Abstract:
An optical recording unit having a compound lens which includes a lens stack. The lens stack encompasses a plurality of lenses, which are inset in a tubular housing. The lens stack includes a spacer ring which accommodates a sealing material or a sealing element and which is braced resiliently against one of the lenses in the tubular housing.

Description:
BACKGROUND INFORMATION 
   Camera modules used in motor vehicle operation are not only subject to high mechanical stresses, but to considerable temperature fluctuations as well, and to the effects of moisture. When a camera module is used that does not have a completely impervious design, under the climatic conditions typical of automotive applications, this can lead, on the one hand, to dew formation in the optical path and, on the other hand, to degradation of the image quality. Moisture condensation is often observed when a high relative air humidity has set in inside of the camera module for a length of time at elevated temperatures, and the camera module is then cooled within a short period of time. 
   Optical recording units constructed from a compound lens and a sensor chip are typically designed to be hermetically sealed. The ingress of moisture resulting from vapor permeability that may be excessive for many of the materials used, can be controlled by using absorbing agents, such as zeolite, for example. Besides gluing the front lens element of a lens stack in place, it is conceivable to use sealing rings, thus, for example, O-ring seals of various materials. This is a practical approach for achieving an impervious design. Compared to fixing in place by gluing, sealing rings made of elastomer material, for example, advantageously make a reconditioning possible. Moreover, when sealing rings are used, there is no risk of contamination. To reliably ensure the sealing action when a sealing ring is used, a defined compression of the sealing ring is produced by the engagement of the threaded connection. Because manufacturing variances are inherent in the structural heights within a lens stack which may include a plurality of lenses, a constant compression is not ensured for all pieces when a sealing ring is clamped between the frontmost lens surface and a contact mass, such as a ring-shaped area in the tubular housing of the compound lens. Moreover, it must be ensured that the sealing ring is not unevenly clamped, which can result, for example, from localized overstraining when the parts are screwed in place during assembly. To ensure the requisite image quality for compound lenses in the application cases mentioned at the outset, it is important to have as few deviations as possible in the positioning of the individual lenses. Thus, the individual lenses of a lens stack require appropriate guidance; the guidance of the individual lenses must not be adversely affected by skewed sealing rings. When a seal rests on a front surface, any encroachment on the available beam diameter is disadvantageous, since the edge thickness of the front lens cannot be arbitrarily reduced. 
   A waterproof camera is described in Japanese Patent Application No. JP 2002 090603. The waterproof camera is equipped with a camera housing which has a front part and a rear part. A lens stack is accommodated inside of the camera housing. Within the lens stack, a hollow space is formed behind the front lens. A lens suited for taking wide-angle shots is placed inside of this hollow space. The front lens of the lens stack is provided at the opening of the lens stack and, in addition, is used as a seal. A projection at the edge of the lens stack is caulked onto the lens side in a thermal joining process. An O-ring seal is provided between the outer periphery of the lens and the peripheral surface of an inner wall for the lens stack to seal off the annular gap therebetween. In addition, another O-ring seal is provided between the outer peripheral surface of the edge part of the lens stack and the inner peripheral surface of the edge part of the front camera housing, in order to seal the clearance space therebetween. 
   From U.S. Pat. No. 5,519,543, an optical system for an inspection device is known. A camera that can be lowered into a bore hole in the earth or into a pipe laid in the earth, includes a tubular body made of thermally insulating material. A front lens group and a rear lens group are disposed at mutually opposing ends of the tubular member. The mutually opposing lens groups at the front and rear ends are thermally isolated from one another, a vacuum chamber, which essentially extends along the optical pathway of the light, being formed in the tubular body. 
   SUMMARY OF THE INVENTION 
   The compound lens design according to the present invention for an optical recording device makes it possible for individual parts to be assembled cost-effectively, including a front lens that is to be inserted imperviously, even while allowing for the typical manufacturing tolerances inherent in the mechanical dimensions of the individual parts. The present invention provides for a spacer ring to be integrated in the lens stack in such a way that it is set in, between the front lens and the second lens positioned downstream thereof. Along with the spacer rings, a sealing ring is inserted, making it possible to achieve both a defined compression for sealing off two surfaces, as well as a precise setting of the nominal clearance between the lenses within the lens stack. 
