US2625856A - Telecentric objective - Google Patents

Description

Jan. 20, 1953 R. M. MULLER 2,625,856

TELECEINTRIC OBJECTIVE Filed Sept. 3. 1949 2 SHEETSSHEET 2 EFL lOOmm Magnification I01 Lens Nd, V Radii ThicKnesses I 1.517 64.4- f t 15.0

[22 -'6|.2 R T592 s 1.0 11 L649 33.8 3 t 6.0

I2 =-5ooo R I s; 59.7 III 1.518 59.5 5 x 11.0

12 =-1o1.3 12-, --79a 53 ISZ I584 4&0 R8 65' k 4.5

Diaphragm Position 49.8mm Frornk Short conjugate 90.4mm From 12. Long conjugate 1050mm From 12 EFL=lO0mm Magnification 20x Lens N4, V Radii Th'1cKnesses I22 =-6Z.2 12 -s94 11 1.649 33.8 5 11 6.0

12., =-s14.7 R 538 5 38.4- 131 1.516 59.5 5 x 11.0 75 2 53 Diaphragm Position 50.7mm From 12 Short conjugate. 85.6 mm From 2 Long conjugctie 205\ mm FromRg INVENTOR R0 BERT M. MULLER Patented Jan. 20, 1953 TELECENTRIC OBJECTIVE Robert M. Muller, Buffalo, N. Y., assignor to American Optical Company,

Southbridge,

Mass., a voluntary association of Massachusetts Application September 3, 1949, Serial No. 113,920

11 Claims.

This invention relates to telecentric lens systems and more particularly to such lens systems constructed and arranged for use in contour projectors and like precision optical measuring instruments.

Telecentric lens systems have particular utility in contour projectors and the like wherein it is desirable that the contours of various tools, workpieces and such may be projected as enlarged images upon a viewing screen at exact predetermined magnifications for measurements or comparison against standard patterns or graphs thereon. Such comparisons can easily be made between or following fabrication steps in the manufacture of tools and the like with much better product control and efliciency being accomplished than is possible otherwise.

A telecentric lens system in a simple form may consist of a positive component and a telecentric stop or diaphragm located at the principal focus of the system on the long conjugate or image side thereof. Thus when an object is positioned for magnification at an object plane on the short conjugate side of the system and illuminated by collimated light substantially paralleling the axis of the system, each principal ray thereof will be caused to pass through the center of the diaphragm aperture and together with the small surrounding bundle of rays coming from the object point of said principal ray at the object plane and passed by said diaphragm aperture will be caused to come to focus upon a viewing screen at the conjugate image plane, there forming an image of predetermined magnification of the object point. The use of such a lens system allows different parts of the tool or workpiece being imaged to appear upon the screen at the same magnification notwithstanding the fact that these parts may be disposed slightly forwardly or rearwardly of the true conjugate plane on the short conjugate side of the system. This is because each telecentric lens system has an unchanging magnification resulting from the fact that the entrance pupil thereof is located at infinity on the object side of the system and the principal rays on that side extend parallel to the optical axis thereof. Such systems, however, are often susceptible of objectionable errors in distortion, particularly when an object field of appreciable size is being considered, and curvature of field.

The present invention embodies a telecentric lens system which is highly corrected for distortion and curvature of field as well as being well corrected for astigmatism, coma spherical and chromatic aberrations. Furthermore, the lens system has a relatively long working .distance in the object space, a condition which is highly desirable in instruments of the type described, and provides at a preselected image plane an image of exact predetermined magnification. The lens system has relatively few elements which are of comparatively fiat curvatures and therefore is less expensive to manufacture and may be easily assembled and adjusted, consisting of only front and rear positive components in axially spaced relation and a diaphragm or stop spaced rearwardly of said components and positioned at the principal focus of the system. The front component in turn consists of a biconvex front ele ment and a negative meniscus rear element spaced slightly axially therefrom. The rear component consists of a biconvex front element and a biconcave rear element spaced slightly axially therefrom.

