JPH0750247B2 - Objective lens system for tilt surface mapping - Google Patents

Description

TECHNICAL FIELD The present invention relates to an objective lens used for fine observation of a liquid column cross section having a free surface in a microgravity environment such as outer space.

[Prior art]

A color-forming layer that can measure the temperature distribution in the cross section of the liquid column with an accuracy of about ± 0.1 ° C by suspending the temperature-sensitive liquid crystal enclosed in microcapsules with a diameter of 5 to 30 μm in the liquid column. Is obtained.

If a tracer such as fine aluminum metal powder is mixed in the liquid column, a diffused reflection fine particle layer capable of measuring the mode of Marangoni convection near the free surface can be obtained.

Accurate measurement of basic properties of fluid physics, for example, for verification and development of semiconductor crystal growth process control theory, fuel flow control theory in jet engine or rocket engine,
Gives a powerful tool.

However, since a liquid column with a completely free surface could not be obtained under the 1G gravitational environment on the earth, there is no strict measuring means such as temperature distribution in the fluid and Marangoni convection, so this kind of precise measurement is not possible. The optical system used was not developed either.

In recent years, various measurements have been performed in a microgravity environment such as outer space, and it has become possible to measure the basic characteristics of new fluid physics that were previously unmeasurable. The objective lens for measurement was required.

[Problems to be Solved by the Invention]

In a microgravity environment, this type of measurement places the liquid in a “suspended” state in the space by the contact surfaces at both ends (upper and lower) as shown in FIG. It is done as it is. This liquid column is illuminated by a thin sheet-like light beam obtained by an appropriate optical optical system for light cutting, and the image of the illuminated cut surface is lit by an objective lens whose optical axis is arranged in a plane perpendicular to the axis of the cylinder. It is formed at a predetermined position and various characteristics of the fluid are examined. At this time, this sheet-like light flux is inclined at an appropriate inclination angle (for example, 45 °) with respect to the optical axis of the objective lens.
) Is projected, and an object image is obtained as a configuration in which the direct light thereof does not enter the objective lens, that is, a configuration of a dark field illumination system.

The objective lens used here is an important factor that influences the measurement accuracy, but there are six difficulties in designing as described below, and measures must be taken to reduce these to a level that does not impair practical use. .

Since the object plane is tilted with respect to the optical axis, the common image plane is also tilted, and non-rotationally symmetric "pseudo-field curvature aberration" is generated.

Since the object surface exists inside the liquid column, a large astigmatism due to the cylindrical refractive power is generated when the image forming light flux passes through the surface of the liquid column.

The free surface of the liquid column is generally an aspherical surface in the generatrix direction, and its aspherical shape changes depending on the viscosity of the liquid.

The refractive index and the spectral dispersion value (Abbe number) of the liquid are not constant depending on the type of test sample liquid, temperature, and temperature distribution.

Since it is necessary to record a moving image (movie) with a certain level of time-dispersion capability, the behavior of the minute light-emitting body that constitutes the subject, the numerical aperture (F number) of the objective lens is the allowable depth of focus required for research. It is desirable that the light is as bright as possible within the range, but this makes it difficult to correct spherical aberration, coma, and axial chromatic aberration related to the aperture.

Since the temperature-sensitive liquid crystal fine particles may be used for the subject, it is necessary to sufficiently remove the chromatic aberration of magnification that may impair the saturation of color development and deteriorate the temperature measurement accuracy.

An object of the present invention is to provide an objective lens that sufficiently satisfies the above problems.

[Means and Actions for Solving the Problems]

Generally, when the object surface is tilted with respect to the optical axis of the objective lens, the tilt angle of the common image plane is determined according to the so-called Scheimpflug condition. Here, the Scheimpflug condition is
"In an ideal thin lens system, if the object plane is tilted,
The both surfaces of the object and the image and the principal surface of the lens intersect on the same straight line. "Under this condition, the object surface and the image surface are inclined in a V shape. Therefore, most of the non-rotationally symmetric "pseudo-image surface curvature aberration" that occurs when the image surface is tilted is removed by tilting the light-receiving element plane placed on the image surface along the tilted image surface. You can Also, the residual field curvature is
It can be removed by setting the negative power in the lens system so that the Petzfar sum of the objective lens becomes almost zero, and correcting the astigmatism as small as possible.

