CA1040466A - Low-glare overhead projector - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An overhead projector is disclosed which has in optical alignment a light source, a transparent surface on which a transparency is laid for pro-jection, and echelon lens structure located between the light source and the surface, and a projection lens for focusing an image of the transparency onto a projection screen. The echelon lens structure comprises at least two linear echelon lenses which are oriented so that their cylinder axes are crossed at an angle greater than 0° and so that the lens structure achieves focusing of an image of the light source, whereby a user of the projector is not disturbed by "propeller"-type concentrated glare emanating from the lens structure.

Description

A particularly useful overhead projector, which is generally des-cribed in United States patent No. 3,126,786, comprises in optical alignment a transparent horizontal state on which transparencies are laid, a source of intense illumination below the stage, an echelon lens structure comprising two annular echelon lenses disposed between the stage and the source of illumination, and a projection lens located above the stage. me echelon lens structure functions as a condensing lens, while the pro~ection lens functions to form an image on a projection screen of the stage and of any transparency laid thereon. Usually a person who is using the projector stands at the side or rear of the projector so that he or she can convenient-ly change transparencies, point to significant portions of a transparency, or make marks on a transparency.
A problem with overhead projectors as described is that a region of concentrated glare is visible to the user when he or she looks at the trans-, parency or uncovered stage. This glare arises because a small portion of the light transmitted by the echelon lens structure is not controlled.
According to the present invention there is provided an overheadprojector comprising in optical alignment a light source, a transparent sur-1face on which a transparency is laid for projection, an echelon lens structure located between the light source and said surface, and a projection lens for focusing an image of said transparency onto a projection screen, characterized in that said echelon lens structure comprises at least two linear echelon lenses which are oriented so that their cylinder axes are crossed at an angle greater than 0 and so that the lens structure achieves focusing of an image of the light source, whereby a user of the projector is not disturbed by "propeller"-type concentrated glare emanating from the lens structure.
In the accompanying drawings:
Figure 1 is a fragmentary cross sectional schematic view of an annular echelon lens structure taken along a plane through the lens, which plane includes the optic axis of the lens;
Figure 2 is a top schematic view of a projector stage;
Figure 3 is a schematic representation of a single linear echelon : ~ .
.. ~ -- 1 ---~040466 lens and a light source and a screen;
Figure 4 shows two orthogonally oriented linear echelon lenses, with a light source and a screen;
Figure 5 is a perspective view of an overhead proJector of the invention;
Figure 6 is a sectional view of the overhead projector shown in Figure 5 taken along the line 6-6 of Figure 5.
Figure 7 is a fragmentary perspective view, partially in section, of the stage and lens structure of the overhead projector of Figure 5.
Figure 8 is a top schematic view of a pair of linear echelon lenses;
Figures 9 and 10 are top schematic views of a structure comprising ; four linear echelon lenses;
Figure 11 is a top schematic view of a projector stage with the lens structure comprising a three element linear echelon lens structure.
Figure 12 is an enlarged exploded schematic perspective sectional view of the stage and lens structure of Figure 11;
Figures 13 and 15 are top schematic views of different lens struc-tures comprising six lens elements;
Figure 14 is an enlarged exploded schematic perspective sectional view of an alternative lens structure having six linear echelon lens surfaces;
Figure 16 is an enlarged schematic cross-sectional view of a further embodiment of a lens structure for the invention.
Figure 1 illustrates the path of controlled and uncontrolled light rays. Each annular increment or ridge 13 of the annular echelon lenses com-prises a particonical working surface 14, a blind riser 15, and outer and inner edges, 16 and 17, at which the working surface and adjacent risers are joined. A ray of light 18 which is not reflected and does not strike the riser is said to be controlled, and the working surfaces are configured to refract all controlled light rays toward a substantially common focus and through the projection lens.
Examples of uncontrolled light rays in the plane illustrated in Figure 1 are identified as light rays 20, 21, and 22. Light rays 20 and 21 .~ ~ - 2 -are uncontrolled because they are reflected rather than refracted at inter-faces between the air and the lens material. Light ray 22 is uncontrolled because it is diffracted at the edge 17. Due to the relatively small size of the source of illumination used in an overhead projector, all the uncontroll-ed (as well as controlled) light rays tend to be nearly coplanar with the optical axis of the echelon lenses ll and 12 in the infinite number of planes extending radially from the optic axis. mus, uncontrolled light rays lie in planes extending radially in all directions from the optic axis of the lenses 11 and 12.
