JP2016095353A - Telecentric lens and image capturing device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a telecentric lens which features capability to adjust focus in response to changes in working distance and greater depth of field and depth of focus which allow for focusing on an entire object even when the object has an irregular surface, and to provide an image capturing device.SOLUTION: A telecentric lens has a common focal point F that coincides with an image-side focal point of an object-side lens group 4 and an object-side focal point of an image-side lens group 5, and is provided with an aperture stop 6 disposed at the focal point F. A variable refractive power liquid lens 7 is disposed in a region on the object side or image side of the aperture stop 6, in which principal rays travel at an angle with an optical axis Z. An aperture diameter of the aperture stop 6 is chosen to be smaller than an aperture diameter of the liquid lens 7.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a telecentric lens used when measuring a dimension of an object to be measured by image processing or the like, and an imaging apparatus using the telecentric lens.

  A telecentric lens is an optical system in which a small aperture is placed at the focal point of the lens so that the principal ray can be regarded as parallel to the optical axis. An object-side telecentric lens that is parallel to the optical axis, an image-side telecentric lens whose chief ray is parallel to the optical axis only on the image side by placing a stop at the object-side focal point of the image-side lens, and the image-side focal point of the object-side lens There is a double-sided telecentric lens in which the principal ray is parallel to the optical axis on both the object side and the image side by placing a stop at a common focal point where the object side focus of the image side lens coincides with that of the image side lens.

The object-side telecentric lens is used in an image pickup apparatus of a dimension measurement system because the size of an image does not change even when the working distance changes.
The image-side telecentric lens is used for a three-plate camera or the like having a color separation prism whose spectral transmission characteristics are affected by the incident angle because the light beam reaching the image plane from the optical system is parallel to the optical axis everywhere on the image plane.
Both-side telecentric lenses have a telecentric effect even when the object side positioning is unstable or the force lens side is loosely mounted. Used when

FIG. 9 shows an example of each ray diagram when the working distance is different in such an imaging apparatus using the conventional double-side telecentric lens.
The imaging device 51 places a stop 54 at a common focal point F in which the image-side focal point of the object-side lens 52 and the object-side focal point of the image-side lens 53 are matched, and the lenses 52 and 53 are arranged along the optical axis Z. It comprises a telescopic lens 55 arranged on both sides and an image sensor 56 arranged on the image plane M thereof.

  When WD = WD0 where the working distance WD to the object B, which is the subject, is equal to the design value WD0, the light emitted from the object B is refracted by the object-side lens 52 as shown in FIG. Only the light that has passed through F reaches the image side lens 53 and forms an image on the image sensor 56 arranged on the image plane M.

At this time, the principal ray CR travels in parallel with the optical axis Z from the object B to the object side lens 52, and intersects the optical axis Z at the common focal point F from the object side lens 52 to the image side lens 53. The light travels diagonally and travels in parallel with the optical axis Z from the image side lens 53 to the image plane M.
The object light BR is diffused along the principal ray CR from the object B to the object side lens 52, travels in parallel with the principal ray CR from the object side lens 52 to the image side lens 53, and is imaged from the image side lens 53. Since the image converges at the imaging point K on the surface M, an in-focus image can be obtained.

  However, since the telecentric lens 55 does not have a large depth of field or depth of focus (hereinafter simply referred to as “depth”), the working distance to the object B and the distance from the image side lens 53 to the image sensor 54 are not limited. There is a problem of defocusing when changes exceed the depth.

FIGS. 9B and 9C are light ray diagrams showing optical path changes when the working distance WD changes.
For the explanation of the optical path change, the object light with WD = WD0 is left with a dotted line, and the object light BR with WD <WD0 and WD> WD0 is shown with a broken line.

In the case of WD <WD0 shown in FIG. 9B, the principal ray CR proceeds in the same manner as in FIG. 9A, but the object light BR has a large diffusion angle from the object B to the object side lens 52, Although the light travels along the principal ray CR from the object side lens 52 to the image side lens 53 and is converged by the image side lens 53, the image plane M shifts backward from the position of the image sensor 56, and the shifted image. An image is formed at an image point K on the surface M.
Therefore, when the working distance to the object B is small and exceeds the depth, the image on the image plane M is out of focus.

When WD> WD0 shown in FIG. 9C, the principal ray CR travels in the same manner as in FIG. 9A, but the object light BR has a small diffusion angle from the object B to the object side lens 52, Although it advances while converging along the principal ray CR from the object side lens 52 to the image side lens 53 and is converged by the image side lens 53, the image plane M shifts forward from the position of the image sensor 56, and the shifted image. An image is formed at an image point K on the surface M.
Therefore, when the working distance to the object B increases and exceeds the depth, the image on the image plane M is out of focus.

For this reason, the refractive power is variable to change the refractive power (= 1 / focal length, refractive power on the opposite side of the stop) outside the image side lens and object side lens (side where the principal ray and the optical axis are parallel). There has also been proposed a mechanism in which a focus adjustment is possible.
However, for that purpose, an image side lens or an object side lens is composed of a plurality of lenses, and a stage for individually moving a specific lens in the Z direction (optical axis direction) is required, which only complicates the structure. In addition, there is a problem that aberration and backlash tend to occur, and the entire telecentric lens becomes large.

