Abstract:
An apparatus for illuminating a document includes an optical element formed of a light-transmissive material of a predetermined refractory index that defines a curved entry surface and an exit surface opposite the entry surface in which light enters the optical element via the entry surface and light exits the optical element via exit surface. A first substantially parabolic surface is defined between a first edge of the entry surface and first edge of the exit surface and a second substantially parabolic surface is defined between a second edge of the entry surface and a second edge of the exit surface such that the first and second substantially parabolic surface are spaced apart more at the exit surface than at the entry surface.

Description:
TECHNICAL FIELD 
   The present disclosure relates to illuminating apparatus used to illuminate hard-copy documents for digital recording, such as in digital scanners, facsimile machines and digital copiers. 
   BACKGROUND 
   In office equipment such as digital copiers and facsimile machines, original hard-copy documents are recorded as digital data using what can be generally called a “scanner.” In a typical scanner, a document sheet is illuminated and the light reflected from the document sheet is recorded by a photosensitive device such as a CCD or CMOS array, to be converted to digital image data. In one embodiment, a narrow strip of the document sheet is illuminated as the sheet is moved through a document handler, or the photosensitive device is moved relative to a platen on which the document sheet is placed. 
   Designing an illuminator for a scanner presents challenges in providing, among other aspects, an even illumination along the narrow strip of the document, as well as providing a suitable illumination profile across the narrow strip. Irregularities in the illumination level in the illuminated area will result in defects in the image data. An overview of the art of designing illuminators for scanners is given in U.S. Pat. No. 6,236,470. 
   One type of illuminator useful in document scanning includes a light-transmissive element that exploits internal reflections to direct light from one or more point sources to emerge in substantially parallel rays from an exit surface of the element toward a document. One known type of optical element, used in various contexts, includes a light-transmissive element that exploits internal reflections to direct light from one or more small sources to emerge in substantially parallel rays from an exit surface of the element. Specifically, one known shape for this purpose is a compound parabolic concentrator, or CPC; uses of the CPC are shown in U.S. Pat. No. 5,255,171 and US Patent Application Publication 2004/0131157. 
   SUMMARY 
   According to one aspect, there is provided an apparatus for illuminating a portion of a document to be recorded. An optical element comprises a light-transmissive material of a predetermined refractory index and defines a curved entry surface, and an exit surface opposite the entry surface. A first substantially parabolic surface is defined between a first edge of the entry surface and a first edge of the exit surface, and a second substantially parabolic surface is defined between a second edge of the entry surface and a second edge of the exit surface. 
   According to another aspect, here is provided an apparatus for illuminating a portion of a document to be recorded. An optical element comprises a light-transmissive material of a predetermined refractory index and defines an entry surface, and a curved exit surface opposite the entry surface. A first substantially parabolic surface is defined between a first edge of the entry surface and a first edge of the exit surface, and a second substantially parabolic surface is defined between a second edge of the entry surface and a second edge of the exit surface. 
   According to another aspect, here is provided an apparatus for illuminating a portion of a document to be recorded. An optical element comprises a light-transmissive material of a predetermined refractory index and defines an entry surface, and an exit surface opposite the entry surface. A first substantially parabolic surface is defined between a first edge of the entry surface and a first edge of the exit surface, and a second substantially parabolic surface is defined between a second edge of the entry surface and a second edge of the exit surface, the parabolic surfaces substantially defining a CPC, wherein the CPC defines a theoretical full length relating to the curvatures of the first substantially parabolic surface and the second substantially parabolic surface. The CPC has an actual length of about 0.5 to about 0.9 of the theoretical full length. 
   According to another aspect, here is provided an apparatus for illuminating a portion of a document to be recorded. An optical element comprises a light-transmissive material of a predetermined refractory index and defines an entry surface, and a curved exit surface opposite the entry surface. A first substantially parabolic surface is defined between a first edge of the entry surface and a first edge of the exit surface, and a second substantially parabolic surface is defined between a second edge of the entry surface and a second edge of the exit surface. The optical element defines a tilt angle in the exit surface thereof, the tilt angle being between five and fifteen degrees from perpendicular to the length of the optical element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified elevational view of a document scanner. 
       FIG. 2  is a sectional view of an optical element in isolation. 
       FIG. 3  is a simplified perspective view of an optical element in isolation. 
       FIG. 4  is a sectional view of an optical element in isolation. 
       FIG. 5  is a sectional view of an optical element in combination with a portion of a platen. 
       FIG. 6  is a sectional view of a pair of optical elements in combination with a portion of a platen. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a simplified elevational view of a document scanner. There is provided a platen  100 , which may have distinct parts, on which a document sheet can be placed for recording therefrom. Also associated with platen  100  is a document handler generally indicated as  102 , which can be of any design known in the art. The document handler sequentially feeds sheets from a multi-page original document past a scan head comprising an illuminator including an optical element  10  a linear array of light sources  20 , and a photosensitive device  30 . The illuminator illuminates a thin strip of the document while the photosensitive device  30 , which includes one or more linear arrays of photosensors, records the reflected light. (There is typically another lens, not shown, interposed between the platen  100  and the photosensitive device  30 .) The scan head can be mounted on a moveable carriage  40 , for recording light reflected from images on sheets placed on the main portion of platen  100 . The optical element is arranged at an angle relative to a surface of the document being recorded. 
     FIG. 2  is a sectional view of an optical element  10  in isolation. In one embodiment, the profile shown in  FIG. 2  is substantially the same along the entire page width of the optical element  10 , i.e., in the dimension coming out of the picture. The optical element  10  can be made of any light-transmissive material, such as glass or plastic, of a predetermined refractory index. 
   The optical element  10  defines an entry surface  12 ; an exit surface  14 ; and a first parabolic surface  16  and second parabolic surface  18 . In each case the more curved portion of each parabolic surface  16 ,  18  is disposed near the entry surface  12 , as shown. The exit surface  14  is the surface through which light is directed toward a document, such as shown in  FIG. 1 . In a practical embodiment, the width (the vertical dimension as shown in the Figure) of entry surface  12  is in a range of about 0.3-2.0 mm; the width of exit surface  14  is in a range of about 2-10 mm; and the length of the parabolic surfaces is in a range of about 10-30 mm. 
   The parabolic surfaces  16 ,  18  need be only generally parabolic in shape. In one practical embodiment, each parabolic surface  16 ,  18  can closely follow the shape of half a true parabola. This profile generally forms what is known as a compound parabolic contractor, or CPC. When the CPC is formed from a solid, light-transmissive member exploiting light refraction and total internal reflection, the CPC is typically known as a “dielectric compound parabolic contractor,” or DCPC, but it is conceivable that the optical effect of the CPC can alternatively be obtained with reflective surfaces. 
     FIG. 3  is a simplified perspective view of an optical element  10  (with the line marked  2 - 2  indicating the section shown in  FIG. 2 ). If the small, point-like light sources (such as  20  in  FIG. 1 ) are in the form of small LED&#39;s, each source  20  can be disposed within a dimple-shaped concavity in entry surface  12 , such as shown as  22 . Alternatively, the concave structure of entry surface  12  can be in the form of a concave channel extending the length of optical element  10 . The entry surface  12  can also be a convex surface. Also alternative, the linear array of point-like light sources could be replaced with a single extended light source, such as a fluorescent lamp, or a single LED extending substantially the length of optical element  10 . 
     FIGS. 4 ,  5 , and  6  show alternate embodiments, each having one or more CPC&#39;s of various designs. In the  FIG. 4  embodiment, the CPC, indicated as  40  but substantially substitutable for the CPC  10  in the above-described embodiment, is in the form of a truncated CPC; that is, the solid member forming CPC  40  includes curved surfaces consistent with a “full” CPC shape, but encompasses only a portion of a total theoretical length L of the CPC shape. 
   In order to calculate the theoretical length L of a DCPC, as well as determine the characteristics of the parabolic surfaces  16 ,  18 , the following equations can be used:
 