When a user of the projector looks at the stage 24 of the projector or at a transparency on the stage, the user's eye "E" receives a light glare emanating from a propeller-shaped area 28. No matter where the user stands around the stage, the user's eye will be in a plane with the optic axis and will see the glare resulting from the uncontrolled light.
Figures 3 and 4 illustrate how the alleviation of disturbing glare is achieved. Figure 3 is a schematic representation of a single linear echelon lens 29, having parallel linear ridges and grooves instead of the annular ridges and grooves of Fresnel lenses previously used in overhead pro-~ectors, and having a cylinder axis 30 extending parallel to the ridges and through the optic center of the lens. The lens 29 is capable of focusing light from a point source 31 to a "line" focus 32. If a screen 33 were placed parallel to the lens, uncontrolled light rays emanating from the sur-face of lens 29, as exemplified by light ray 34, would form a band of light 35 along the screen. me band 35 would have a width equal to the length of the line focus 32 and would extend laterally to either side of the line focus 32.
Figure 4 shows two orthogonally oriented linear echelon lenses 37 and 38 positioned in parallel and aligned with a source of light 39 to focus light gathered from said source to the focus 40 which is the image of the light source 39. If a screen 41 were placed parallel to the lenses, an 30 observer would see two bright strips 42 and 43 formed by the uncontrolled light. Each of the strips 42 and 43 is caused by a different one of the lenses 37 and 38, and the strip caused by each lens is narrowed because the ~40466 other lens acts to actually focus the uncontrolled rays. Each strip is found to be approximately as wide as the corresponding dimension of the focus 40 on the plane of paper 41. Some uncontrolled light rays fall within the quadrants between the strips 42 and 43, but this light is not intense enough to be dis-turbing to a user viewing the lenses or a stage of a projector from the normal position E. Typically, in a preferred overhead projector of the invention, the strips are approximately 4 centimeters wide at the focal plane or plane of paper 41. So long as the user is in a normal position and is looking at the stage from a position between the symmetry or cylinder axes of the lenses, such as axes 44 and 45, and not in a direction perpendicular to an axis of a lens element, a substantially reduced amount of glare is received from the echelon lens structure when viewing the stage or a transparency on the stage.
Preferably a projector of the invention includes at least four separate linear echelon lenses, each lens comprising a smooth planar surface and a surface formed with parallel ridges and grooves. The four lenses are generally oriented in pairs so that the axis of one lens of a pair is sub-stantially perpendicular to the axis of the other lens of the pair, and each pair is oriented so that the axes of one pair lie at an angle of between 5 and 45 degrees relative to the nearest axes of the other pair. However, linear echelon lenses can also be arranged in other arrangements according to the invention, such as triplets in which the axis of each lens forms an angle of about 60 to the axes of the other lenses.
~ The use of crossed cylinder lenses is known to achieve the general -I effect of a single spherical or aspherical lens; see the publication by I. Pitman of London, titled "Technical Optics" by Louis E. Martin, P. 312 et seq.. Further, crossed linear echelon lenses have previously been used; see ; Cooper, United States Letters Patent No. 3,580,661 issued May 25, 1971, and also Maloff, United States Letters Patent No. 2,726,573 issued December 13, 1955, where rear projection screens are described.
However, none of these prior art references suggests that the glare problem associated with the annular echelon lenses in overhead projectors can be reduced by the use of crossed linear echelon lenses. Instead, the projec-~"~t~ _ 4 _ :` -~`~ 104~)466 tor art has tended to ignore the problern, or to use means of lirnited effec-: tiveness, such as shields between the stage and the user's eyes or other approaches to overcome the glare problem, approaches which are rnore expensive and which reduce the arnount of light transmitted through the condensing lens structure.
The illustrative overhead projector of the invention 45 shown in Figure 5 includes a base 46 and a projection lens 47, which is supported above the base on a rod 48. As shown in Figure 6, the base 46 includes side walls 50, a bottom wall 51, a projection lamp 52, a transparency supporting surface or stage 53, and a linear echelon lens structure 54. As also shown in Figure 6, the projection lens 47, comrnonly known as the "pro~ection head,"
includes a pair of sirnilar simple positive meniscus lenses, 55, 56, , ~ .