Therefore, recently, a lens that achieves focusing of a telecentric lens using a liquid lens that can change the refractive power without changing the position of the lens has been proposed (Patent Document 1).
The imaging device 61 in FIG. 10 is arranged on a telecentric lens 65 in which a liquid lens 64 is arranged at a common focal point F of the object side lens 62 and the image side lens 63 arranged on the optical axis Z, and on the image plane M thereof. It consists of an image sensor 66.

  When WD = WD0 where the working distance WD to the object B is equal to the design value WD0, the refractive power (P) of the liquid lens 64 is set to 0, so that the object B comes out of the object B as shown in FIG. The light is refracted by the object side lens 62, and the light passing through the liquid lens 64 reaches the image side lens 63 and forms an image on the image plane M.

At this time, the principal ray CR travels in parallel with the optical axis Z from the object B to the object side lens 62, and passes through the liquid lens 64 from the object side lens 62 and reaches the image side lens 63. The lens travels diagonally so as to intersect the optical axis Z at F, and travels parallel to the optical axis Z from the image side lens 63 to the image plane M.
The object light BR diffuses along the principal ray CR from the object B to the object side lens 62, travels in parallel with the principal ray CR from the object side lens 62 to the image side lens 63, and passes from the image side lens 63 to the image. Since the image converges at the imaging point K on the surface M, an in-focus image can be obtained.

FIGS. 10B and 10C are ray diagrams showing changes in the optical path when the working distance WD changes.
In order to show the difference in the optical path depending on the presence or absence of the liquid lens 64, the light path of the object light when the liquid lens 64 is not provided for the light emitted from the liquid lens 64 is indicated by a dotted line. The optical path of the refracted object beam BR is indicated by a broken line.

In the case of WD <WD0 shown in FIG. 10B, an image can be formed on the image sensor 66 by swinging the refractive power of the liquid lens 64 to the plus side.
At this time, although the chief ray CR travels in the same manner as in FIG. 10A, the object light BR has a large diffusion angle from the object B to the object side lens 62, and the principal ray CR from the object side lens 62 to the liquid lens 64 is principal. It proceeds while diffusing along the light beam CR.
Since the refracting power is swung to the plus side by the liquid lens 64, the diffused light incident on the liquid lens 64 is refracted and collimated and converges on the image forming point K on the image sensor 66 by the image side lens 63 and is connected. In order to form an image, the image plane M can be made to coincide with the imaging surface of the image sensor 66, and a focused image is formed.

In the case of WD> WD0 shown in FIG. 10C, an image can be formed on the image sensor 66 by swinging the refractive power of the liquid lens 64 to the minus side.
At this time, although the chief ray CR travels in the same manner as in FIG. 10A, the object light BR has a small diffusion angle from the object B to the object side lens 62, and the main light CR from the object side lens 62 to the liquid lens 64 is principal. It proceeds while converging along the light beam CR.
Since the refractive power of the liquid lens 64 is swung to the minus side, the convergent light incident on the liquid lens 64 is refracted and collimated, and converges on the image formation point K on the image sensor 66 by the image side lens 63. In order to form an image, the image plane M can be made to coincide with the imaging surface of the image sensor 66, and a focused image is formed.

However, in this optical system, since the liquid lens 64 is arranged at the position of the focal point F, it is not possible to arrange an aperture.
In the telecentric lens, it is necessary to set the aperture diameter small in order to ensure a certain depth, but when the liquid lens 64 is used without using the aperture, the liquid lens 64 also serves as the aperture. The aperture diameter depends on the aperture of the liquid lens 64.
Therefore, such an optical design cannot be performed when it is desired to freely select the aperture of the diaphragm regardless of the aperture of the liquid lens 64.
In addition, by using a liquid lens, the depth becomes shallow even if the focus can be adjusted. Therefore, when an object having large irregularities on the surface is observed, there is a possibility that the focus is partially shifted.

  In addition, this type of imaging device is often used by being incorporated into a production line for precision equipment installed in a clean room of a factory where temperature and humidity are controlled. When the time elapses, the temperature inside the imaging apparatus changes, which may cause the refractive index of the liquid lens to change and blur the focus.

JP 2012-58435 A

  Therefore, the present invention can not only adjust the focus in response to changes in the working distance, but also increase the depth so that an object with large unevenness on the surface can be focused as a whole, and more preferably Therefore, it is a technical subject to enable proper and quick focusing even when the refractive index of the liquid lens changes with temperature change.

  In order to solve this problem, the present invention provides an object-side lens group that causes a principal ray on the object side to be incident parallel to the optical axis by focusing on the image-side focus, and to focus on the focus. Provided with one or both of image side lens groups that emit principal rays that have passed through the diaphragm in parallel to the optical axis on the image side by disposing a stop at the side focal point, the light transmitted through the lens group in advance In the telecentric lens that forms an image on the image plane at a set position, a refractive power (in the region where the chief ray travels obliquely with respect to the optical axis on the object side or the image side of the stop is formed on the optical axis. = 1 / focal length), and an aperture diameter of the diaphragm is selected to be smaller than an aperture diameter of the liquid lens.