let a=exit surface ½ width;
 
a′=entrance surface ½ width;
 
n=refractive index of DCPC.
 
θ i ′=sin −1 ( a′/a )
 
theoretical “full” length  L =( a+a ′)/tan θ i ′.
 
(The parabolic surfaces of such a DCPC are defined as follows:
 
 f=a ′*(1+sinθ i ′)
 
R=2f
 
Δ y =±( a′−f *sin θ i ′)
 
Δ z=−f *cos θ i ′.)
 
   Once the theoretical full length L of a DCPC is determined, a portion of the theoretical full DCPC can be used. In one practical embodiment, the actual length L′, such as shown in  FIG. 4 , is between about 0.5 and about 0.9 of the theoretical full length L of the DCPC. When selecting what portion of the full DCPC to use, material from either the entrance or exit “ends” of the DCPC can be in effect “removed,” but there is more design latitude in removing material from the wider, exit end. 
   The  FIG. 5  embodiment shows a CPC  50 , once again substantially substitutable for the CPC  10  in the above-described embodiment, which defines a “tilt” in the exit surface  54  thereof which is about ten degrees, or more broadly between five and fifteen degrees, from perpendicular to a (theoretical or actual) length L of the CPC  50 . In one practical implementation, the DCPC  50 , having the ten-degree tilt in the exit surface  54 , is disposed at about 37 degrees relative to normal to the platen  100 . The specific tilt of the exit surface  54  and the angle with regard to the platen  100  may be determined with regard to overall illumination objectives of the apparatus, such as how large a band of a document is desired to be illuminated, or a desired profile of illumination across such a band. 
   The  FIG. 6  embodiment shows a pair of CPC&#39;s  60 , each once again substantially substitutable for the CPC  10  in the above-described embodiment. Each CPC  60  defines a relatively shallow radius, here shown as a concavity but which could alternatively be a convexity, in the exit surface  64  thereof. In one practical implementation, the concave radius is about 20 mm, each CPC is disposed at about 37 degrees relative to normal to the platen  100 , and the adjacent edges of the exit surfaces of the two CPC&#39;s are about 6.4 mm apart. The specific dimensions, angles, and other parameters of a practical implementation may be determined with regard to overall illumination objectives of the apparatus, such as how large a band of a document is desired to be illuminated, or a desired profile of illumination across such a band. 
   In practical implementations of any of the above-described embodiments, other considerations affecting desirable designs include a distance of the CPC to a target; a gap length between the light sources and the CPC; as well as specific characteristics of the LED&#39;s or other light sources, such as the angles of emission thereof. 
   Although the  FIG. 6  embodiment shows the use of multiple CPC&#39;s and accompanying sets of light sources, any of the above-described embodiments can be adapted for scanner with two or more CPC&#39;s directing light to substantially the same small area of a document to be illuminated. One common reason to use two CPC&#39;s with accompanying sets of light sources is to suppress shadows on the recorded image caused by paste-ups in the original document on the platen. 
   The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.