JPH03267707A - Defocusing detecting device - Google Patents

Abstract

PURPOSE:To obtain sufficient defocusing detection sensitivity by a compact optical system by forming an annular photoelectric conversion surface and further dividing it equally into four parts. CONSTITUTION:Reflected light from an object surface 1 is made incident on a photodetecting element 4A by passing through an aspherical lens 2a, a spherical lens 2, and a cylindrical lens 3 and the element 4A is arranged at a focusing position where circular converged light from the lens 3 is photodetected when the object surface 1 is at a specific position. The lens 3 deforms the converged light into an elliptic shape as defocusing is caused. The element 4A is formed annularly and divided equally into the four parts 4-1 to 4-4, the internal diameter 2RB is made less than the external diameter 2RS of the circular converged light at the time of focusing, and the respective parts 4-1 to 4-4 are connected to a differential amplifier to output a defocusing detection signal. Consequently, if defocusing is caused, the ratio of the projection area of the elliptic converged light on a divided surface in the major-axis direction and the projection area in the minor-axis direction becomes large, so that defocusing detection sensitivity can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a defocus detection device due to astigmatism, and particularly to a defocus detection device suitable for use in optical distance detection, focus servo systems, and the like.

[Prior Art] As a conventional device of this type, for example, a measurement and control Vo
1.26. There is one shown in Nc2 (February 1986) [Distance detection method for robots]. With this device,
As shown in FIG. 3, a defocus detection device is used to detect the displacement of the object plane 1 from a predetermined position. For this reason, light is irradiated from a light source (not shown) onto the object surface 1, and the reflected light from the object surface 1 is used as defocus detection target light through the spherical lens 2. The light passes through a cylindrical lens 3 and enters a light receiving element (photoelectric conversion surface) 4.

At this time, the cylindrical lens 3 is placed in the convergent optical path from the spherical lens 2 and gives an astigmatism effect to the imaging system, while the light receiving element 4 It is placed at a focal point position where it receives a perfectly circular convergent light from the lens 3.

As shown in FIG. 4, the light receiving element 4 is composed of four-division photodiodes (4D-PD) PDI to PD4, and each of the photodiodes PDI to PD4 is formed into a square shape of the same size. .

Here, the deviation (a
or the position shown by C), a defocus occurs in the reflected light from the object surface 1 (the defocus detection target light) that is converged by the spherical lens 2, and the converged light from the cylindrical lens 3 is deforms from a perfectly circular state (see symbol b□ in FIG. 4) to an elliptical shape (see symbol a1 or C□ in FIG. 4). The above-mentioned light receiving element 4 is divided into four equal parts according to the long axis direction and the short axis direction of this elliptical shape.

Further, a differential amplifier 5 is connected to the light receiving element 4, and a defocus detection signal (defocus amount of the light to be detected) δ is outputted from the differential amplifier 5.

Here, δI=(I□+L)−(I3+I4).

In addition, ■, 〜, respectively indicate the photodiode PDI〜
This is the output from PD4.

With the above configuration, when the object surface 1 is at the predetermined position [b], the defocus detection target light is in a focused state and the convergent light from the cylindrical lens 3 is on the light receiving element 4 as shown in FIG. As shown, it converges in a perfect circular shape. Therefore,
The output δ from the differential amplifier 5 becomes O.

On the other hand, when the object surface 1 shifts from the predetermined position to position a or C, the defocus detection object light becomes out of focus, and the convergent light from the cylindrical lens 3 is moved onto the light receiving element 4. It converges in an elliptical shape as indicated by the symbol a1 or C1 in FIG. That is, depending on the positive or negative direction of the deviation of the object surface 1, the astigmatism of the cylindrical lens 3 results in elliptical shapes that are perpendicular to each other.

Therefore, the output δ■ corresponding to the direction and amount of deviation is obtained from the differential amplifier 5. For example, when the object plane 1 shifts toward position a, the convergent light on the light receiving element 4 becomes an ellipse indicated by the symbol a1, and the output δ from the differential amplifier 5
■ becomes a positive value, while if the object plane 1 shifts to the position C side, the convergent light on the light receiving element 4 becomes an ellipse indicated by the symbol C□, and the output δ from the differential amplifier 5 is a negative value.

[Problems to be Solved by the Invention] However, in the conventional defocus detection device described above,
The defocus detection sensitivity is determined by the optical path length Q0 on the image side. Q (see FIG. 5) increases as the ratio of the major axis diameter/minor axis diameter of the elliptical convergent light on the light receiving element 4 increases. In other words, in the conventional apparatus, in order to obtain a certain level of sensitivity, an optical path length longer than 100 nm is required, which is a factor leading to an increase in the size of the apparatus.