In the imaging device in which an imaging element is arranged at the image plane position of the telecentric lens, the principal ray travels obliquely with respect to the optical axis on the object side or the image side of the diaphragm on the optical axis of the telecentric lens. A liquid lens with adjustable refractive power is arranged in the area to be
A focus system is provided that adjusts the refractive power of the liquid lens by changing a control voltage for driving the liquid lens to form an object with a different working distance at the image plane position.

  This focus system outputs a control voltage for imaging a subject at the image plane position based on the relationship between the refractive power of the liquid lens at a reference temperature and the control voltage according to one or more preset working distances. When the control voltage is output in the focus mode and the preset focus mode, the refractive power changed according to the liquid lens temperature detected by the temperature sensor based on the relationship between the liquid lens temperature and the refractive power is used as the refractive power at the reference temperature. A temperature correction mode that outputs a correction control voltage to match the image and a numerical value that represents the in-focus state for each image based on the output signal of each pixel while continuously taking images by changing the control voltage Auto focus mode that outputs the control voltage output when the value reaches the maximum or minimum as the focus voltage; It is provided.

  In the telecentric lens according to the present invention, since the liquid lens capable of adjusting the refractive power is disposed on the optical axis on the object side or the image side of the diaphragm, by adjusting the refractive power of the liquid lens according to the working distance, The convergence angle / divergence angle of the light beam passing through the liquid lens changes, and the light beam can be imaged at the image plane position.

  Further, since the object light passes through a diaphragm having an aperture diameter smaller than that of the liquid lens, the depth of the object light can be increased as compared with the case where the liquid lens is disposed at the diaphragm position. In other words, since the working depth focused by the liquid lens can be centered, the depth of focus can be increased before and after that, even when observing an object with large irregularities on the surface, partial defocusing occurs. It is difficult and can focus on the whole.

  Furthermore, when used as an image pickup apparatus in which an image pickup element is arranged at the image plane position of such a telecentric lens, the control voltage for driving the liquid lens is changed, the refractive power of the liquid lens is adjusted, and the working distance is different. Since a focus system for focusing the subject on the image plane position is provided, it is possible to automatically focus.

In this case, if the preset focus mode is set, the object light is coupled to the image plane position based on the relationship between the refractive power of the liquid lens at the reference temperature and the control voltage according to one or more preset working distances. Since the control voltage for imaging is output, the focus of the image sensor arranged on the image plane can be adjusted when the working distance to the object is determined in advance.
At the same time, when the temperature correction mode is executed, a correction control voltage that matches the refractive power changed according to the liquid lens temperature detected by the temperature sensor with the refractive power at the reference temperature based on the relationship between the liquid lens temperature and the refractive power. Is output, so there is no occurrence of blurring due to the temperature change of the liquid lens.

  Furthermore, if the auto focus mode is set, a numerical value indicating the in-focus state is calculated based on the output signal of each pixel, and the control voltage output when the numerical value becomes maximum or minimum is output as the in-focus voltage. Therefore, even if the working distance to the object is unknown, autofocus is performed.

  This autofocus mode can also be executed after setting a preset working distance in the preset focus mode. The focusing time can be adjusted by this, and the processing time until the focusing state is achieved can be shortened compared with the case where the focusing is performed in the autofocus mode from the beginning.

Explanatory drawing which shows the both-sides telecentric lens which concerns on this invention. The ray diagram. The flowchart which shows the process sequence of a focus mode setting means. The flowchart which shows the process sequence of autofocus mode. The flowchart which shows the process sequence of preset focus mode. The flowchart which shows the process sequence of temperature correction mode. Explanatory drawing which shows the object side telecentric lens which concerns on this invention. Explanatory drawing which shows the image side telecentric lens which concerns on this invention. Explanatory drawing which shows the conventional both-side telecentric lens. Explanatory drawing which shows the conventional double-sided telecentric lens using a liquid lens.

In this example, not only can the focus be adjusted in response to a change in the working distance, but the depth can be increased so that the entire surface can be focused even on an object with irregularities on the surface. In order to achieve the purpose of being able to focus properly and quickly even when the refractive index of the liquid lens changes with the change,
An object side lens group that makes the principal ray on the object side incident parallel to the optical axis by concentrating the stop on the image side focal point and condensing on the focal point, and the aperture on the object side focal point A telecentric lens provided with either one or both of the image side lens group for emitting the principal ray that has passed through the image side in parallel with the optical axis;
In an imaging apparatus in which an imaging element that images light transmitted through the lens group is arranged at a preset image plane position on the optical axis,
On the optical axis of the telecentric lens, a liquid lens having an adjustable refractive power (= 1 / focal length) is arranged in a region where the chief ray travels obliquely with respect to the optical axis on the object side or the image side of the stop. And
A focusing system that changes the control voltage for driving the liquid lens, adjusts the refractive power of the liquid lens, and forms an object with a different working distance at the image plane position;
The focus system
A preset focus mode for outputting a control voltage for forming an image of the subject at the image plane position based on the relationship between the refractive power of the liquid lens at the reference temperature and the control voltage in accordance with one or more preset working distances;
When outputting the control voltage in the preset focus mode, based on the relationship between the liquid lens temperature and the refractive power, a correction for matching the refractive power changed according to the liquid lens temperature detected by the temperature sensor with the refractive power at the reference temperature. Temperature compensation mode for outputting control voltage,
While the image is continuously picked up by changing the control voltage, a numerical value representing the in-focus state is calculated for each image based on the output signal of each pixel, and is output when the numerical value becomes maximum or minimum. And an autofocus mode that outputs the control voltage as a focusing voltage,
Focus mode setting means for performing focusing individually or in combination with each mode is provided.