Next, the large astigmatism caused by the cylindrical refractive power of the surface of the liquid column is caused by the objective lens facing the liquid column sample, the front group lens arranged in close proximity, and the light receiving element. Dividing into a group lens, a cylindrical refraction surface of the opposite sign that cancels the cylindrical refraction power of the surface of the liquid column, as close as possible to the surface of the liquid column in the front lens group,
Alternatively, by disposing the toric refracting surface, it is possible to remove most of the adverse effects of the liquid column surface refracting power on the final imaging performance.

Also, "imaging performance due to the change in the aspherical free surface of the liquid column due to the viscosity of the sample liquid column" and "the change due to the type, temperature, and temperature distribution of the sample liquid to be tested regarding the liquid refractive index and dispersion value" For deterioration of, the average value of these change factors is adopted as the standard value in lens design.

The objective lens is divided into a front lens group and a rear lens group through a relatively long distance, and the distance between them is readjusted to provide a "macro photography effect".

The subject field of view is limited to the center of the liquid column as much as possible.

If measures such as these are taken, it can be reduced to the extent that there is no practical problem.

The F number of the objective lens can be made bright by adopting a lens type having a large aperture aberration correction potential, for example, a Gauss type photographic objective lens for the rear lens group.

Regarding the chromatic aberration of magnification of the whole objective lens system, a lens having sufficient achromatic correction potential is mainly used for the rear lens group, and at the same time, an achromatic refracting surface is appropriately introduced in the front lens group system, or a low refractive index surface is used. By taking measures such as using a dispersive glass material so as to minimize the occurrence of chromatic aberration relating to the chief ray in the front lens group, the entire system can be corrected.

Also, if an appropriate cylindrical refractive surface or toric refractive surface is introduced into the rear lens group, there will be a surplus in correcting astigmatism due to the liquid column cylindrical refractive power, which further improves the performance of the entire objective lens system. Clearly, it can be improved.

The objective lens according to the present invention is, as described above,
Since both surfaces of the image are optical systems that are tilted substantially according to the Scheimpflug condition, in the direction along the tilt angle, a large amount of “image-conditioning condition” deviation with origin asymmetry, that is, “a kind of distortion” is inevitably generated. However, it is easy to obtain the amount in advance by calculation or actual measurement. Therefore, if the spatial correspondence between the object and the image is calibrated and displayed by image processing technology, it is practical. It doesn't matter.

Based on the above consideration, in the present invention, as the configuration of the objective lens system used for mapping the object surface tilted with respect to the optical axis in the substantially cylindrical liquid column onto the light receiving element,
A front group lens disposed in the vicinity of the liquid column and a rear group lens disposed on the light receiving element side are provided, and the radius of curvature of a cross section perpendicular to the generatrix in the central portion of the liquid column is R _C , The liquid side refracting power in the vertical plane and the focal length on the air side of the composite system of the front lens group are F ₁ , the focal length of the rear lens group in the vertical plane is F ₂ , and the front lens group and the liquid lens normalized by _RC When the focal length of the combined system of column refracting power is F (F = F ₁ / R _C ), and the entrance pupil position of the front lens group measured from the object point is ENP, (1) 1.0 ≦ | F ₂ / F ₁ (2) −6.0 ≦ F ≦ −1.0 (3) −5.0 ≦ ENP / F ₁ ≦ −0.7 The configuration is adopted so as to satisfy the condition.

Next, the operation of each condition to be satisfied by the present invention will be described.

