Abstract:
An apparatus for the thermal compensation of an optical system is disclosed. The apparatus comprises a housing, at least one optical element adapted to be displaced relative to the housing, and at least one piston-and-cylinder unit positioned directly between the housing and the optical element. The piston-and-cylinder unit acts on the position of the optical element within the housing. It contains a fluid. The coefficients of volumetric thermal expansion of the piston, the cylinder and the fluid are selected such that for a predetermined change in temperature of the apparatus a defined relative displacement between the piston and the cylinder takes place which compensates the change of the optical properties of the optical system caused by the change in temperature. The fluid is a polymer system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to the field of optical systems.  
         [0002]     More specifically, the invention is related to the filed of thermal compensation of an optical system.  
         [0003]     Still more specifically, the invention is related to an apparatus for the thermal compensation of an optical system, comprising a housing, at least one optical element adapted to be displaced relative to the housing, and at least one piston-and-cylinder unit positioned directly between the housing and the optical element, the piston-and-cylinder unit acting on the position of the optical element within the housing and containing a fluid, wherein the coefficients of volumetric thermal expansion of the piston, the cylinder and the fluid are selected such that for a predetermined change in temperature of the apparatus a defined relative displacement between the piston and the cylinder takes place which compensates the change of the optical properties of the optical system caused by the change in temperature.  
       BACKGROUND OF THE INVENTION  
       [0004]     Optical systems are temperature sensitive. Under the influence of temperature the geometry of lenses (radii, thickness, diameter) changes as well as the refractive index of the lens material used. The surrounding structure (the mounts and the barrel supporting the optical elements) change likewise. These changes effect a deterioration of the imaging quality.  
         [0005]     In this context various approaches have become known for compensating this negative influence of changes in ambient temperature.  
         [0006]     A first approach consists in purposefully combining materials for the components of the optical system with different coefficient of volumetric thermal expansion, such that the different volumetric thermal expansions just compensate one another. This approach, however, has substantial effects on the design of the components. It is, moreover, quite limited in its efficiency because the thermal coefficients of solids only differ little, and, therefore, the components have to be quite voluminous for obtaining noticeable compensation effects. Further, the selection of materials becomes limited thereby.  
         [0007]     A second approach works with actuators which purposefully change the position of one or several optical elements within the system during a change in temperature, such that the imaging errors caused by the temperature change are compensated.  
         [0008]     In this context one has already used apparatuses in which a bi-metallic element changes its shape as a function of temperature and an actuating force is derived therefrom. The forces generated thereby are, however, quite small and allow only small amounts of displacement for the optical elements.  
         [0009]     On the other hand one has also been working with temperature sensors for controlling an actuator, for example a motor in connection with a pinion-and-rack drive unit, a spindle drive unit, a piezo drive unit or the like. By doing so one may obtain high actuating forces and long distances of displacement, however, one has to put up with the fact that the dimensions of the units require substantial space. Moreover, such systems require an electrical power supply which is not available for many optical systems, for example telescopes, and which is not desired either.  
         [0010]     Finally, apparatuses have become known utilizing a piston-and-cylinder unit in which one takes advantage of the substantial difference in thermal coefficients between solids on the one hand and liquids on the other hand.  
         [0011]     Document EP 1 081 522 B1 discloses a temperature-compensated objective lens for a film camera. In this objective lens the optical components are not displaced. Only the index ring is rotated relative to the rotatable distance setting ring for compensating the scale values on the distance setting ring which would be faulty otherwise. The actuator used for that purpose comprises a wax motor with a cylinder and a piston moving temperature-dependent with the coefficient of volumetric thermal expansion of the wax, and thereby rotates the ring. The wax motor rotates the ring in dependence of the temperature acting on it which expands or contracts, respectively, a liquid contained therein. The ring is biased in a circumferential direction by a compression spring, thereby overpowering by pressure any play that might exist.  
         [0012]     This prior art apparatus has the disadvantage that the adjustment of the optical elements required for focusing is effected indirectly because the wax motor acts on the adjustment ring, and, therefore, also influences its reading.  
         [0013]     Document U.S. Pat. No. 4,525,745 A discloses a similar apparatus for the thermal compensation of a focusing unit in a projection objective lens. The apparatus comprises a piston-and-cylinder unit with a cylinder and a piston that is likewise biased by springs. Her, too, the optical elements are adjusted indirectly, namely by means of a lever or a cam drive.  
         [0014]     Document U.S. Pat. No. 3,162,664 A discloses still another such apparatus having likewise a spring-biased piston and an indirect adjustment via coupling elements.  
         [0015]     Document U.S. Pat. No. 4,919,519 A discloses a fluid thermal compensation system for an objective lens. This system provides for a piston-and-cylinder unit directly between the housing and a lens of the objective lens. A liquid, namely a mixture of 66% ethylene glycol (with inhibitors) and 34% water is contained in the piston-and-cylinder unit cavity. This liquid is selected because of its low freezing point of −65° C. The liquid has a coefficient of volumetric thermal expansion of 540×10 −6 /°C.  
         [0016]     In such systems liquids have substantial drawbacks. These draw-backs of liquids in particular consist in that already a small loss of liquid results in the formation of little gas bubbles so that the thermal compensation system becomes inoperable as a whole, and, on the other hand, the leaking liquid contaminates optical surfaces.  
       SUMMARY OF THE INVENTION  
       [0017]     It is, therefore, an object underlying the invention to improve an apparatus of the type specified at the outset, such that the afore-mentioned drawbacks are avoided. In particular, an apparatus shall be provided that has a reliable thermal compensation over long time use, and which is simple to manufacture.  
         [0018]     In an apparatus of the type specified at the outset this object is achieved in that the fluid is a polymer system.  
         [0019]     The object underlying the invention is, thus, entirely solved.  
         [0020]     The use of a polymer system instead of a liquid namely has the advantage that due to the higher viscosity, as compared to that of prior art hydraulic fluids, fluid will practically not escape in case of a leakage, such that one has not to be afraid of either an impairment to the thermal compensation system due to the formation of gas bubbles, or a contamination of the optical elements.  