An imaging apparatus 1 shown in FIG. 1 includes a both-side telecentric lens 2 and an imaging element 3 disposed at the position of the image plane M.
The bilateral telecentric lens 2 is provided with a stop 6 at a common focal point F in which the image side focal point of the object side lens (object side lens group) 4 and the object side focal point of the image side lens (image side lens group) 5 are matched. The respective lenses 4 and 5 are arranged along the same optical axis Z.

Since the stop 6 is disposed at the common focal point F that is the image side focal point of the object side lens 4, the principal ray CR on the object side is incident on the lens 4 in parallel with the optical axis Z and is collected at the focal point F. Lighted.
In addition, since the stop 6 is disposed at the common focus F that is the object-side focus of the image side lens 5, the principal ray CR that has passed through the stop 6 is refracted by the lens 5, and the optical axis Z and The light is emitted in parallel.
Thus, the principal ray CR on the object side is incident on the object side lens 4 so as to be parallel to the optical axis Z, refracted so as to intersect the optical axis Z at the common focal point F, and oblique to the optical axis Z. Further, the light is refracted again by the image side lens 5 and emitted parallel to the optical axis Z on the image side.

On the optical axis Z, the principal ray CR is refracted into a region S where the principal ray CR travels obliquely with respect to the optical axis Z on the object side or the image side (in this example, the object side) of the diaphragm 6 close to the diaphragm 6. A liquid lens 7 with adjustable force (= 1 / focal length) is arranged.
The diaphragm 6 is smaller than the aperture diameter of the liquid lens 7, and in this example, an aperture diameter that can be variably adjusted is used so that the depth can be increased by adjusting the aperture diameter. It has become.
A beam splitter 8 for performing coaxial epi-illumination is disposed between the object side lens 4 and the liquid lens 7, and a light source 9 is disposed on the branched optical axis.

  The liquid lens 7 is a control in which two kinds of immiscible liquids having different refractive indexes are sealed in a cylindrical disk-shaped container having an upper bottom surface and a lower bottom surface as light incident / exit surfaces and applied between two annular electrodes. By changing the voltage between 30 and 55 V, for example, the contact position between the liquid interface and the inner peripheral surface of the container is changed. As a result, the uneven shape of the liquid interface is changed to adjust the refractive power. .

FIG. 2A shows a ray diagram when the relationship between the working distance WD and the design value WD0 of the telecentric lens 2 is WD = WD0. In this case, the refractive power (P) of the liquid lens 7 is set to 0 (for example, control voltage = 42V).
The light from the object B is refracted by the object side lens 4, and only the light passing through the liquid lens 7 reaches the image side lens 5 and forms an image on the image plane M.
At this time, the chief ray CR travels in parallel with the optical axis Z from the object B to the object side lens 4, and passes through the liquid lens 7 from the object side lens 4 to reach the image side lens 5. The lens travels diagonally so as to intersect the optical axis Z at F, and travels parallel to the optical axis Z from the image side lens 5 to the image plane M.
On the other hand, the object light BR diffuses along the principal ray CR from the object B to the object side lens 4, and travels in parallel with the principal ray CR from the object side lens 4 through the liquid lens 7 to the image side lens 5. Since the image-side lens 5 converges to the image formation point K on the image plane M, an image in focus is obtained by the image pickup device 3 placed on the image plane M.

FIGS. 2B and 2C are ray diagrams showing changes in the optical path when the working distance WD changes.
In order to show the difference in the optical path depending on the presence or absence of the liquid lens 7, the optical path of the object light when the liquid lens 7 is not provided for the light emitted from the liquid lens 7 is indicated by a dotted line. The optical path of the refracted object beam BR is indicated by a broken line.

In the case of WD <WD0 shown in FIG. 2B, an image can be formed on the image plane M by swinging the refractive power of the liquid lens 7 to the plus side.
At this time, the chief ray CR travels in the same manner as in FIG. 2A, but the object light BR has a large diffusion angle from the object B to the object side lens 4, and the main light CR from the object side lens 4 to the liquid lens 7 It proceeds while diffusing along the light beam CR.
Since the refractive power of the liquid lens 7 is swung to the plus side, the diffused light incident on the liquid lens 7 is refracted and collimated so that the image side lens 5 converges to the imaging point K on the image sensor 3. Since the image is formed, the image plane M and the image sensor 3 are maintained in a matched state, and a focused image is formed.

