Abstract:
A lens has a lens body ( 1 ), reinforcing extensions ( 3   a   , 3   b ) and a clear aperture ( 9 ) surrounded on top and bottom by lens body. The lens is suitable for spot scanning. The lens is made of a water absorbing material, such as most polymers, particularly acrylate polymers. Aluminum sheets as vapor barriers are attached on each side of the lens body. This results in excellent resistance to change or distortion in high humidity environments.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to lenses of configuration for light scanning through the lens, such as f-theta lens, the lenses being of material which changes or distorts the optical function with increases in humidity. This invention renders such lenses resistant to such changes or distortions.  
         BACKGROUND OF THE INVENTION  
         [0002]    Materials forming lenses that change or distort with increases of humidity are typically plastics, especially acrylic plastics. Some hydrocarbon plastics resist water intake and are used in special situations for lenses, but the material are expensive and are difficult to mold to form the lens.  
           [0003]    The prior art has addressed this problem in U.S. Pat. No. 5,409,360 to Nakanishi, but the solution presented in that patent is one which significantly restricts the characteristics and form of the lens since it is based on the premise that a lens fully permeated with water does not exhibit major distortion. Accordingly, the better lenses in accordance with this patent would be tall and thin.  
           [0004]    Lenses of this invention may differ little from ordinary form for scan lenses, but have vapor barriers applied to render the lens highly resistant to increases in humidity.  
         DISCLOSURE OF THE INVENTION  
         [0005]    A lens of this invention has a long dimension, an optical transmission dimension perpendicular to the long dimension, and a height dimension perpendicular to both the long dimension and the optical transmission dimension. The lens has a clear aperture along the long dimension of at lease 35 millimeters (mm), the clear aperture having a height that is less than 32 mm. The clear aperture is surrounded top and bottom by lens body having height on each side of at least 1.5 mm.  
           [0006]    On the top and bottom of this lens is a layer of a vapor barrier, which may be a sheet of metal or other imperious material attached by adhesive. Such lenses are very stable optically with changes in humidity up to almost saturation humidity. Where a lens is thin in the light transmitting direction, the barrier might be omitted at such thin points, as the significant optical distortion does not occur where the lens is thin enough to become fully saturated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The details of this invention will be described in connection with the accompanying drawings, in which  
         [0008]    [0008]FIG. 1 is a perspective view showing the lens with vapor barrier;  
         [0009]    [0009]FIG. 2 is a is a second perspective view from a higher angle than FIG. 1 showing vapor barrier sheet spaced away for purposes of illustration,  
         [0010]    [0010]FIG. 3. a top view looking down on the light transmission plane showing a cross section of the lens through the clear aperture;  
         [0011]    [0011]FIG. 4 is a side view from the optical transmission path before application of a vapor barrier;  
         [0012]    [0012]FIG. 5 is a bottom view looking up on the light transmission plane before application of a vapor barrier;  
         [0013]    [0013]FIG. 6 is a front view from the optical transmission path showing in dotted outline the clear aperture region before application of a vapor barrier;  
         [0014]    [0014]FIG. 7 is a side view of FIG. 5 from the right side,  
         [0015]    [0015]FIG. 8 is a sectioned view through the center of the lens, also of FIG. 5 from the right;  
         [0016]    [0016]FIG. 9 is a plot of data from a representative application without the vapor barrier of this invention; and  
         [0017]    [0017]FIG. 10 is a plot of data from the same representation application as that of the FIG. 9 data with the vapor barrier of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The lens of this invention is shown illustratively in FIG. 1. It is characterized by being relatively long and narrow. It has an optical body  1  and planar extensions  3   a  (FIG. 2) and  3   b , which provide added rigidity. Extensions  3   a  and  3   b  are molded integrally with optical body  1  for molding efficiency and for immediate, strong contact with body  1 . The lens has a top barrier layer  5   a  and a bottom barrier layer  5   b , attached by adhesive in this embodiment.  
         [0019]    As illustrated in FIG. 2, barrier layers  5   a  and  5   b  are substantially coextensive with the surface of extensions  3   a  and  3   b  respectively, and extensions  3   a  and  3   b  have portions that cover optical body  1 . Accordingly, the top and bottom surfaces in the height dimension of optical body  1  are covered by barrier layers  5   a  and  5   b  in the complete lens, such as shown in FIG. 1.  
         [0020]    [0020]FIG. 3 illustrates, by cross section through the lens, the optical body  1  and the lower planar extension  3   b . Although this invention is understood to be generally applicable to the type of lenses used for scanning, for purposes of this illustrative embodiment, the outer surface  1   a  of body  1  has a circular radius of 235 mm and the inner surface  1   b  of body  1  has a circular radius of 382 mm. With respect to FIG. 3 the length dimension of the lens is the horizontal dimension (the x direction in the insert), and the light transmission dimension is the vertical dimension (the y direction in the insert). The linear, horizontal dimension of the curved lens part is 152 mm.  
         [0021]    Non-curved, rectangular end parts  1   c  and  1   cc  are for position referencing and other support of the lens.  
         [0022]    The major dimensions of FIG. 3 are generally to scale. It is apparent that the optical body  1  is relatively thick throughout its length, although the thickness increases significantly between the middle and thinner ends. Such a lens has positive optical power. A diverging or negative lens would be thinner in the middle than at the ends. This invention applies to all scan lenses in general.  
         [0023]    [0023]FIG. 4 is a back, side view with respect to FIG. 3 but not sectioned and without the barrier layers. Supporting parts  1   c  and  1   cc  are rectangular and extend across the lens. Part of the outer surface of parts  1   c  and  1   cc  may be a reference or critical dimension from which other part of the lens are to be located. Such referencing is standard and therefore will not be further discussed.  