With reference to the front component, it should be noted that the small axial spacing between the adjacent surfaces on the front and rear elements of these surfaces is such that slight amounts of axial adjustment may be employed to advantage in controlling errors of distortion, in a distortion corrected system. which may be introduced into the system during the manufacture and assembly of the elements thereof. By using air spaced doublets a sumcient number of surfaces were provided in the system to afford convenient control of the aberration of the system. It was desirable for reasons of design to keep these air spacings of the doublets small while allowing, with reference to the front doublet, for the required final adjustment when necessary for distortion correction in the manufactured objective without having the centers or the edge portions of the elements touch and limit such adjustment. In this manner, slight variation in thicknesses, radii and optical properties which might occur during the manufacture of the components of the objective and which might introduce distortion into the system may be compensated for. Also the system is such that the spacing between the front and rear components may be varied slightly while the preselected long conjugate is maintained fixed to adjust the magnifled image to the predetermined magnification required upon the viewing screen of the instrument with relatively little influence upon the distortion of the system. Actually, with the long conjugate of the system maintained at its preselected value, the spacing between the elements of the front component and the spacing between the front and rear components mutually efiect both magnification and distortion in the system. However the first spacing has great effect upon distortion with only slight eflect upon magnification while the spacing between the components has a material effect upon the magnification and very little eifect upon distortion. of course such changes will have an effect, but only slightly, upon the working distance or short conjugate distance of the system. The importance of such conditions in a lenssystem such as is employed on a contour projector is that the problem of adjusting the system to its finished working condition is greatly simplified. Instead of having to attempt the simultaneous balancing of the adjustment of the spacing between the components and the spacing of the elements of the front component, one may initially adjust the spacing of said elements to adjust distortion to its minimum value and then may carefully adjust the spacing between the components for correct screen magnification, after which only a very slight readjustment of the first spacing may be needed to produce an acceptable finished objective.

It is accordingly an object of the present invention to provide a telecentric lens system for use in contour projectors and like instruments which is composed of relatively few optical elements of types that are relatively easy to fabricate and assemble while providing a system which is highly corrected for distortion and curvature of field and well corrected for astigmatism, coma, spherical and chromatic aberrations, and which has a relatively long working distance on the short conjugate side of the system.

It is a further object of the invention to provide a lens system in which the pre-assigned long conjugate distance may be maintained while allowing slight adjustments to be made for obtaining predetermined magnification with very little effect upon distortion and for obtaining thereafter in a simple manner minimum distortion with substantially no material effect upon magnification to thereby compensate for inaccuracies in the physical or optical properties of the lens elements which might be introduced into the system during the ordinary commercial methods of fabrication and assembly used, to be made in a simple manner.

Other objects and advantages of the invention will become apparent from the detailed descrip- 7 tion which follows when considered in conjunction with the accompanying drawing in which:

Fig. 1 is a diagrammatic side view of a lens system embodying the invention;

Fig. 2 is a table illustrating an embodiment of the invention;

Fig. 3 illustrates a second embodiment of the invention; and

Fig. 4 illustrates a. third embodiment of the invention.

In each of the embodiments of the invention, it will be noted a lens system has been shown which consists of a front positive component and a rear positive component arranged in axially spaced relation and rearwardly thereof is a telecentric diaphragm or stop. Each component is an air spaced doublet, the front component having a front biconvex element and a rear negative meniscus element closely fitting the front element, the rear component having a, biconvex front element and a biconca-ve rear element slightly spaced therefrom.

In each embodiment, based upon a focal length of 100 mm. for the system, a space of approximately 1 mm. has been allowed between the elements of the front component to allow an axial adjustment thereof if such is necessary for distortion correction in the system. Normally no more than one quarter to one half millimeter adjustment in either direction from this preferred spacing will be required for reducing the distortion to a minimum value. Since the specing between the components in the several examples lies between 30 to 40 mm., adequate adjustment for manufacturing errors for effecting predetermined screen magnification while the long conjugate remains unchanged may be had by 2 /2 to 3 millimeters of movement of the front component in either direction from the preferred spacing thereof.

A preferred embodiment for a telecentric lens system having a 1.6x magnification and figured on the basis of a focal length of mm. is as follows:

and wherein N is the index of refraction, V is the index of dispersion, R1 to Rs are the radii, ti to t4 are the thicknesses and $1 to s: are the spaces between the elements of the system taken from front to rear thereof. In this system when a negative lineal field up to 13.08 mm. was measured on the short conjugate side and the short conjugate measured 143.7 mm. from the surface R1 the distortion measured on the screen at 1.603x paraxial magnification was between 002 and -.009 mm. The long conjugate measured 217.1 mm. and the diaphragm distance measured 56.9 mm. from the rear surface Ra of the system.