When WD> WD0 shown in FIG. 2C, the image can be formed on the image plane M as shown in FIG. 2C by swinging the refractive power of the liquid lens 7 to the minus side.
At this time, the chief ray CR travels in the same manner as in FIGS. 2A and 2B, but the object light BR has a small diffusion angle from the object B to the object side lens 4, and the liquid from the object side lens 4 is liquid. It proceeds while converging along the principal ray CR up to the lens 7.
Since the refractive power of the liquid lens 7 is swung to the negative side, the convergent light incident on the liquid lens 7 is refracted and collimated and converges to the image point K on the image sensor 3 by the image side lens 5. Since the image is formed, the image plane M and the image sensor 3 are maintained in a matched state, and a focused image is formed.

  In addition, since the aperture diameter of the diaphragm 6 is selected to be smaller than the aperture diameter of the liquid lens 7, the aperture diameter of the diaphragm 6 can be set to be larger than that of the liquid lens 7 even in the telecentric lens 2 that is shallow enough to pass through the liquid lens 7. By setting an arbitrary size smaller than the aperture, it is possible to configure the telecentric lens 2 having a deep focus adjustment.

A focus system 10 that drives the liquid lens 7 and adjusts its refractive power is composed of a microcomputer 11 or the like incorporated in the imaging apparatus 1, and stores necessary programs and data in advance and temporarily stores calculation results. And an arithmetic unit 13 that performs necessary data processing based on input data.
The microcomputer 11 is connected to the temperature sensor 15 for measuring the temperature of the liquid lens 7, the image sensor 3, and the switches SW <b> 0 to SW <b> 3 on the input side via the I / O port 14. The driver 16 of the liquid lens 7 is connected to.
The temperature sensor 15 is preferably incorporated in the liquid lens 7 to measure the internal temperature or the surface temperature in terms of accuracy. However, the temperature of the liquid lens 14 is not contacted from the outside or the ambient temperature is measured. Also good.
When the driver of the liquid lens 7 is formed of an annular substrate or the like and is integrally formed on the light incident / exit side end surface of the liquid lens 7, the temperature sensor 15 may be provided on the annular substrate, or may be formed on the annular substrate. The aperture diameter of the light transmission hole may be an appropriate size smaller than the aperture diameter of the liquid lens 7, and this may be used as the diaphragm 6.

  The focus system 10 includes an autofocus mode M1 that automatically sets an optimum focus position according to a working distance to an object, a preset focus mode M2 that sets a refractive power according to a preset working distance, Focus mode setting means is provided for executing a temperature correction mode M3 for adjusting a focus shift due to a temperature change when executing the preset focus mode M2 alone or in combination.

<Focus mode setting means>
FIG. 3 is a flowchart showing the processing procedure of the focus mode setting means.
After turning on the main switch SW0, the switches SW1 to SW3 corresponding to the respective modes M1 to M3 are operated.

First, in step STP1, the state of the switch SW1 is determined. If it is on, the process proceeds to step STP2, and if it is off, the process proceeds to step STP7.
In step STP2, the state of the switch SW2 is determined. If it is off, the process proceeds to step STP3 to determine the autofocus mode M1, and if it is on, the process proceeds to step STP4.

  In step STP4, the state of the switch SW3 is determined. If the switch SW3 is off, the preset focus mode M2 and the autofocus mode M1 are continuously executed in step STP5. If the switch SW3 is on, the temperature correction mode M3 is accompanied in step STP6. Continuous execution of the preset mode M2 and the autofocus mode M1 is determined.

When the process proceeds from step STP1 to step STP7, the state of the switch SW2 is determined. If it is off, the process returns to step STP1, and if it is on, the process proceeds to step STP8.
In step STP8, the state of the switch SW3 is determined. If it is off, the preset focus mode M2 is determined in step STP9. If it is on, the preset mode M2 with the temperature correction mode M3 is determined in step STP10.

<Auto focus mode M1>
FIG. 4 is an explanatory diagram showing a processing procedure in the autofocus mode M1. The autofocus mode M1, while the control voltage CV _N is changed from the initial value V0 by a predetermined unit step [Delta] V, and captures an image MG _N pickup element 3, and 8 pixels adjacent the all pixels the sum Si of the absolute value of luminance difference calculated in, calculates the sum ΣSi of Si as the in-focus evaluation value a _N, and outputs the control voltage when the value becomes the maximum as the focus voltage.

Specifically, resetting the index N = 0 in step STP11, the control voltage _{CV N (= V0 + N ·} ΔV) at step STP12 captures an image MG _N is output to the liquid lens 7, the process proceeds to step STP13.
In step STP13, the focus evaluation value A _N is calculated for the image MG _N and stored in a predetermined storage area, and the maximum value MX (A _N ) of the focus evaluation value A _N stored so far is calculated. The value N _MX of N corresponding to the maximum value MX (A _N ) is stored as the maximum value index X.