For example, in order to obtain an elliptical convergence light of 50x150μ on the light receiving element 4 when a 5μ heel defocus occurs, as shown in FIG. The focal length f of each cylindrical lens 3 is 5 mm,
50 mm and −20 wm, the optical path length Q1 between the spherical lens 2 and the cylindrical lens 3 needs to be about 45 mm, and the optical path length Q2 between the cylindrical lens 3 and the light receiving element 4 needs to be about 10 am. In FIG. 5, the aspherical lens 2a is placed between the object surface 1 and the spherical lens 2 in order to convert the reflected light from the object surface 1 into a beam having a diameter of 5IIIm.

An object of the present invention is to provide a defocus detection device that improves the S/N ratio regardless of the optical path length and makes it possible to obtain sufficient defocus detection sensitivity with a more compact optical system.

[Means for Solving the Problems] In order to achieve the above object, a defocus detection device of the present invention includes a photoelectric conversion surface capable of receiving defocus detection target light that has passed through a cylindrical lens, and the photoelectric conversion surface is arranged at a position to receive a perfectly circular convergent light from the cylindrical lens when in focus, and detects the defocus amount of the defocus detection target light based on the output from the photoelectric conversion surface, wherein the photoelectric The conversion surface has an annular shape and is divided into four equal parts according to the long axis direction and the short axis direction of the convergent light from the cylindrical lens, which deforms into an elliptical shape as the focus shift of the focus shift detection target light occurs. , and the inner diameter of the annular photoelectric conversion surface is set smaller than the outer diameter of the perfectly circular convergent light.

[Function] In the defocus detection device of the present invention described above, the amount of defocus of the defocus detection target light is detected based on the output from the annular photoelectric conversion surface, as in the conventional device. If a deviation occurs, the area projected by the elliptical convergent light onto the dividing plane of the photoelectric conversion surface corresponding to its major axis direction,
The larger the ratio of the photoelectric conversion surface corresponding to the minor axis direction to the area projected on the dividing plane, the higher the defocus detection sensitivity. Therefore, when elliptical convergent light of the same size is incident on a conventional photoelectric conversion surface and an annular photoelectric conversion surface, the annular photoelectric conversion surface with the central part cut out circularly. The above-mentioned area ratio is larger for the surface, and defocus detection sensitivity is improved.

[Embodiments of the Invention] Hereinafter, a defocus detection device as an embodiment of the present invention will be explained with reference to the drawings. FIG. b) is a schematic side view showing the overall configuration. Note that this embodiment also describes a case where the apparatus of this embodiment is used to detect a positional deviation of the object surface 1 from a predetermined position.

As shown in FIG. 1(b), the overall configuration of this embodiment is almost the same as that of the conventional device. Light is irradiated from a light source (not shown) to the object surface 1, and light is reflected from the object surface 1. The light is passed through an aspherical lens 2a, a spherical lens 2. Through the cylindrical lens 3, the light receiving element (
The light is incident on the photoelectric conversion surface (photoelectric conversion surface) 4A.

The light-receiving element 4A is arranged at a focal point position where it receives a perfectly circular convergent light from the cylindrical lens 3 when the object surface 1 is at a predetermined position, and is also arranged at a focal point position where it receives a perfectly circular convergent light from the cylindrical lens 3.
) is formed as shown. In other words, the light receiving element 4A
is formed into an annular shape and is divided into four equal parts (photodiode ) It is divided into 4-1 to 4-4. In addition, the inner diameter 2·RB of the annular light receiving element 4A is the outer diameter 2 of the perfectly circular convergent light C1 at the focused point.
・Rs (Rs is the half value III of the light intensity distribution) 1 For example, RB=α・R8 (α=0.8
~1).

Note that a differential amplifier 5 (see FIG. 4) is connected to each portion 4-1 to 4-4 of the light-receiving element 4A, as in the conventional device, and a defocus detection signal (focus The amount of defocus of the light to be detected for deviation) δ1 is output. Here, δ■=(L+L)−(L+I, ). In addition, ■, ~ engineering.

are the outputs from the equal portions 4-1 to 4-4, respectively.

Since the defocus detection device as an embodiment of the present invention is configured as described above, the device of this embodiment basically operates in the same manner as the conventional device and outputs the defocus detection signal δ. It has become so.

In this embodiment, the light receiving element 4A is formed as shown in FIG. 1(a), and the operation of this light receiving element 4A will be described in detail below.

Generally, noise mainly consisting of shot noise is usually superimposed on the defocus detection signal δI=(I□+1.)−(I,+I4) from the light receiving element 4A (differential amplifier 5).
The S/N ratio is a factor that determines the sensitivity of the defocus detection system. Since the shot noise n3 has a magnitude proportional to the square root of the light intensity, if we set ns:, , /T, then
The S/N ratio is as follows.