A second preferred embodiment for a telecentric lens system having a 10x magnification and figured on the basis of a focal length of 100 mm. is as follows:

Lens N V Radii Thicknesses Si- 1.0 R3= 59. 2 II 1.649 33.8 lg= 6.0

S1=39. 7 R 51. 1 III 1. 518 59. 5 t =ll.0

Rs=l01. 3

S 0.6 R 79. 3 IV 1. 584 46. 0 t 4. 5

In this system when a lineal field up to +19.95 was used on the short conjugate side and the short conjugate measured substantially 90.4 mm. from the front surface R1 the distortion measured on the screen at slightly less than 10X paraxial magnification was between +.07 mm. from exactly ten times the object size. The long conjugate measured substantially 1050 mm. and

Lens N V Radii Thicknesses I 1.517 64.4 t1=l5.0

S1 38. Be 53.8

IV 1.584 46.0 tu- 4.5

In this system when a lineal field up to +1843 mm. was used on the short conjugate side and the short conjugate measured substantially 85.6 mm. from the front surface R1 the distortion measured on the screen at slightly less than 20x paraxial magnification was between +.10

mm. from exactly twenty times the object sizes The long conjugate measured substantially 2051 mm. and the diaphragm distance measured 50.7 mm. from the rear surface Rs of the system. It

-is interesting to note that the focal length of the front component in all three modifications given above was taken at 154 mm. and for the rear components was taken at 202 mm.

It should be noted that in the modifications given above the short conjugate (based upon an EFL of 100 mm.) falls between approximately 85 and 144 mm. which distance will be useful in providing a relative long working distance for instruments of the character described. Also it will be seen that the spacing provided between the elements of the front component in the examples given is approximately between 1% and 5% of the spacing between the front and'rear components, 2.5% being preferred. While the same glasses have been employed as a matter of convenience in the three preferred embodiments given above, it should be noted that the invention is not limited to the use of such glasses.

Other known glasses could have been similarlyemployed with suitable allowances being made in the computations therefor. The important factors of the invention are, however, that the long conjugate of the system may be predetermined and may remain fixed, after which the spacing 81 adjusted to give minimum distortion, the spacing s: adjusted to give correct image magnification with very slight effect upon the distortion and finally the spacing s1 readjusted only slightly to return the distortion of the system to a minimum without material eifect upon the magnification.

Having described the invention, I claim:

1. A telecentric projection lens system of predetermined magnification and relatively long working distance and highly corrected for distortion and curvature of field, said lens system con-- sisting of a front positive component, a rear positive component in axially aligned spaced relation space r arwardly of locama 9.

5...! g, r tan between said rear m, the

5 mm diaphragm being between substantially 49 and 57 percent of the focal length of said system, and the distance between said front and rear components being between substantially 30 and 40 percent of the focal length of said system, said front component being a doublet consisting of a biconvex element and a negative meniscus of higher refractive index rearwardly thereof, the adjacent surfaces of said element and said meniscus being of closely matching curvatures allowing a nesting thereof, said rear component being a doublet consisting of a biconvex element and a biconcave element of higher refractive index adjacent the rear thereof, the spacing between said front and rear components being of a predetermined amount and axially adjustable slightly relative to each other to provide said predetermined image magnification while the rear conjugate distance remains substantially unchanged, the elements of said front component being spaced between substantially 1 and 5 percent of the distance between said front and rear components from each other and axially adjustable slightly in either direction relative to each other so as to reduce distortion to a minimum, when errors of magnification and distortion occur in the system due to manufacturing inaccuracies or the like.

2. A telecentric lens system according to claim l in which the radius of curvature of the front surface of the meniscus element of the front component is substantially equal to the radius of curvature of the rear surface of the biconvex element adjacent thereto.