         [0021]     In particularly preferred embodiments of the invention the polymer system is a reactive polymer system being liquid in the non-cured state, and having a consistency between that of a gel and that of an elastomer in the cured state.  
         [0022]     This measure has the advantage that during the manufacture of the optical system the polymer system, due to its low viscosity in the non-cured state, may be filled into the thermal compensation system in a simple manner, and that after curing it assumes the desired higher viscosity in situ.  
         [0023]     It is, further, preferred when the polymer system is selected from the group consisting of: silicones, polyurethanes, acrylates, epoxies, urethaneacrylates, epoxyacrylates, polysulfides, hot-melt adhesives, hot-melt resins, ketone resins, colophonium derivates, waxes.  
         [0024]     These substances, the enumeration of which being not to be understood as a limitation, have turned out to be particularly appropriate during practical tests.  
         [0025]     In embodiments of the invention the polymer system is an addition crosslinking two-component casting compound.  
         [0026]     This measure has the advantage that during the manufacture of the optical system the polymer system may be produced in a simple manner by mixing the two components with each other.  
         [0027]     According to the invention, a particularly good effect is achieved in that fillers are admixed to the polymer system.  
         [0028]     This measure has the advantage that the properties of the polymer system may be purposefully adjusted.  
         [0029]     Preferred as fillers are nano particles, in particular SiO 2  particles having a particle size of between 5 and 20 nm, preferably of 10 nm.  
         [0030]     The preferred optical elements are lenses or groups of lenses. The invention, however, may likewise be used in connection with other optical elements, for example aperture stops or mirrors.  
         [0031]     The optical system, preferably, is an objective lens, for example for a camera or a telescope.  
         [0032]     In embodiments of the invention the piston and the cylinder are each configured sleeve-shaped and coaxial to the optical element.  
         [0033]     This measure has the advantage that a particularly compact design is obtained which, as compared to conventional apparatuses without thermal compensation requires only a very small additional space.  
         [0034]     In this context it is particularly preferred when the piston and the cylinder surround the optical element.  
         [0035]     In the context of the present invention the kinetic alternative is preferred in which the optical element is connected to the cylinder and the housing is connected to the piston.  
         [0036]     In a further group of embodiments the piston is biased relative to the cylinder by means of a spring.  
         [0037]     This measure has the advantage that a remaining play, likewise a play caused by the required gaskets within the piston-and-cylinder unit is suppressed.  
         [0038]     Another embodiment of the invention is characterized in that the piston-and-cylinder unit comprises a cavity for the fluid, and that the cavity is subdivided into a plurality of axial chambers in a circumferential direction.  
         [0039]     This measure has the advantage that in the case of unfavorable length conditions one avoids that the piston tilts within the cylinder because the fluid expands homogenously within the chambers.  
         [0040]     Moreover, a measure is preferred, according to which the piston-and-cylinder unit comprises a cavity for the fluid, and the cavity is connected to an auxiliary cavity.  
         [0041]     This measure has the advantage that the expanding fluid volume may be positioned at an arbitrary location within the optical system. This results in additional design options, and, due to a larger volume, larger expansions to be exploited.  
         [0042]     Further advantages will become apparent from the description and the enclosed drawings.  
         [0043]     It goes without saying that the features mentioned before and those that will be explained hereinafter may not only be used in the particularly given combination but also in other combinations, or alone, without leaving the scope of the present invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]     Embodiments of the invention are shown in the drawing and will be explained in further detail throughout the subsequent description.  
         [0045]      FIG. 1  is a schematic depiction of a piston-and-cylinder unit for explaining the invention; and  
         [0046]      FIG. 2  is a partial, cross-sectional view, of an embodiment of the apparatus according to the invention, exemplified with respect to an objective lens. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     In  FIG. 1  reference numeral  10  as a whole designates a hydraulic adjustment unit. Adjustment unit  10  comprises a sleeve-shaped piston  12  which slides within a likewise sleeve-shaped cylinder  16  via gaskets or seals  14   a,    14   b.  With corresponding annular shoulders piston  12  and cylinder  14  define an annular cavity  18  containing a fluid  20 . According to the invention, fluid  20  is a polymer system.  
         [0048]     Preferably, the polymer system is selected from the group consisting of: silicones, polyurethanes, acrylates, epoxies, urethaneacrylates, epoxyacrylates, polysulfides, hot-melt adhesives, hot-melt resins, ketone resins, colophonium derivates, waxes.  
         [0049]     Further preferred is the use of 2K silicone rubber.  
         [0050]     By using within the thermal compensation system a reactive polymer system having a consistence between that of a gel and that of an elastomer, the essential drawbacks of conventional hydraulic fluids are avoided. These drawbacks of liquids for example consist in that in view of sealing problems only highly fluorinated liquids may be used which in the long run behave indifferently in relation to the elastomers of the gaskets. A good sealing effect is essential in the present context because on the one hand already a small loss of liquid results in the formation of little gas bubbles so that the thermal compensation system becomes inoperable as a whole, and, on the other hand, the leaking liquid contaminates optical surfaces.  
         [0051]     The polymer system used according to the invention, preferably, is an addition crosslinking two-component casting compound. This casting compound is filled into the thermal compensation system in its non-cured state which is highly facilitated by the low viscosity in that state. Only after the filling in the casting compound is crosslinked, for example with the help of heat or of ultraviolet light.  
         [0052]     By properly selecting the casting compound and guiding the crosslinking process, the consistency of the cured polymer system may be set in wide ranges from gel-like over elastomer-like to brittle. For each individual application, the modulus of elasticity, the glass transition range, and the coefficient of volumetric thermal expansion are of importance.  
         [0053]     The following coefficients of volumetric thermal expansion (each in 10 −6 /° C. units) have roughly been determined for the present application:  
                                                       Silicones:   250-300           Silicone gels:   300-350           Polyurethane:   200-300           Epoxy resin:   60-80           Polycarbonate (CR 39):   100-120                      
 
