but since the image moves in the longitudinal direction of the slit, the image is almost invisible on the array sensor 23 arranged perpendicularly thereto. This is because it doesn't seem to move.

Further, in this case, the prism edge of the pupil splitting prism 22 is within the plane of the paper in FIG. (However, as already mentioned, in FIG. 1, for convenience of explanation, the edge of the pupil splitting prism 22 is shown perpendicular to the plane of the paper.) The field diaphragm 19 uses a roof-shaped prism 22 to direct its image onto the array sensor 23. When two images are formed, their role is to prevent them from overlapping each other and to prevent the light beams from the two pupils from mixing.

Also, for example, the aperture diaphragm 10 and the relay lens 13
If a positive lens and a negative lens of appropriate power are made removable with a turret etc. between
2 is a strongly negative diopter (in this case, the inserted lens is a positive lens), or when the eye 12 to be examined is a strongly positive diopter (in this case, the inserted lens is a negative lens), the amount of movement of the relay lens 13 is No need to make it bigger.

(Effects of the Invention) As described above, according to the focus detection device of the present invention,
The optotype projection optical system does not need to be linked with the observation optical system and is fixed, so the device itself becomes very simple, and focusing information can be obtained through the focusing optical system itself, improving accuracy. do.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical system and electric block of a fundus camera having an embodiment of the present invention, and FIG. 2 is a diagram showing only the focusing information detection optical system and observation optical system in FIG. 1. Fig. 3 is an enlarged plan view of the ring slit, Fig. 4 is an enlarged plan view of the index plate with an index, Fig. 5 is an enlarged perspective view of the pupil-dividing roof prism, and Fig. 6 is the focus information detection optical system. An enlarged view of the main parts, Figure 7 is a diagram showing the intensity distribution of the light image on the array sensor, Figure 8 is a detailed diagram of the electric block in Figure 1, Figure 9 is a diagram showing the driving state of the array sensor, FIG. 10 is a flowchart for explaining the operation of the microcomputer used in FIG. (Explanation of symbols of main parts), 7...Dichroic prism, 8...Relay lens for ring slit, 9...Perforated mirror, 10...Aperture diaphragm,
11...Objective lens, 13...Relay lens, 1
4... Dichroic mirror, 16... Light source for index projection, 17a, 17b... Relay lens for index projection system, 18... Index plate, 19... Field diaphragm, 2
0... Field lens, 21... Re-imaging lens, 22... Pupil division roof prism, 23... Array sensor, 24... Drive means, 25... Calculation means, 26... Motor drive means, M... …motor.

Claims (1)

[Scope of Claims] 1. A focus detection device for a fundus observation apparatus comprising an index projection optical system that projects an index onto the fundus of an eye to be examined, and a focus information detection means that detects focus information, comprising: the index projection optical system; The system includes an index fixed on the optical axis, a light source that illuminates the index from behind, and a projection optical system that makes the light source conjugate near the pupil of the eye to be examined and projects the index onto the fundus of the eye to be examined. , and the focus information detection means includes a dividing imaging means for dividing a light beam from an exit pupil of an observation optical system of the fundus observation device to form a plurality of images of an index to be projected onto the fundus of the eye to be examined. and a light receiving means for detecting the image intensity in the dividing direction of the plurality of index images formed by the dividing image forming means, and a light receiving means for detecting the image intensity in the dividing direction based on the signal from the light receiving means. A calculation means for detecting a focusing state by extracting a phase component by Fourier transform of intensity, and a servo means for driving a focusing optical system of the fundus observation device based on the output of the calculation means. Features a focus detection device.