WO1991002995A1

WO1991002995A1 - A method of arranging the ray path in an optical instrument

Info

Publication number: WO1991002995A1
Application number: PCT/SE1990/000540
Authority: WO
Inventors: Rolf STRÖMBERG
Original assignee: Stroemberg Rolf
Priority date: 1989-08-22
Filing date: 1990-08-21
Publication date: 1991-03-07
Also published as: SE467941B; JPH04507307A; EP0489095A1; SE8902786D0; SE8902786L; AU6293090A

Abstract

The present invention comprises a way to arrange the light path in a monocular, binocular or similar instrument (7), to obtain in practice an instrument as compact and ergonomical as possible, at the same time as best possible optical data are maintained and the internal mirrors (1-4) can be given smallest dimensions. The light path is located near a plane, but at least two (2; 3) of the image erecting mirrors are located on each side of this plane. The light from the objective (6) strikes these two mirrors (2; 3) in turn, and image tilt is eliminated by inclining the intersection line for said two mirrors (2; 3) to the plane.

Description

A Method Of Arranging The Ray Path In An Optical Instrument,

Technical field

The present invention comprises a way to arrange the light path in a monocular, binocular or similar instrument, to obtain in practice an instrument as compact and ergonomical as possible, at the same time as best possible optical data are maintained and the internal mirrors can be given smallest dimensions. The light path is located near a plane, but at least two of the image erecting mirrors are located on each side of this plane. The light from the objective strikes these two mirrors in turn, and falling lines are avoided by inclining the intersection line for said two mirrors to said plane. Prior art

In binoculars or telescopes where a from the instrument 's objective produced, real image can be examinated through a positive ocular, is an unavoidably requirement that the light path is turned 180 degrees around the optic axis (image erection). If this is not the case, the image is seen upside- down through the ocular, which is unacceptable in most cases.

Image erection takes place in most telescopes in the conventional manner via the first Porro system. A disadvantage with this system is that the telescope cannot be made very compact when the objective focal length is high. image erection by means of a lens between objective and ocular is also known. The disadvantage with this, is that the instrument becomes very long, at the same time as the image erecting lens may introduce aberrations.

Another principle for image erection is described in US pat. 3,298,770. An advantage with this concept is that the instrument can be made very compact, in spite of high objective focal length, and another advantage is that the size of the internal mirrors are minimized. In spite of that has this concept hardly experienced any acceptance, because a so called roof mirror pair must be used, which needs very great precision and hence is expensive to produce. For highest demands on a telescope, the following requirements can be listed:

1 ) The entering and the exiting optical axes ought to be parallel, which makes possible for the user to see the image in the same direction as the original object.

2) For greatest simplicity should only four plane mirrors be utilized for image erection.

3) No roof mirror pairs should be used. -r. For good optic properties should the objective focal length be ratner long, but the outer size of the instrument ought to be rather small, which means that the ray path should make a compact instrument possible.

5) The instument casing may be rather wide which makes gripping with both hands easy, but the length should not exceed the width by any greater value; ot¬ herwise the ergonomy becomes reduced. At the same time ought the height of the instrument be as small as possible.

6) Since the ocular in most practical cases has rather large outer diameter, ought this to be located relatively near the symmetry plane of the instrument ca- sing. Otherwise does the ocular protrude from the instrument, which destroys the look and the handling properties of the instrument. An example of this defect is shown in fig 6a. As a comparison is the outer symmetry for an instrument ac¬ cording to the present invention shown in fig 6b.

7) It is also desirable that the internal mirrors are as small as possible, which minimizes size, weight and production costs for these.