         [0024]    Element  7  is a gate for injection molding. It may be removed from the final lens if desired.  
         [0025]    The insert in FIG. 4 is to define the long dimension of the lens as x and the height dimension of the lens as z. The major dimensions of FIGS.  3  though  5  are generally to scale.  
         [0026]    [0026]FIG. 5 is a bottom view with respect to FIG. 3. It illustrates bottom extension  3   b  as having a center, rectangular small locating extension  3   bb  for locating the lens during use. Extension  3   bb  is not shown in other views for purposes of clarity and because alternative elements to locate the lens are entirely consistent with this invention. Similarly, FIG. 5 shows small bumps  1   d  and  1   dd  on supporting parts  1   c  and  1   cc  respectively. These are alternative locating elements. Such locating elements are standard and therefore will not be further discussed.  
         [0027]    [0027]FIG. 6 is a view toward the lens as light exits the lens. It is a front side view with respect to FIG. 3, but not sectioned and without the barrier layers. Two, spaced positioning bumps  1   d  and  1   dd  are on opposite ends.  
         [0028]    Shown in dotted outline in FIG. 6 is the optical aperture  9 , which is the part of the lens used for accurate optical scanning. In ordinary use, light might only be directed to the optical aperture  9 , since that is the part of the lens to be used for focusing. In this embodiment, the optical aperture is generally centered in the lens, both laterally and in height, is 145 mm in the long direction and 5 mm in the height direction. The entire height of the lens, exclusive of the vapor barriers, is 14 mm.  
         [0029]    Accordingly, in this embodiment the linear length of the optical aperture is almost 30 times the height of the optical aperture. Similarly, in this embodiment the height of the lens body is almost three times the height of the optical aperture. However, it is understood that lens body as little as 1.5 mm in the height dimension on each side of the optical aperture is effective, although preferably that dimension should be at least 2.5 mm on each side. Material near the outside of the mold exhibits optical distortion and is therefore unsuitable for use as part of the optical aperture.  
         [0030]    [0030]FIG. 7 is a side view of FIG. 5 and FIG. 8 is also a side view with respect to FIG. 5, but sectioned in the center of the lens. They are generally to scale and therefore illustrate the overall dimensions. Surface  1   a  is shown as concave. Specifically, it may be a true toric. In other lenses it might be convex, and the variations in curvature along the length define the focusing of the lens. Similarly, surface  1   b  is shown concave and is typically toric or quasi toric. Although defining the surfaces of a lens is specialized and complex, with respect to this invention it may be standard and will not be further discussed.  
         [0031]    Barrier layers  5   a  and  5   b  in this embodiment are 0.178 mm thick aluminum foil with acrylic adhesive on the surface which contacts the surfaces  3   a  and  3   b  respectively. The remainder of the lens as described, which are lens body  1 , extensions  3   a  and  3   b , end parts  1   c  and  1   cc , location extension  3   bb , and bumps  3   d  and  3   dd  are a single integral member formed of polymethyl methacrylate by simple injection molding or by injection molding with pressure assist, as is standard in the molding art. Barrier layers  5   a  and  5   b  are applied by simply bringing the adhesive side of the barrier into direct contact with the surfaces  3   a  and  3   b  respectively.  
         [0032]    A typical use for the lens of this invention is part of a light spot scanning system on a photoconductive surface, as illustrated in FIG. 10 of the foregoing U.S. Pat. No. 5,408,360. A small spot of light is moved across the clear aperture to apply a small spot of light sequentially across the photoconductive surface.  
         [0033]    Lenses of this invention may be excellent f-theta lenses. Such lenses are well known in the art. They are used to direct a spot of light across a light sensitive surface with the light moving linearly with the angle (hence the reference to theta, a symbol commonly used for angle). Without the adjustment by the focusing of an f-theta lens, the line would move in accordance with the cosine of the angle.  
         [0034]    Since the internal sides of surfaces  3   a  and  3   b  not over body  1  are not covered with a vapor barrier, the essential advantage is understood to result from the top and bottom of body  1  having a vapor barrier, coupled with the optical aperture being within body  1  and surrounded in the height dimension on both top and bottom by body having height of at least 1.5 mm. Similarly, where body  1  is thin in the optical transmission direction in certain lenses, the barrier layer may not be necessary over such locations as significant optical distortion is not normally realized where a lens becomes saturated across it full width in the optical transmission direction.  
         [0035]    Alternative barrier layers can include any treatment or application, which is or becomes impervious to water. This would include paints and possibly surface treatments by oxidation.  
         [0036]    With respect to FIG. 9 and FIG. 10 data was obtained from an optical system employing this invention originally at ambient temperature and humidity and then subjected to 60 degrees C. and 90 percent relative humidity. The spot size was measured both at the location representative of the photoconductive drum where the spot was focused at the ambient conditions and at 1 mm equally spaced locations both plus and minus from that location. At ambient conditions the spot size measured becomes larger as the measurement point departs from the location representative of drum location.  
         [0037]    At high humidity conditions the spot size at the photoconductor (drum plane  0 ) increased due to focal shift about 70 percent in size (about 70 um to about 120 um). This data resulted in curves shown in FIG. 9. As shown in FIG. 9, the day 13 and day 15 results of the 15 day test show and improvement, believed to be the result of the lens approaching saturation. Where a vapor barrier was used, the results varied very little over a similar 15-day test, as shown in FIG. 10, thus clearly showing the value of this invention. (the curves are so close together that they could not be meaningfully shown with their identifying symbol. The curves are very close together at the 0 drum plane.) Accordingly, this invention is believed to have a wide applicability and variability in form, particularly with respect to how the vapor barriers are formed.