   Following the design approach of the present invention, a defined compression is able to be applied by positioning a spacer ring, along with the sealing ring integrated in the same, underneath the front lens of a lens stack when twisting the front lens into place using a securing ring. The design approach of the present invention limits the influence of manufacturing variances on the compression and, to be precise, limits the influence of those tolerances inherent in the manufacturing of the spacer ring and of the tubular housing. However, the design approach of the present invention makes it possible to compensate for the thickness tolerances of each of the lenses and of the spacer ring, as well as for variations in the length of the tubular housing. 
   In the approach provided by the present invention for integrating a spacer ring which also forms a retainer for the sealing ring, the available beam diameter is not encroached on, since the edge thickness of the front lens cannot be arbitrarily reduced. 
   In the compound lens design according to the present invention, the sealing ring integrated in the spacer ring is not located directly underneath the securing ring to be twisted on, so that it is not subjected to any twisting strain or localized overstraining. The guidance or centering of the front lens is still ensured, as before, by the inner wall of the tubular housing. Moreover, a visual inspection may be made through the front lens to check the sealing ring and the spacer ring for correct positioning; handling during assembly is also substantially facilitated. 
   Instead of integrating an elastomer ring to be inserted into the spacer ring, a sealing material may alternatively be injection molded directly onto the metal ring. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the cross section through a lens stack of a compound lens according to the present invention. 
       FIG. 2  illustrates a representation of the sealing region on an enlarged scale. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a compound lens in whose tubular housing  1  a lens stack  7  is inset. Tubular housing  1  includes a threaded section  2 , in which a securing ring  17  is screwed into position. Securing ring  17  includes an external thread  18  which is complementary to threaded section  2  and which fixes a first lens  8  (front lens) of lens stack  7  in position in tubular housing  1 . Lens stack  7 , which is accommodated in tubular housing  1 , encompasses first lens  8  (front lens) already mentioned, a subjacent second lens  9 , another third lens  10 , as well as a fourth lens  11 . 
   Hermetically closing an aperture  29  is fourth lens  11 , which is inset at the bottom of tubular housing  1 . Located above fourth lens  11  is a diaphragm ring  4 , which may have a shaping  5  adapted to the particular optical application. On its upper side, diaphragm ring  4  has a plane face  26 , on which a third lens  10  rests flat. A second, convexly and concavely curved lens  9  rests on third lens  10 . Second lens  9  and third lens  10  are guided inside of a first receptacle  6  of tubular housing  1 , in such a way that they are centered by a first centering surface  27 . 
   In addition, tubular housing  1  has a second receptacle  19 . First lens  8  (front lens) is inset in second receptacle  19 , which is delimited by second centering surfaces  28 . Front lens  8  has a convex curvature  20 . In the peripheral regions of its convexly curved exterior side, first lens  8  is fixed in place by a hold-down edge  16  of securing ring  17 . In response to securing ring  17  being screwed down, first lens  8  (front lens) is preloaded by resilient tongues against a spacer ring  12  inset in a hollow space  25  between first lens  8  and the top side of second lens  9 . Spacer ring  12 , which is preferably fabricated from a metallic material, includes a toroidally extending upper part  13 . Upper ring part  13  has a first contact face  14  in which a sealing ring  24  is inset. Sealing ring  24  rests, on the one hand, against first contact face  14  of upper ring part  13 ; on the other hand, against second centering surface  28  of second receptacle  19  on the top side of spacer ring  12 . Spacer ring  12 , for its part, is braced by its bearing edge  23  against the convexly curved side of second lens  9 . 
   To complete this description, it should also be mentioned that the outside surface of tubular housing  1  is denoted by reference numeral  30 , and the aperture of the tubular housing in which fourth lens  11  is situated, is denoted by reference numeral  29 . 
   By employing the spacer ring between first lens  8  (front lens) and second lens  9  and inserting a sealing ring  12 , it is possible to achieve both a defined compression for sealing off two surfaces, as well as a precise setting of the nominal clearance between lenses  8 ,  9  of lens stack  7 . The clearance between first lens  8  and second lens  9  is precisely defined by spacer ring  12 . Lens stack  7  illustrated in  FIG. 1  is assembled from four lenses and a diaphragm ring, beginning with fourth lens  11  which is the first to be inserted in tubular housing  1 , the thickness tolerances of diaphragm ring  4 , of third lens  10 , of second lens  9 , and of first lens  8  are cumulated. If sealing ring  24  accommodated on spacer ring  12  were located between securing ring  17 , first lens  8  (front lens) and tubular housing  1 , the result would be a pronounced variation in the attainable compression of sealing ring  24 , due to the manufacturing variance inherent in the lens thicknesses in lens stack  7 . Because only small sealing ring cross sections are feasible, an impervious fitting of lenses  8 ,  9 ,  10  and  11  fabricated from glass is not ensured, given the manufacturing variance to be expected. 