Then, until the calculated focus evaluation value _{A N} of the required number (e.g. seven pieces of N = Less than six) at step STP 14, returned as N = N + 1 in step STP11 in step STP15, it reaches N = 6 At the time, the process proceeds to step STP16.
At step STP16, it is determined that the change trend of the focus evaluation value A _N, the process proceeds to step STP17 it is determined that the increase, if X> 3, determines that the downward trend if X <3 Then, the process proceeds to step STP22. If X = 3, the process proceeds to step STP28.

If it is determined in step STP16 that there is an increasing tendency and the process proceeds to step STP17, N = N + 1 is set, and in step STP18, the control voltage CV _N (= V0 + N · ΔV) is output to the liquid lens 7.
In step STP19, the focus evaluation value A _N is calculated for the captured image MG _N , and the maximum value MX (A _N ) and the maximum value index X of the focus evaluation value A _N are stored.

Then, at step STP20, the focus evaluation value A _N is to determine whether the peaked, it is determined whether or not a N = X + 3. That is, the determination is made based on whether three focus evaluation values A _N smaller than the maximum value MAX (A _N ) are output in succession.
If it is still in an increasing trend, it is determined No, so the process returns to step STP17 to continue the process. If Yes, the process proceeds to step STP21 and the maximum value MAX (A _N ) at that time is output. The control voltage CV _X is output as the focusing voltage FV.

On the other hand, when it is determined in step STP16 that the rate is decreasing, N = −1 is set in step STP22, and the control voltage CV _N (= V0 + N · ΔV) is output to the liquid lens 7 in step STP23.
In step STP24, calculates a focus evaluation value _{A N} for captured image MG _N, until now calculates the maximum value MAX _{(A N)} of the stored focus evaluation value _{A N,} the N at that time The value is stored as the maximum value index X.

Then, at step STP25, the focus evaluation value A _N is to determine whether the peaked, it is determined whether or not a N = X-3. That is, the determination is made based on whether three focus evaluation values A _N smaller than the maximum value MAX (A _N ) are output in succession.
If it is still in an increasing trend, it is determined No, so that N = N−1 in Step STP26 and the process returns to Step STP23. If Yes, the process proceeds to Step STP27 and the maximum value MAX (A _N ) at that time is returned. The control voltage CV _X at the time of outputting is output as the focusing voltage FV, and the process is terminated.

If it is determined in step STP16 that X = 3, the three focus evaluation values A0 to A2 and A4 to A6 on both sides thereof are lower than the central focus evaluation value A3. in step STP28, the maximal value-related index X = 3 on the basis of the N values of the focus evaluation value A3 as a maximum value, and outputs a control voltage CV ₃ as an in-focus voltage FV, the process ends.

<Preset focus mode M2>
FIG. 5 is an explanatory diagram showing a processing procedure in the preset focus mode M2.
This preset focus mode M2 outputs a control voltage for imaging the subject at the image plane position based on the relationship between the refractive power of the liquid lens at the reference temperature and the control voltage according to one or more preset working distances. To do.

  As a method, the working distance of each workpiece is specified by some method and a control voltage is output, and the focusing voltage FV obtained in the autofocus mode M1 is stored in advance to control at a specific refractive power. Although the voltage may be output, FIG. 4 shows the former processing procedure.

In this case, a WD-CV conversion table in which the relationship of the control voltage CV imaged on the image plane M is previously set in the memory 12 according to the working distance WD.
When the work arrives at the imaging position, the processing shown in FIG. 4 is started. In step STP31, the control symbol CC recorded at the specific position of the work is read by the control symbol reading-only camera, and the process proceeds to step STP32.

In step STP32, the working distance WD of the work is specified with reference to a preset CC-WD conversion table corresponding to the read control symbol CC, and the process proceeds to step STP33.
In step STP33, the control voltage CV is determined with reference to the WD-CV conversion table in correspondence with the specified working distance WD, and in step STP34, the control voltage CV is output and the process is terminated.

  When a control voltage at a specific refractive power is output by storing the focus voltage FV obtained in the autofocus mode M1, the relationship between the control symbol recorded at a specific position of the workpiece and the focus voltage FV is obtained. What is necessary is just to memorize | store and output the control voltage FV when the said control symbol is read.

<Temperature correction mode M3>
FIG. 6 shows a processing procedure in the temperature correction mode M3.
The temperature correction mode M3 is a mode in which the control voltage CV output when the preset focus mode M2 is executed is corrected according to the temperature of the liquid lens 7, and is always executed accompanying the preset focus mode M2.
In the preset focus mode M2, the control voltage CV is specified according to the working distance WD. However, since the refractive power of the liquid lens 7 changes with temperature even if the control voltage is constant, the control voltage corresponding to the working distance WD. Strictly speaking, CV depends on temperature.

Therefore, the temperature is changed in advance, and for each temperature T, a control voltage CV (T) corresponding to the working distance WD is obtained by experiment, and the relationship is stored in the memory 12 as a WD-CV (T) conversion table. .
When the temperature correction mode M3 is started, the temperature T of the liquid lens 7 is detected by the temperature sensor 15 in step STP41, and the WD-CV (T) conversion table corresponding to the temperature T is read from the memory 12 in step STP42. It is.
Next, in step STP43, the WD-CV conversion table referred to in step STP33 of the preset focus mode M2 is set, and the process returns to step STP41. When the temperature changes, the processes in steps STP41 to S43 are repeated.