';;-, :':

~ - 5 -and a mlrror 57.
The linear echelon lens structure 54, which is shown best in Figure 7, includes linear echelon lenses 59 and 60 supported in a frame member 61 between flanges 62 and 63. The frame member 61 also supports the stage 53 on a flange 64. In general the linear echelon lenses will be held together in one air-tight sealed structure to prevent dust from entering between the lenses. Often, the indivi-dual echelon lenses will be provided with planar borders around their edges, and these borders will be adhered together, as e.g. by solvent sealing by thermal fusion or other suitable techniques. While the lenses S9 and 60 each cylinder have a rectangular outline, the/axes of the lenses are not necessarily parallel to the edges of the lenses.
me structure shown in Figure 7 is useful, but a preferred linear echelon lens structure includes more than two linear echelon lenses to minimize aberration problems that are inherent with crossed cylinder lenses of rela-tively large aperture. Figures 8 and 9 illustrate the effect of crossed-cylinder aberration and the improvement in crossed-cylinder aberration achieved by the use of four lenses respectively. Figure 8 is a top view of the two-lens structure 54 pictured in Figure 7, having a cylinder axes 74 and 75. Light passing through the lens structure 54 ~ithin the area generally defined by the four-lobéd curve 72 is focused to a relatively small area within an image circle 73 (on the focal plane located a suitable distance above the lens structure). Light passing through the corners of the lens structure outside of curve 72 is bent somewhat too much, causing it to image outside of circle 73 but within the four-lobed curve 76. For example, a ray of light passing through the lens structure at point 77 intersects the image plane at point 78. If corrective measures are not taken, the image generally defined by curve 76 may be too large to pass unobstructed through the projec-tion lens, causing portions of the projected image of the stage to appear colored or dark.
Figure 9 illustrates the use of an alternative lens structure which includes two pairs of crossed linear echelon lenses positioned to alleviate crossed-cylinder aberration.
The cylinder axis of each lens of a pair -- 80 and 81, and 82 and 83, respectively -- is at substantially right angles to the other axis of the pair. But the cylinder axes of the lenses of one pair form an angle theta (e) to the nearest axes of the lenses of the other pair, thereby causing areas of relatively high refracting power to be positioned over areas of relatively low refracting power.
The light rays that are brought to a substantially common focus by such an arrangement pass through the lens structure within a curve that may be represented by the curve 84.
Curve 84 circumscribes a larger area than the curve 72 in Figure 8, is not 80 deeply lobed, and hence affords a larger usable condensing area 85 from similar lens sheets.
The angle e is preferably at least 5 degrees to minimize Mbire'patterns and at least 10 degrees to obtain a significant increase in the extent of the useful area 85.
The optimum angle for increasing the useful area 85 and to minimize Moire patterns would be 54. However, the angle 45 has a disadvantage in that the low-glare regions are t~40466 smaller than desirable. Angle ~ may be optimized to produce an acceptable range as illustrated in Figure 10, which figure is a top view like Figure 9 of a four element linear echelon lens structure. A small light source (not shown) is located below the structure. Light from the source passes through the lens structure and is brought to a focus within an area indicated by circle 86 on a spaced plane 87. The uncontrolled light impinges on plane 87 within four strips 88, 89, 90, and 91 of somewhat different width, but approximately as wide as the image circle 86.
Because the user's eyes are typically at a height above the lens struature corresponding approximately to the position of the image plane 87 and image circle 86, the angles gamma (~) and omega (~) define regions, hereinafter referred to as low-glare regions, within which the user' 8 eyes are not in the path of a significant amount of uncontrolled light.
To obtain a sufficiently large useful area of the lens structure, to reduce moire patterns, and to obtain sufficiently large low-glare regions, the angle ~ is optimized between 25 and 35. The orientation of the cylinder axes of the lenses relative to the exterior outline of the lens structure may be altered without changing ~ in order to obtain suitable orientation of the low-glare regions.
Figures 11 and 12 illustrate a three-lens condensing lens of the invention. The three cylinder axes 96, 97, and 98 are positioned to intersect at angles of substantially 60 when the projector stage is viewed-from above as in Figure 11. Looking at the lenses, in an '~'`'' _g_ io40:s66 exploded sectional view as in Figure 12, the structure comprises a cover sheet 99 which may be used as the stage and this cover sheet may be formed of glass, polymethyl methacrylate or polycarbonate. It is secured around its edges to the border of the upper lens 100, which lens is oriented with its cylinder axis 96 perpendicular to the front and rear edges of the projector stage. Lens 101 has a border and is sealed between lens 100 and a third lens 102. Lenses 101 and 102 have their respective cylinder axes 97 and 98 rotated with respect to each other and to the axis 96. This triplet lens structure has been found to be capable of achieving a focusing effect similar to that achieved by annular echelon lenses.
Pigures 13 and 14 show a lens structure useful in the invention having a pair of triplet lenses. Six ridged surfaces are formed on five sheets 110, 111, 112, 113, and ,, 114; sheet 112 has linear ridges on each surface, and each side of the sheet is regarded as a lens, herein. Such a :
structure reduces the number of refractive surfaces in the lenses and thereby reduces the losses due to reflection at the interfaces. The 8iX ridged surfaces have cylinder axes represented as axes 104, 105, 106, 107, 108, and 109. The axes 104, 105, and 106 of one triplet are each at 60 to one another; the axes 107, 108, and 109 of the second triplet are at 60 to one another; and axes of the two triplets are separated at an angle beta (~) of about 10.
Low-glare regions of 50 are thus obtained at the front corners of the stage.
In the lens structure of Figures 13 and 14, the smooth flat upper surface of the lens 110 may be utilized ~' 9 ~04~466 ~s the stage for the projector. To protect the polymeric material from which the lens element 110 may be formed, a transparent hard coat material llS may be applied to the smooth upper surface of the lens 110. This hard coat material may be, for example, a silicon backbone fluoro-polymer such as sold by E. I. dePont deNemours of Wilmington, Delaware under the trade dk~4yn~tion "Abcite."
Figure 15 discloses a six-lens lens structure suitable for use in the pre~ent invention, comprising three pairs of linear echelon lenses. Within each pair, the cylinder axes of the lenses are perpendicular to one another. me axes 117 and 118 of one pair are at an angle of 10 to 15 from the axes 121 and 122 of the second pair.
The third pair has its axes 119 and 120 10 to 15 from the axes 121 and 122 of the second pair. With the pairs of lenses oriented as described, the low-glare regions on the projector will have an angular extent of 60 to 70.
Figure 16 discloses a preferred embodiment of a condensing lens structure appropriate for the present invention. mis structure utilizes two pairs of linear echelon lenses with the pairs combined to provide a cylinder axis orientation similar to the orientation illustrated in Figure 10. In this structure however the upper lens 125 is provided with the ridges and grooves in a central recess 126, thus providing with a mating lens 127 having a similar recess 128 a cavity therebetween. In this cavity, formed upon the joining of the borders of the lenses 125 and 127, :"
may be disposed thin lenses 129 and 130 which form the other two lenses of the structure. The lenses are preferably formed with ridges and grooves such that lenses 125 and ~i'' ~o ,~_ 1~40466 129 form one optical pair and lenses 127 and 130 form the other optical pair of the lens structure. In this structure the ridged echelon surface of the upper pair of lenses 125 and 129 are disposed toward the source of light and the echelon surfaces of the lenses 127 and 130 are on the sides of the sheets away from the source.
In the lens structures useful in the present invention, the particular placement of the lenses with respect to each other and the light source have some preferred orientation. It is however not an inflexible orientation and the manner in which the lens elements of a four-element lens structure are positioned may vary. For example, if a four-lens structure comprises lenses 1, 2, 3, and 4 with 1 and 2 comprising one pair of orthogonal lenses and 3 and 4 comprising the second pair, then the lenses may be stacked in various orders, such as 1, 2, 3, 4, or, 1, 3,