   In the design according to the present invention, as illustrated in  FIG. 1 , the sealing ring is not located directly underneath securing ring  17  to be twisted on, so that the danger of a twisting strain or of localized overstraining is eliminated. The guidance or centering of first lens  8  (front lens) in second receptacle  19  is still ensured by second centering surface  28 . This permits a larger available beam diameter to pass through first lens  8  (front lens), thereby making it possible to reduce disadvantageous vignetting. The intensity in the image plane is typically not homogeneous, but rather decreases towards the peripheral region proportionally to cos 4  φ. In this context, φ denotes the field angle measured relative to the optical axis. If the path of rays in the compound lens is limited by another diaphragm in addition to aperture diaphragm (diaphragm ring  4 ), then the incident radiation intensity may fall off more appreciably toward the image edge than is given by the relation cos 4  φ. This additional shading toward the edge is termed vignetting, as mentioned above. By using securing ring  17  to fix first lens  8  (front lens) in position in tubular housing  1 , it is possible, through this first lens  8 , to visually control the correct position of both spacer ring  12 , as well as of sealing ring  24  in the tubular housing  1 . 
   In place of sealing ring  24  accommodated on spacer ring  12  in  FIG. 1 , a sealing material could also be extruded directly onto spacer ring  12  and, depending on the sealing operation, be used in place of sealing ring  24  shown in  FIG. 1 . 
   Spacer ring  12  illustrated in the representation according to  FIG. 1  rests, on the one hand, with its bearing edge  23  on the convexly curved top side of second lens  9  and, on the other hand, is braced against second centering surface  28  of second receptacle  19 . First lens  8  (front lens) rests, on the one hand, with its concavely curved top side on the top side of upper ring part  13  and, on the other hand, compresses the elastic material of sealing ring  24 . Since this elastic material is held by a cage-type retainer formed by the top side of spacer ring  12 , first contact face  14 , as well as second centering surface  28  of second receptacle  19 , the elastic material, whether it be a sealing ring  24  or extruded sealing material, is directly contacted in response to the concave rear side of first lens  8  (front lens) making contact, so that it forms a very effective seal, even when securing ring  17  is screwed on lightly. In this context, it is ensured that first lens  8  (front lens), as well as second lens  9 , third lens  10 , and also fourth lens  11  are centered by centering surfaces  27  and  28 , respectively, of first receptacle  6  and of second receptacle  19 . The design approach according to the present invention makes it possible to simultaneously seal off two surfaces, which are oriented at right angles to one another, in tubular housing  1 . The design approach according to the present invention makes it advantageously possible to ensure that no ambient air is able to penetrate through the gap between first lens  8  (front lens) and tubular housing  1  and, from there, into hollow space  25 . 
   The sealing region is shown on an enlarged scale in the representation according to  FIG. 2 . 
   It is inferable from the representation in the figure that spacer ring  12  rests with its bearing edge  23  on second lens  9 . For its part, second lens  9  is accommodated in tubular housing  1 . Upper ring part  13  of spacer ring  12  fixes sealing ring  24 , which, on the one hand, abuts on first contact face  14  of upper ring part  13  and, on the other hand, rests on the top side of spacer ring  12 . Both a first sealing surface  32 , as well as a second sealing surface  33 , which extend at right angles to one another, may be sealed off by sealing ring  24 . As a result of the deformation of sealing ring  24 , which is accommodated inside of the cage-type retainer of spacer ring  12 , both the gap between first lens  8  (front lens) and second centering surface  28  in tubular housing  1 , as well as the gap between the inside of first lens  8  (front lens) may be sealed off. It is thus ensured that no ambient air and no moisture are able to penetrate into hollow space  25  accommodating spacer ring  12  (compare representation according to  FIG. 1 ) along the outer edge of first lens  8  (front lens) and second centering surface  28  of tubular housing  1 .