         [0054]     wherein, as is well known, the coefficients of volumetric thermal expansion for aluminum are about 25, for steel are about 10-14, and for optical glass (BK 7) are about 7-8, also in 10 −6 /° C. units.  
         [0055]     The properties of the polymer system may be purposefully modified by the admixing of fillers. Preferred as fillers are nano particles, for example SiO 2  particles having a particle size of between 5 to 20 nm, preferably of 10 nm.  
         [0056]     Piston  12  and cylinder  14  consist of a solid material, in particular a metal. It is assumed that they both consist of the same material having a coefficient of volumetric thermal expansion designated α. The coefficient of volumetric thermal expansion of fluid  20  is designated β. In  FIG. 1 , further, the axial length of cavity  18  is designated as L, the outer diameter of piston  12  as D 1  and the inner diameter of cylinder  16  as D 2 .  
         [0057]     As the coefficient of volumetric thermal expansion β of fluid  20  is essentially higher as compared to the coefficient of volumetric thermal expansion α of piston  12  and cylinder  16 , a change in temperature causes a relative axial movement between piston  12  and cylinder  16 . In order to obtain a desired change in length ΔL for a predetermined temperature change ΔT, one has to calculate the required axial length L of cavity  18 .  
         [0058]     The volume V of cavity  18  is: 
 