The condition (1) is a condition regarding power distribution between the front and rear lens groups of the objective lens. Since most of the aberration correction potential of this objective lens is in the rear lens group, in order to improve the imaging performance of the entire system, give the rear lens group a relatively short focal length and make it relatively excessive. It is a bad idea to configure the system so that a light beam with a large aperture and a wide angle of view enters. If this ratio falls below the lower limit of 1.0, it will be difficult to sufficiently remove coma aberration mainly in the oblique light flux aperture.

The condition (2) is an essential condition that enables almost all of the large astigmatism due to the cylindrical refractive power of the liquid column surface to be corrected by the front lens group including the liquid column refractive power alone. Is.

The refractive power distribution in the plane perpendicular to the generatrix in the central portion of the liquid column is selected within the range indicated by these conditions, and the refractive power in the plane parallel to the generatrix is defined by When the point aberration is determined to be almost zero, the other various aberrations generated in the front lens group remain within a range that can be sufficiently removed by the rear lens group. F is the lower limit of condition (2) (-
In the state of less than 6.0), compared with the cylindrical refractive power of the surface of the liquid column, the cylindrical refractive power of the opposite sign in the front group or toric refractive power in the front group, which should cancel this, is insufficient,
Significantly insufficient correction of astigmatism remains in the front lens group alone, and the amount thereof exceeds the astigmatism correction power of the rear lens group. Similarly, when F exceeds the upper limit value (-1.0) of the condition (2), the cylindrical refractive power or the toric refractive power of the opposite sign in the front lens group becomes excessive, and the front lens group is significantly corrected. Excessive astigmatism is generated, and the amount thereof becomes a level exceeding the astigmatism correction power of the rear lens group.

The condition (3) defines the entrance pupil position of the front lens group, that is, the entire objective lens system. As is well known, generally, the various aberration amounts of the imaging optical system make a significant difference depending on whether or not the position of the entrance pupil is properly selected. In particular, when an object surface such as this objective lens is tilted in the liquid column, if the entrance pupil position is erroneously determined, total reflection occurs when the chief ray exits from the liquid column surface into the air. The field of view is narrowed, and significant spherical aberration and chromatic dispersion occur, and as a result, astigmatism of the entire objective lens system, field curvature,
The chromatic aberration of magnification will remain excessive.

In this condition, specifically, the entrance pupil position is set to −5 of the focal length F ₁ of the front lens group on the optical axis of the objective lens on the side opposite to the liquid column surface on the objective lens side with respect to the object point in the liquid internal space. .
It means to put it in the range of 0 times to -0.7 times. That is, as long as the entrance pupil is placed within this range, the narrowing of the visual field and the occurrence of excessive aberrations as described above can be avoided.

Now, when the entrance pupil position exceeds the upper limit value −0.7F ₁ , the chief ray exiting from the maximum object point does not form a sufficient convergence angle after exiting the surface of the liquid column, and therefore the front group facing the liquid column. The effective diameter of the first lens of the lens becomes excessively large, and as a result, the aperture of the front lens group with respect to the principal ray optical path becomes large, and an excessive burden is imposed on correction of aperture aberration of the front lens group, particularly off-axis high-order coma aberration. After all, the deterioration of the imaging performance of the entire system becomes unacceptable. Also, when the entrance pupil position falls below the lower limit value -5.0F ₁ , the chief ray emerging from the maximum object point exits the surface of the liquid column. Since the light is incident on the first lens of the front lens group with an excessively large convergence angle, the negative refracting power of the optical path of the chief ray in the front lens group must be strengthened more than necessary. As a result, the field curvature of the entire system is overcorrected, and the deterioration of the imaging performance cannot be ignored. Further, in order to reduce this deterioration as much as possible, the working distance between the surface of the liquid column and the first lens of the front group tends to be short, but the extreme shortening is not preferable in use of the objective lens.

〔Example〕

 Examples of the objective lens according to the present invention will be shown below.