3. A telecentric projection lens system of predetermined magnification and highly corrected for distortion and curvature of field, said lens system consisting of a front positive component, a rear positive component in axially aligned spaced relation thereto, and a telecentric diaphragm spaced rearwardly of said rear component and located substantially at the principal focus of said system, said front component being a doublet consisting of a biconvex element and a negative meniscus of higher refractive index rearwardly thereof. said rear component being a doublet consisting of a biconvex element and a biconcave element of higher refractive index adjacent the rear thereof, the spacing between said front and rear components being substantially between 0.3 and 0.4 times the focal length (F) of the lens system, and the diaphragm distance from the rear surface of said rear component being substantially between 0.5 and 0.6 times the focal length of said system, the elements of said front component being spaced a relatively small predetermined distance from each other so as to allow small amounts of axial adjustment therebetween for removing distortion from said system when such occur therein due to manufacturing inaccuracies or the like.

4. A telecentric projection lens system of predetermined magnification and highly corrected for distortion and curvature of field, said lens system consisting of a front positive component, a rear positive component in axially aligned and predetermined spaced relation relative thereto, and a telecentric diaphragm spaced rearwardly of said rear component and located substantially at the principal focus of said system, the distance between said rear component and said diaphragm being between substantially 49 and 57 percent of the focal length of said system, and the distance between said front and rear components being between substantially 30 and 40 percent of the focal length of said system, said front component being an air spaced doublet consisting of a biconvex element and a negative meniscus of higher refractive index spaced between substantially 1 and 5 percent of the distance between said front and rear components for allowing slight axial adjustment therebetween in either direction, said rear component being a doublet consisting of a biconvex element and a biconcave element of higher refractive index adjacent the rear thereof.

5. A telecentric lens system according to claim 4 in which the adiacent surfaces of the elements of said rear component are air spaced and are of closely matching curvatures allowing a nesting relation therebetween.

6. A telecentric lens system according to claim 4 in which the object plane on the short conjugate side of said system is spaced between approximately .8 and 1.5 times the focal length of said system forwardly of the first lens surface thereof thus providing a relatively long working distance for said system.

7. A telecentric lens system according to claim 4 in which the elements of said rear component are separated a relatively small predetermined distance from each other.

8. A telecentric lens system according to claim 4 in which the radius of curvature of the front surface of the meniscus element of the front component is less than the radius of curvature of the rear surface of the biconvex element adjacent thereto.

9. A telecentric lens system of predetermined magnification and highly corrected for distortion and curvature of field, said lens system being substantially in accordace with the specification shown in the following table:

and wherein N is the index of refraction, V is the index of dispersion, R1 to Rs are the radii, tl to t4. are the thicknesses and $1 to ss are the spaces between the elements of the system taken from front to rear thereof.

10. A telecentric lens system of predetermined magnification and highly corrected for distortion and curvature of field, said lens system being substantially in accordance with the specification shown in the following table:

Lens N V Radii Thicknesses R|=+0. 7 F I 1.52 64 h-O. 2 F

Rg -O. 6 F

81 0- 01 F Rg -O. 6 F II 1.65 34 tz=0. 06 F 8:=0. 4 F Rs=+0.5 F III 1.52 59 t =0.1 F

I R|--l. 0 F

a =0.0l F R1=0. 8 F IV 1. 58 46 i4=0.05 F

lav-+1.7 F

and wherein N is the index of refraction, V is the index of dispersion, R1 to Re are the radii, t1 to t4 are the thicknesses and s1 to so are the spaces between the elements of the system taken from front to rear thereof.

11. A telecentric lens system of predetermined magnification and highly corrected for distortion and curvature of field, said lens system being substantially in accordance with the specification shown in the following table:

and wherein N is the index of refraction, V is the index of dispersion, R1 to Rs are the radii, t1 to it are the thicknesses and $1 to s: are the spaces between the elements of the system taken from front to rear thereof.

ROBERT M. MULLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,484,853 Warmisham Feb. 26, 1924 1,843,519 Richter Feb. 2, 1932 2,394,959 Wynne Feb. 12, 1946 2,500,046 Schade Mar. 7, 1950 FOREIGN PATENTS Number Country Date 465,010 Germany Sept. 3, 1928 42,449 France May 2, 1933 (1st addition to 706,347)

OTHER REFERENCES Hardy and Perrin text, The Principles of Optics, pages 74, 104, published by McGraw-Hill Book Co., 1932, New York. Copy in Division 7.

Johnson text, Photographic Optics, page 157, section 62, published 1909 by Ward 8: Company, 34 Craven Street, Charing Cross W. C., London, England. Copy in Division 7.

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