  The above is an example of the configuration of the present invention, and the operation thereof will be described next in the case of measuring the dimensions of the workpiece conveyed on the production line in which the imaging device 1 is incorporated.

By turning on all the switches SW1 to SW3 with the main switch SW0 turned on, the preset mode M2 and the autofocus mode M1 accompanied by the temperature correction mode M3 are continuously executed.
First, the temperature correction mode M3 is executed, and a WD-CV (T) conversion table corresponding to the temperature T detected by the temperature sensor 15 is set.
In this state, the preset mode M2 is executed, and every time the imaging position where the workpiece arrives is reached, the control symbol CC recorded at the specific position of the workpiece is read, and the working distance WD corresponding to the control symbol is specified. A control voltage CV corresponding to the working distance WD is output.
Therefore, since the control voltage CV (T) corresponding to the temperature at that time is output according to the working distance WD of the arrived work, the refractive power of the liquid lens 7 is maintained regardless of the temperature difference from the reference temperature T0. Set to the optimal value.

Next, the autofocus mode M1 confirms whether or not the subject is actually in focus. If not, the image is further finely corrected based on the image signal, so that an image in focus can be obtained with certainty.
By combining the preset focus mode M2 and the autofocus mode M1 in this way, the approximate focus is achieved in the preset focus mode M2, so that the in-focus time is faster than when focusing is performed only in the autofocus mode M1. There is a merit.

  In the description of the above embodiment, the case where the object side lens 4 has a single lens configuration and the image side lens 5 has a plurality of lens configurations has been described, but the number of lenses constituting each is arbitrary.

FIG. 7 shows a second embodiment according to the present invention. In addition, about the part which is common in FIG. 1, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted.
The imaging device 21 according to this example includes an object-side telecentric lens 22 and an imaging element 3 disposed at the position of the image plane M.
The object side telecentric lens 22 is provided with an object side lens (object side lens group) 23 and an imaging lens 24 along the optical axis Z, and an aperture 6 is provided at the image side focal point F of the object side lens 23. The light passing through the diaphragm 6 is imaged on the image plane M by the imaging lens 24.

  Thus, the principal ray CR on the object side is incident on the object side lens 23 so as to be parallel to the optical axis Z, is refracted so as to intersect the optical axis Z at the focal point F, and is inclined with respect to the optical axis Z. After traveling and passing through the diaphragm 6, the light reaches the image formation point K on the image plane M which is refracted again by the image formation lens 24 and coincides with the image sensor 3.

On the optical axis Z, close to the diaphragm 6 and refracted into a region S where the principal ray CR travels obliquely with respect to the optical axis Z on the object side or image side (in this example, the object side) of the diaphragm 6. A liquid lens 7 capable of adjusting the force is disposed.
The diaphragm 6 is smaller than the aperture diameter of the liquid lens 7, and in this example, an aperture diameter that can be variably adjusted is used.
A beam splitter 8 for performing coaxial epi-illumination is disposed between the object side lens 4 and the liquid lens 7, and a light source 9 is disposed on the branched optical axis.

  According to this, even when the working distance WD is changed by the same procedure as in the first embodiment, the refraction of the liquid lens 7 is performed so that the image pickup device 3 and the image plane M coincide with each other and the image is formed on the image plane M. Since the force can be adjusted, the image captured by the image sensor 3 is not out of focus.

FIG. 8 shows a third embodiment of the present invention. In addition, about the part which is common in FIG. 1, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted.
The imaging device 31 according to this example includes an image-side telecentric lens 32 and an imaging element 3 disposed at the position of the image plane M.
The image side telecentric lens 32 is provided with an objective lens 33 and an image side lens (image side lens group) 34 along the optical axis Z, and an aperture 6 is disposed at the object side focal point F of the image side lens 34. The light that has passed through the objective lens 33 and the diaphragm 6 is refracted by the image side lens 34 and is imaged on the image plane M.

  As a result, the principal ray CR on the object side incident on the objective lens 33 is refracted so as to intersect the optical axis Z at the focal point F, travels obliquely with respect to the optical axis Z, and passes through the aperture 6. The light is refracted again by the image side lens 34 and travels parallel to the optical axis Z, and reaches an image forming point K on the image plane M.

On the optical axis Z, in the vicinity of the stop 6, on the object side or the image side (in this example, the image side), the principal ray CR has a refractive power in a region S that travels obliquely with respect to the optical axis Z. An adjustable liquid lens 7 is arranged.
The diaphragm 6 is smaller than the aperture diameter of the liquid lens 7, and in this example, an aperture diameter that can be variably adjusted is used.
A beam splitter 8 for performing coaxial epi-illumination is disposed between the object side lens 4 and the liquid lens 7, and a light source 9 is disposed on the branched optical axis.

  According to this, the refractive power of the liquid lens 7 can be adjusted so as to form an image on the image plane M even when the working distance WD changes by the same procedure as in the first embodiment. There is no blurring when taking an image.