 V=π/ 4( D 2 2   −D 1 2 ) L    [1]
 
         [0059]     The change in volume ΔV F  of fluid  20  (coefficient of volumetric thermal expansion β) for a change in temperature ΔT is: 
 
Δ V   F   =βΔTV=βΔTπ/ 4( D 2 2   −D 1 2 ) L    [2]
 
         [0060]     The change in volume ΔV H  of cavity  18  (coefficient of volumetric thermal expansion α) for a change in temperature ΔT is: 
 
Δ V   H =(π/4( D 2′ 2   −D 1′ 2 ) L ′)−(π/4( D 2 2   −D 1 2 ) L ),   [3]
 
         [0061]     wherein D‘′ and D 2 ′ are the diameters of piston  12  and cylinder  16 , and L′ is the length of cavity  18  at the temperature having changed by ΔT.  
         [0062]     With 
 
 D 1 ′=D 1(1 +αΔT ) und  D 2 ′=D 2(1 +αΔT )   [4]
 
and 
 
 L′=L+ΔL    [5]
 
         [0063]     one obtains  
                     Δ   ⁢           ⁢     V   H       =       ⁢     (       π   /   4     ⁢     (           (     1   +     α   ⁢           ⁢   Δ   ⁢           ⁢   T       )     2     ⁢     (       D   ⁢           ⁢     2   2       -     D   ⁢           ⁢     1   2         )     ⁢     (     L   +     Δ   ⁢           ⁢   L       )       -                             ⁢       (       D   ⁢           ⁢     2   2       -     D   ⁢           ⁢     1   2         )     ⁢   L     )     =               =       ⁢         π   /   4     ⁢     (       D   ⁢           ⁢     2   2       -     D   ⁢           ⁢     1   2         )     ⁢     (           (     1   +     α   ⁢           ⁢   Δ   ⁢           ⁢   T       )     2     ⁢     (     L   +     Δ   ⁢           ⁢   L       )       -   L     )       =                 =       ⁢       π   /   4     ⁢     (       D   ⁢           ⁢     2   2       -     D   ⁢           ⁢     1   2         )     ⁢     L   ⁡     (           (     1   +     α   ⁢           ⁢   Δ   ⁢           ⁢   T       )     2     ⁢     (     1   +     Δ   ⁢           ⁢     L   /   L         )       -   1     )                       [   6   ]             
 
         [0064]     As the changes in volume ΔV F  and ΔV H  are equal, [3] and [6] may be equated, too, and one obtains for L: 
 
 L=ΔL /((βΔ T+ 1)/(1 +αΔT ) 2 −1)   [7]
 