It turns out that none of the heretofore known principles for image erection meets all of the above listed requirements. The object of the present invention is therefore to make possible an instrument which meets all the requirements above. An instrument according to the present invention may be compared to an instrument made according to the above mentioned US pat. 3,298,770 with regard to size and other properties, but has the advantage that expensive roof mirror pairs are avoided. Description of the figures

The invention is described below in connection with the figures 1 -6, where fig 1 shows the light path of the instrument in perspective, fig 2 shows an end wiew and fig 3 shows a wiew in elevation, fig 4 shows a wiew from the above, fig 5 shows a tunnel diagram of the light path, fig 6a shows, in elevation, an irregular instrument casing, and fig 6b shows, in elevation, the symmetry which is made possible for an instrument with a light path according to the invention.

Certain common optical terms are not explained here, but reference is made to US MIL HDBK 141 , in which many design rules are present. Description of an embodiment

Reference is now first made to fig 1 , where a monocular telescope 7 with optics according to the invention is described briefly. The central ray (CS from now on) is defined as a to telescope 7 entering light ray, which before entrance into the instrument is coaxial with the optical axis for objective 6. Central ray CS passes objective 6, is reflected in mirrors 1 , 2, 3, and 4 in turn, before it finally exits through ocular 5. The ocular optic axis is for best results, according to normal optic design rules, coaxial with exiting central ray CS. Through ocular 5 can an erect image be seen, precisely as in a conventional telescope. The reflecting plane for mirror 1 is from now on designated 1 ', the reflecting plane for mirror 2 is designated 2 ', for mirror 3, 3 ', and for mirror 4, 4 ' .

To simplify the description below, a plane SP in the telescope is now defined. Plane SP contains the optic axis for objective 6, and a point P, located precisely between the points where central ray CS strikes mirror 2 and 3, respectively. The telescope casing 8 can preferably be made symmetrical with regard to plane SP. The ocular optic axis is in the described case, but need not be, parallel with the optic axis for objective 6. All optical elements are normally secured to casing 8.

The interesting thing about the invention, which makes possible that all requirements (1 -7) listed above simultanously can be fulfilled, is the unique way the mirrors 1 -4 are arranged in inside casing 8, and therefore is reference now made to the tunnel diagram in fig 5. A number of prism-tunneldiagram are found in said MIL HDBK 141 . The tunnel diagram used here in fig 5 is the same in principle, the only difference is that front surface mirrors here corresponds to the prism reflecting surfaces. A common deign rule for hand held telescopes is that as much as 50% vignetting is allowed at the field stop. If this design rule is used, together with the self-evident requirement that all of the light from the objective should reach the field center, the border lines BL1 and BL2 are attained by optical dimensioning, in fig 5. Every one of the mirrors 1 -4 must be dimensioned so large so they reach from line BL1 to BL2 in the tunnel diagram, to make sure that at least 50% of the light from objective 6 reaches the field stop, and all of the light from the objective reaches the field center.

From fig 4, the observation can be made that the distance between mirror 1 and objective 6 (length L) in essence determines the overall length of the telescope. Large value of L means a long telescope, but on the other hand ought distance L not to be too small, because mirror 1 must then be made unnecessary large (the distance between the lines BL1 and BL2 in fig 5 increases with decreasing distance to objective 6). Hence, the length L should be chosen as large as possible with regard to practical limits, so the instrument do not become too long. A good compromise may be that distance L amounts to approximately 40% of the objective focal length.

Further: the tunnel diagram in fig 5 shows that the distance between the lines BL1 and BL2 is small, far away from objective 6. The mirrors 2, 3, and 4 SI IUUIU therefore be located far away from objective 6, measured along me umiudt ray CS, because they then do not need to be so large. In the shown embodiment according to the invention, all of the mirrors 2 - 4 has small sizes, because they are located relatively far away from objective 6, measured along the central ray CS. This possibility to minimize the sizes of the mirrors is a definite advantage compared to the first Porro system, where two of the four reflecting surfaces normally must be made relatively large.