2, 4. The position of the cylinder axis however does not change when the order of the lenses is changed.
As an example of practice of the invention, a useful projector was prepared including a condensing lens structure comprising four 4-millimeter-thick sheets of polymethyl methacrylate, each provided on one of its faces with ridged linear echelon lens increments. The cylinder axes of the four lenses were arranged substantially as shown in Figure 10, the angle ~ measuring 30 and the angles A and B on each side of the center line of the stage measuring 15. The lenses were identical to each other, being an analog of an approximately F 1.1 plano-convex standard cylinder lens. The ridges were at a : frequency of 40 per centimeter. The lower lenses had the ." ~i ridges on the upper side away from the light source, the upper two lenses had the ridged surfaces facing the light source, i.e., face downward, and the upper surface of the upper lens served as a stage, thereby eliminating the need for a separate sheet of material to serve as a stage.
The linear echelon lonses aro generally made by embossing, pressing or molding a synthetic polymeric material such as polymethyl methacrylate. Other u~eful polymeric materials are cellulose acetate butyrate and polycarbonate.
me protective coating or sheet of rigid material on the mooth upper surface of the upper oloment is most important in the structures wh-re the len~es are formed of polycethyl methacrylate. Also, as shown in Figure 16, the lens structure may have a sheet 131 of gla8B or optically clear polymeric material, such a~ clear polycarbonate, adhered to the smooth upper ~urf~ce of the len~ 125. ~hi~
sheet 131 provides a rigid support for the lens structure and a hard surface upon which to place transparencies without danger of marring or scratching the surface.