2 to 4 are sectional views showing the lens arrangement of each embodiment in the meridian plane (plane perpendicular to the axis of the liquid column). FIG. 2 shows Embodiment 1 and FIG. Example 2 and FIG. 4 show Example 3
Respectively. In the first embodiment, the front lens group is composed of an achromatic triplet lens and includes three toric refracting surfaces. In the second embodiment, two cylindrical lenses made of a low-dispersion glass material are arranged orthogonally in the front lens group. In the third embodiment, the front group is composed of one lens having a toric refracting surface and one group of achromatic doublet lenses.

Numerical data of each example will be shown below. In the data, R _Y and R _X are the curvatures of the respective lens surfaces in the meridional plane and the plane orthogonal thereto (the plane including the axis of the liquid column and the optical axis). Radius, D
Is the surface spacing, N is the refractive index, and ν _d is the Abbe number. Also,
The surface without the numerical value of R _X is an axisymmetric surface.

Since the change in the basic property value of the fluid physics is remarkable especially near the free surface of the liquid column, in each of the embodiments of the present invention, the axial (0% image height) point located in the deep part of the liquid column and the vicinity of the free surface of the liquid column at the same time. It is designed to be good even at (85% to 100% image height) points. This fact is shown in FIGS. 5, 6, 7, and 8.

5A to 5E show image height maximum of Example 1, 85%, 70
%, 50% and on-axis line image intensity distribution function and MT
F （Modulation Transfer Function） ・ PTF （Phase Tran
It is a figure which shows a sfer function) curve.

6A to 6E show image height maximum of Example 2, 85%, 70
%, 50% and on-axis point spread function and MT
It is a figure which shows F * PTF curve.

7A to 7E are image height maximums of Example 3, 85%, 70
It is a figure which shows the point image intensity distribution function and a wave optical MTF * PTF curve in%, 50%, and an axis.

FIG. 8 is a diagram showing a correction state of the field curvature aberration of the third embodiment.

Note that FIG. 9 is a graph showing the distortion aberration in the width direction of the liquid column (that is, the meridional direction) caused by the tilt of both the object and the image in the objective lens of Example 3, and as described above, the image processing is performed. When using technology to correct distortion, the correction amount may be calculated based on this curve.

〔The invention's effect〕

According to the present invention, it is possible to obtain a lens system having a sufficient image forming performance as an objective lens of an apparatus for precisely measuring a basic physical property value of fluid.

[Brief description of drawings]

FIG. 1 is a diagram showing a liquid columnar sample suspended on a lid and a bottom surface by surface tension in a microgravity environment, a sheet-like illumination light for optically cutting the liquid column, and an arrangement of an objective lens, FIGS. 2 to 4 Is a diagram showing the lens arrangement of Examples 1 to 3 of the present invention, FIGS. 5A to 5E are diagrams showing the image forming performance of Example 1, and FIGS. 6A to 6E are the image forming performances of Example 2. Figure, first
7A to 7E and FIG. 8 are diagrams showing the image forming performance of the third embodiment, and FIG. 9 is a calibration curve of the distortion aberration in the liquid column width direction caused by the inclination of both the object and image sides of the third embodiment.

Claims (1)

[Claims] 1. An objective lens system for mapping an object surface, which is tilted with respect to an optical axis, in a liquid column having a substantially columnar shape onto a light receiving element, the front lens group being arranged in proximity to the liquid column. And a rear group lens disposed facing the light receiving element, and a radius of curvature of a cross section perpendicular to a generatrix in a central portion of the liquid column.
R _C , liquid column refracting power in the vertical plane and the air side focal length of the composite system of the front lens group are F ₁ , the focal length of the rear lens group in the vertical plane is F ₂ , and before normalized by R _C When the focal length of the combined system of the group lens and the liquid column refractive power is F (F = F ₁ / R _C ), and the entrance pupil position of the front group lens measured from the object point is ENP, (1) 1.0 ≦ | F ₂ / F ₁ | (2) -6.0 ≤ F ≤ -1.0 (3) -5.0 ≤ ENP / F ₁ ≤ -0.7 An objective lens for tilt object surface mapping which is characterized by satisfying the following conditions.