Here, it is assumed that the shape of the convergent light is flattened due to a slight defocus, and the convergent light has an elliptical shape as shown by the symbol C2 in FIG. 1(a) and enters the light receiving element 4A. At this time, assuming that the minor axis diameter of the ellipse C2 is 2Rs・(1-i) and the major axis diameter is 2R3・(1+t), the high forces 11~■ from each photodiode 4-1 to 4-4 4 is approximately proportional to the area 81 to S4 that the convergent light projects onto the surface of each photodiode 4-1 to 4-4, and when E is sufficiently small, each area S to S4 is below. Equations (2), (3
).

51=82? (π/4)・Ra2・(1+β・i)−(π/4)
・Ra” ・=(2)S,=S4 Mi(π/4)・R52・(1-β-E)−(π/4)・
Ra'' - (3) Therefore, the total power of the convergent light is ■., R
Let B=α・R3, and I i=I a・S i/ (x
・R5)'' and the above formulas (2) and (3), (
Equation 1) becomes the following equation (4).

ζ7°"0"9 Here, K in the S/N ratio is when the present invention is not applied (
In other words, when α=0), it corresponds to the S/N ratio of 1
/f near represents the coefficient of improvement in the S/N ratio due to the adoption of the one-ring-shaped light receiving element 4A.

Therefore, when the annular light-receiving element 4A having a circularly hollowed-out central portion is used, the detection sensitivity is reliably improved compared to the conventional one.

For example, in order to improve the S/N ratio by 6 dB, 1/
JV'? = 2, α”=0.866 (RB=0°8
66・Rs).

Next, the specific optical path length etc. when such a light receiving element 4A is applied to an actual optical system will be explained with reference to FIG. 1(b).

For example, an aspherical lens 2a, a spherical lens 2. The focal length f of the cylindrical lens 3 is 5+i+*, 2, respectively.
5 mm, −10 m, the optical path length L between the spherical lens 2 and the cylindrical lens 3 is about 25 mm, and the optical path length L2 between the cylindrical lens 3 and the light receiving element 4A.
and about 51, when a focus shift of 5 μm occurs,
An elliptical convergent light of 70×1 and 0 μm is projected onto the surface of the light receiving element 4A. In other words, in order to make the optical path length etc. smaller than in the conventional device, the flatness of the convergent light on the surface of the light receiving element 4A is considerably smaller than in the conventional device.

However, in the device of this embodiment, the annular light receiving element 4A
Since the S/N ratio of the defocus detection signal δ is improved as described above, sufficient detection sensitivity can be obtained even if the optical path length is short.

As described above, according to the device of this embodiment, the S/N ratio of the defocus detection signal is improved regardless of the optical path length by the annular light receiving element 4A having a hollowed-out central portion. , sufficient defocus detection sensitivity can be obtained with a more compact optical system.

In the above embodiment, the optical system on the object surface 1 side is composed of the aspherical lens 2a and the spherical lens 2, but instead of these, an optical system having a Cassegrain mirror may be used.

Further, in the above embodiment, the light receiving element itself was formed in an annular shape, but as shown in FIG.
The light-receiving element 4A may be formed by covering it with a mask member (shaded area) 6 and providing an annular exposed area. In this case as well, the same effects as in the above embodiment can be obtained.

[Effects of the Invention] As described in detail above, according to the defocus detection device of the present invention, the S of the defocus detection signal is Since the /N ratio is improved, sufficient defocus detection sensitivity can be obtained with a more compact optical system.

[Brief explanation of drawings]

1 and 2 show a defocus detection device as an embodiment of the present invention, FIG. 1(a) is a plan view showing the light receiving element (photoelectric conversion surface), and FIG. 1(b) is a plan view showing the light receiving element (photoelectric conversion surface). FIG. 2 is a schematic side view showing the overall configuration, FIG. 2 is a plan view showing a modified example of the light receiving element, FIGS. 3 to 6 show a conventional defocus detection device, and FIG. FIG. 4 is a schematic side view showing the overall configuration, FIG. 4 is a plan view showing the light receiving element, and FIGS. 5 and 6 are diagrams for explaining the specific optical path length and size of convergent light. In the figure, 1 - object surface, 2 - spherical lens, 2a - aspherical lens, 3 - cylindrical lens, 4A - light receiving element (photoelectric conversion surface), 4 - 1 to 4 - 4 - equal part (photodiode), 6-Mask member.

Claims (1)

[Claims] It includes a cylindrical lens and a photoelectric conversion surface that can receive the defocus detection target light that has passed through the cylindrical lens, and is arranged at a position where the photoelectric conversion surface receives a perfectly circular convergent light from the cylindrical lens when the photoelectric conversion surface is in focus. In the defocus detection device that detects the defocus amount of the defocus detection target light based on the output from the photoelectric conversion surface, the photoelectric conversion surface has an annular shape and the defocus detection target light The convergent light from the cylindrical lens, which deforms into an elliptical shape due to the occurrence of defocus, is divided into four equal parts according to the long axis direction and the short axis direction, and the inner diameter of the annular photoelectric conversion surface is A defocus detection device characterized in that the diameter is set smaller than the outer diameter of a perfectly circular convergent light.