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Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An overhead projector comprising in optical alignment a light source, a transparent surface on which a transparency is laid for projection, an echelon lens structure located between the light source and said surface, and a projection lens for focusing an image of said transparency onto a projection screen, characterized in that said echelon lens structure comprises at least two linear echelon lenses which are oriented so that their cylinder axes are crossed at an angle greater than 0° and so that the lens structure achieves focusing of an image of the light source, whereby a user of the projector is not disturbed by "propeller"-type concentrated glare emanating from the lens structure.

2. A projector according to claim 1 in which the linear echelon lens structure comprises two linear echelon lenses oriented so that the cylinder axis of one lens lies at an angle of between 85° and 90° to the cylinder axis of the other lens.

3. A projector according to claim 1 in which the linear echelon lens structure comprises three linear echelon lenses oriented so that the cylinder axis of any lens lies at an angle of between 55° and 65° to the cylinder axes of the other two lenses.

4. A projector according to claim 1 in which the linear echelon lens structure comprises four linear echelon lenses oriented in two pairs so that the cylinder axis of one lens of a pair lies at an angle of between 85° and 90° to the cylinder axis of the other lens of the pair, and the pairs are oriented relative to each other so that the smallest angle between their respective axes is between 5° and 45°.

5. A projector according to claim 1 in which the linear echelon lens structure comprises six linear echelon lenses oriented in pairs so that the cylinder axis of one lens of a pair is at an angle of between 85° and 90° to the cylinder axis of the other lens of the pair, the pairs being oriented relative to each other so that the smallest angles of the cylinder axes of one pair relative to the cylinder axis of the other pairs are between 5° and 20°.

6. A projector according to claim 1 in which the linear echelon lens structure comprises six linear echelon lenses oriented in triplets so that the cylinder axis of any lens of a triplet lies at an angle of between 55° and 65° relative to the cylinder axes of the other lenses of the triplet, and the angles between the cylinder axes of one triplet and the cylinder of the other triplet are between 5° and 10°.

7. A projector according to any of claims 2-6 in which two of the echelon lenses of the linear echelon lens structure are part of an integral sheet, the echelon lens ridges of one lens being on one surface of the sheet and the echelon lens ridges of the other lens being on the opposite surface of the sheet.

8. A projector according to claim 1 in which an even number of echelon lenses are included in the linear echelon lens structure, and half of those lenses are oriented with their ridged surfaces positioned toward said surface on which transparencies are laid and half are oriented with their ridged surfaces positioned away from said surface.

9. A projector according to claim 8 wherein the half of the lenses that are oriented with their ridged surfaces toward said surface on which transparencies are laid are the lenses that are most removed from the surface.

10. A projector according to claim 4 wherein the smallest angle between the respective axes of the two pairs of lenses is between 25° and 35° and the smallest angle is bisected by a center line of said surface on which transparencies are laid.

11. A projector according to any of claims 1-3 wherein a sheet of glass or clear polymeric material is bonded to a non-ridged surface of one of the echelon lens.

12. A projector according to claim 1 wherein a non-ridged surface of the echelon lens located farthest from the light source faces away from said source and serves as the surface on which transparencies are laid for projection.

13. A projector according to claim 12 wherein a sheet of glass or clear polymeric material is bonded to the smooth surface of said farthest lens.