         [0065]     For the design of  FIG. 1  the diameters D 1  und D 2 , and, hence, their tolerances, have no influence on the thermal compensation. The length L required for a change in length ΔL when temperature changes by ΔT solely depends on α and β.  
         [0066]     Because the contribution from a is small, [7] may be simplified to read: 
 
 L=ΔL /(βΔ T ).   [8]
 
         [0067]     Example: when piston  12  and cylinder  16  are made from aluminum (α=24×10 −6 /K) and fluid  20  is oil (β=10 −3 /K), then for a desired change in length ΔL=0.2 mm for a temperature change ΔT=20K the required length is obtained from equation [7] as L=10.54 mm, and from equation [8] as L=10 mm. The deviation of about 5% is acceptable, such that in practice one may calculate with simpler equation [8]. The example shows that length L for the desired change in length ΔL is very short, thus enabling a compact design.  
         [0068]      FIG. 1  shows an apparatus with two O-ring seals  14   a  and  14   b.  Instead of these O-rings one may of course also use other kinds of slide seals or gaskets as are known in the field of hydraulics. Moreover, membrane seals, which do not slide, may likewise be used.  
         [0069]     Irrespective of the kind of seal used, the problem of lost motion may arise which results in a hysteresis within the thermal compensation. The O-rings used are namely compressed in a radial direction for obtaining a sufficient sealing effect. Thereby, the elastic O-rings are broadened in an axial direction. For enabling such an axial broadening to happen, the groove housing the O-Ring must be axially broader as the O-Ring diameter. Accordingly, when the fluid becomes warmer or colder, respectively, the O-rings first move to the respective opposite groove wall before the piston and the cylinder start to move relatively to one another.  
         [0070]     In the apparatus shown in  FIG. 2 , this disadvantage is avoided.  
         [0071]      FIG. 2  shows an optical system  30 , namely an objective lens. System  30  has a housing  31 . An optical element  32 , being a group of lenses in the embodiment shown, is housed axially displaceable within housing  31 .  
         [0072]     The lens group is held on opposite ends by means of holding rings  34   a  and  34   b  which are bolted to a sleeve  36  surrounding the lens group. Sleeve  36 , in turn, is bolted to a sleeve-shaped cylinder  38  which is journalled axially displaceable within housing  31 . A sleeve-shaped piston  40  is provided between sleeve  36  and cylinder  38 . Piston  40  and cylinder  38 , together with seals  42   a  and  42   b  surround a cavity  44 . A fluid  46  is contained within cavity  44 . Fluid  46  may be filled into cavity  44  via an opening which may be closed by a closure bolt  48 . Piston  40  is bolted to housing  31 .  
         [0073]     Axial and circumferentially distributed pull springs  52  are provided between piston  40  and cylinder  38 .  
         [0074]     For the piston-and-cylinder unit  38 ,  49  the same holds true as already explained in connection with  FIG. 1 . The above discussed problem in connection with the lost motion caused by the seals is solved through the pull springs  52  because they bias piston  40 , and, thereby, overpower the lost motion or the hysteresis, respectively, by compression.  
         [0075]     In the case of a change of temperature, cylinder  38  together with the lens group move relative to piston  40  being rigidly connected to housing  31 .  
         [0076]     Starting from the system of  FIG. 2 , various advancements may be provided.  
         [0077]     According to a first advancement one takes into account that under unfavorable length conditions, in particular for very short lengths of guide, cylinder  38  and piston  40  may tilt relative to one another. In order to avoid that, cavity  44  may be subdivided into several, preferably three or four axial chambers by providing several circumferentially distributed and radially meshing axial ridges. When fluid  46  gets warmer, it expands simultaneously and uniformly within all such chambers, thereby generating a parallel movement and, hence, an axial guide without the risk of tilting.  
         [0078]     According to a second advancement an additional cavity is provided being connected to cavity  44 . The additional cavity may be positioned at an arbitrary location within system  30 . The larger volume, thus obtained results in a bigger expansion amount of the fluid, such that longer distances and/or higher forces of displacement may be obtained. The above calculation would, of course, have to be modified accordingly.