  The present invention can be applied to an imaging apparatus for imaging an object whose working distance changes and a telecentric lens used for the imaging apparatus.

DESCRIPTION OF SYMBOLS 1 Imaging device 2 Telecentric lens 3 Imaging element 4 Object side lens 5 Image side lens 6 Aperture 7 Liquid lens B Object M Image surface F Common focus Z Optical axis CR Main ray

Claims (9)

An object side lens group that makes the principal ray on the object side incident parallel to the optical axis by concentrating the stop on the image side focal point and condensing on the focal point, and the aperture on the object side focal point One or both of image side lens groups that emit the principal ray that has passed through the image side in parallel with the optical axis are provided, and the light transmitted through the lens group is imaged on an image plane at a preset position. In telecentric lenses,
On the optical axis, a liquid lens whose refractive power can be adjusted is disposed in a region where the principal ray travels obliquely with respect to the optical axis on the object side or the image side of the stop,
A telecentric lens, wherein an aperture diameter of the diaphragm is selected to be smaller than an aperture diameter of the liquid lens.   The telecentric lens according to claim 1, wherein the stop is a variable aperture stop. An object side lens group that makes the principal ray on the object side incident parallel to the optical axis by concentrating the stop on the image side focal point and condensing on the focal point, and the aperture on the object side focal point A telecentric lens provided with either one or both of the image side lens group for emitting the principal ray that has passed through the image side in parallel with the optical axis;
In an imaging apparatus in which an imaging element that images light transmitted through the lens group is arranged at a preset image plane position on the optical axis,
On the optical axis of the telecentric lens, a liquid lens whose refractive power can be adjusted is arranged in a region where the principal ray travels obliquely with respect to the optical axis on the object side or the image side of the stop,
The aperture diameter of the diaphragm is selected to be smaller than the aperture diameter of the liquid lens,
An imaging apparatus comprising: a focus system that changes a control voltage for driving the liquid lens to adjust a refractive power of the liquid lens to form an object with a different working distance at the image plane position.   The focus system outputs a control voltage for focusing a subject on the image plane position based on a relationship between a refractive power of the liquid lens at a reference temperature and a control voltage in accordance with one or more preset working distances. The imaging apparatus according to claim 3, further comprising a focus mode.   When the control system outputs the control voltage in the preset focus mode, the focus system changes the refractive power changed according to the liquid lens temperature detected by the temperature sensor based on the relationship between the liquid lens temperature and the refractive power at the reference temperature. The imaging apparatus according to claim 4, further comprising a temperature correction mode that outputs a correction control voltage that matches the force.   The focus system calculates a numerical value representing an in-focus state for each image based on an output signal of each pixel while continuously capturing images by changing the control voltage, and the numerical value is maximum or minimum. The imaging apparatus according to claim 3, further comprising an autofocus mode in which a control voltage output at the time of occurrence is output as a focusing voltage. An object side lens group that makes the principal ray on the object side incident parallel to the optical axis by concentrating the stop on the image side focal point and condensing on the focal point, and the aperture on the object side focal point A telecentric lens provided with either one or both of the image side lens group for emitting the principal ray that has passed through the image side in parallel with the optical axis;
In an imaging apparatus in which an imaging element that images light transmitted through the lens group is arranged at a preset image plane position on the optical axis,
On the optical axis of the telecentric lens, a liquid lens whose refractive power can be adjusted is arranged in a region where the principal ray travels obliquely with respect to the optical axis on the object side or the image side of the stop,
The aperture diameter of the diaphragm is selected to be smaller than the aperture diameter of the liquid lens,
A focus system that changes the control voltage for driving the liquid lens, adjusts the refractive power of the liquid lens, and forms an object with a different working distance at the image plane position;
The focus system
A preset focus mode for outputting a control voltage for forming an image of the subject at the image plane position based on the relationship between the refractive power of the liquid lens at the reference temperature and the control voltage in accordance with one or more preset working distances;
When outputting the control voltage in the preset focus mode, based on the relationship between the liquid lens temperature and the refractive power, a correction for matching the refractive power changed according to the liquid lens temperature detected by the temperature sensor with the refractive power at the reference temperature. Temperature compensation mode for outputting control voltage,
While the image is continuously picked up by changing the control voltage, a numerical value representing the in-focus state is calculated for each image based on the output signal of each pixel, and is output when the numerical value becomes maximum or minimum. And an autofocus mode that outputs the control voltage as a focusing voltage,
An imaging apparatus comprising: a focus mode setting unit that executes each of the modes independently or in combination. The focus mode setting means includes
Single mode setting means for selectively executing the preset focus mode or the autofocus mode;
Continuous mode setting means for continuously executing the preset focus mode and the autofocus mode;
Temperature correction mode setting means for selecting whether or not to execute the temperature correction mode when the preset focus mode is executed by the single mode setting means or the continuous mode setting means;
The imaging apparatus according to claim 7, further comprising: The imaging apparatus according to claim 3, wherein the diaphragm is a variable aperture diaphragm.

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