Requirement 5 above stipulated that the telescope for ergonomic reasons should be as thin as possible, ie the smallest outer dimension of the telescope should be as small as possible. For the design at hand , it is unavoidable because of common optical reasons that the mirrors 2 and 3 ends up "above" each other, and these two mirrors should therefore be as small as possible. If mirrors 2 and 3 in the tunnel diagram are located near point P1 , where the distance between the lines BL1 and BL2 reaches a minimum, becomes also the height which these mirrors takes up in the telescope, minimized. Even this can be fulfilled with the optics according to the invention, which is shown in the tunnel diagram in fig 5, where it is clear that mirror 2 and 3 are located near point P1 . It follows also from the need for minimal height that the mirrors 2 and 3 substantially should be located on each side of the symmetry plane of the instrument casing, and that none of these mirrors should be placed^' unnecessary far away from plane SP. This requirement may be fulfilled and is fulfilled, which is shown in figs 2 and 3.

Requirement 6 above stipulated that the optical axis for ocular 5 should be located in or near plane SP. It turns out that even this requirement may be fulfilled with optics according to the invention. The expedient to bring about this, is to angle line CS downward, towards plane SP, after reflection in mirror 3, so that mirror 4 and hence ocular 5 can be placed near plane SP. It follows from require¬ ment 1 above, that the ocular optical axis and the point where CS strikes mirror 4, should be placed at the same distance L1 from plane SP. In the shown case has L1 = approximately 5 mm been chosen, but L1 = 0 may very well be valid. Fig 6b shows, in elevation, the symmetry that the exterior of the instrument may exhibit when L1 = 0.

It has now been shown that the locations for the mirrors 1 -4 inside telescope casing 8, produces a for given objective focal length very compact and particularly thin instrument, where the dimensions of the internal mirrors can be minimized, and where the ocular is located at the best place. Requirements 1-7 are therefore fulfilled. As a conclusion, with some realistic measures mentioned, tins, can ut. said: seen from the ocular side of the telescope, central ray CS is reflected by mirror 1 oblique forward-downward inside the telescope, so that CS strikes mirror 2 approximately 12 mm below plane SP. Mirror 2 reflects CS forward- upward in the telescope so that CS strikes mirror 3 approximately 12 mm over plane SP. Mirror 3 throws in turn CS oblique forward-downward in the telescope, so that CS strikes mirror 4 at a location near or in plane SP. If CS strikes mirror 4 at a point lying in plane SP, the ocular optic axis should according to requirement 1 ) above also be located in plane SP, and therefore can the exterior of the telescope then with advantage be made wholly symmetrical. The angular position for mirror 4 must therefore be adjusted so that CS, after reflection in mirror 4, is parallel with in the instrument entering CS, and by that plane SP.

One problem remains, namely to adjust the optics so that falling lines do not occur. If falling lines are present, is for example a through the instrument observed horisontal line not seen wholly horisoπtal, but tilted in some direction.

For clearity in the description is now an angle c< defined, an imaginary axis AX, and a line RL. Line RL (roofline) is an imaginary intersection line for the reflecting planes 2 and 3. Angle - is defined as the angle between line RL and plane SP. Imaginary axis AX is defined as the axis which intersects point P and forms the same angle to the reflecting plane 2, as to reflecting plane 3, and which is orthogonal to intersection line RL.

It turns out that the image tilt may be varied, and hence be adjusted down to zero, by adjustment of the angle o , which in practice preferably is done in such a way that the mirrors 2 and 3 as a unit are adjusted by small angular motions around the imaginary axis AX, until the right value for angle ©•^*. is reached.

It is evident that in the in fig 3 already correctly adjusted telescope, RL is tilted an angle ø< relative plane SP. In practice is it necessary in the shown telescope that mirror 2 and 3 are oriented so, that line RL is tilted in the direction shown in fig 3. It can be mentioned that in a prototype according to the concept above, the value of «< became approximately 15. This value may of course vary, depending of the optics used, but in practice must the value of -< lie somewhere between 3*and 50? Values outside this interval are not suitable in practice. Finally, the mirror 4 must be adjusted so, that from mirror 4 outgoing CS is parallel with in the instrument incoming CS. This last adjustment also affects the image tilt a little, which means that adjustment of mirror 2 and 3 according to above must be performed again, and so on. A method to manually, through settings in turn, adjust the optics is by that shown. This method works, but calculation, preferably by a computer, may be preferable. The method above can then be simulated, whereby an iterative method of calculation is obtained. Prisms are as a rule not suitable in a telescope according to the present invention, because the light path is irregular; front surface plane mirrors are preferred. With a high reflection coating, the overall transmission of the telescope can approach 100%, and an another advantage in practice is that "ghost reflections", which the glass surfaces of the prisms in the light path can give rise to, are wholly missing.

It should be noted that the monocular optics described above very well can be used in a binocular. Two similar but not necessarily identical systems are then combined.

Of course can the light path be "mirrored", so that CS, seen from the ocular side of the telescope, is mirrored forward-upward by mirror 1 , thrown downward by mirror 2, and forward-upward by mirror 3. The angles /b and Y can of course be changed from their in figure 4 shown values, too.

Even other modifications can be performed according to the invention. One conceivable such is that some or several parts of the optics can be made movable or adjustable relative the instrument casing, to make possible for example optical image stabilization. This movability or adjustability for some part of the optics is only meaningful if it takes place around a central position, which should be considered to define the light path according to the present invention . Another modification may be, that the optic axis of ocular 5 is not parallel with the optic axis for objective 6. This is not generally recommended, but may be an advantage in some cases. Lenses between the mirrors can if so wished be added, and such modifications, obtained through added optic elements, should be considered covered by the invention as long as the basic concept, described in the claims, is applied. Ocular 5 may be omitted and replaced by a film plane or the like.

Claims

Way to arrange the light path in an optical instrument such as telescopes, tele objectives or the like (7), in which the entering and exiting optic axes are sub- stantially parallel, comprising at least casing (8), objective (6), and mirrors ( 1 -4 ) with first { 1 ' ) second ( 2 ' ), third ( 3 ' ) and fourth { 4 ' ) plane reflecting sur¬ faces, where the reflecting surfaces ( 1 '- 4 ' ) performs image erection, and where the central ray (CS) is reflected in turn in the first ( 1 ' ), second ( 2 ' ), third ( 3 ' ) and fourth ( 4 ' ) plane reflecting surface, the second ( 2 ' ) and third ( 3 ' ) reflecting surface, together with the optic axis for the objective (6) defines a plane (SP) in such a way so the plane (SP) contains the optical axis for the objective (6) and a point (P), which lies halfway between the two points where the central ray (CS) strikes the second ( 2 ' ) and the third ( 3 ' ) reflecting surface, characterized in that the central ray (CS) strikes the second ( 2 ' ) re- spectively the third { 3 ' ) reflecting surface at points each lying at least 4 millime- o o ters from, and on each side on the plane (SP), and that 3 < ° < 50 is valid, where ~< constitutes the angle as the intersection line (RL) for the second ( 2 ' ) and the third ( 3 ' ) reflecting surface forms to the plane (SP), the reflection point for the central ray in the fourth reflecting surface is nearer an imaginary plane (HP) than the reflection point for the central ray in the third reflecting surface , where the plane (HP) contains the objective optic axis, together with the central ray (CS) where it runs between the first and the second reflecting surface.

2. Way according to claim 1 , characterized in that the value of the angle ( 3 ) between entering ray to, and exiting ray from the first reflecting surface is less than 85?

3. Way according to claim 1 , characterized in that the symmetry plane for the exterior of the casing (8) of said instrument substantially coincides with said plane (SP).

4. Way according to any of the claims 1 -4, characterized in that some or several parts of the optics (1 -7) for purposes of image stabilization are movable or ad¬ justable around a central position or normal position relative the instrument casing (8), the central position or normal position defines the ray path according to claims 1 -4.