Prismatic Lighting Panel

Abstract

A 1/8-inch plastic lighting panel having recessed conical prisms on 1/4-inch centers. The cones are one-tenth inch deep.

Description

BACKGROUND OF THE INVENTION

This invention relates to plastic prismatic lighting panels used for controlling the distribution of light from a light source. Such panels are widely used in overhead fluorescent lighting fixtures. Their primary purpose is to reduce direct glare by controlling the angles at which light emerges from the panel.

The theory of prismatic panels is well known, and is discussed in McPhail U.S. Pat. No. 2,474,317. Briefly, light entering an upper plane surface of the panel is refracted downward through the lower surface of the panel or reflected upward through the upper surface by prismatic elements making up the lower surface. If the prismatic elements have straight sides which make the proper angle with the normal of the panel, virtually all of the light which would otherwise emerge at high angles relative to the normal of the panel is reflected by the prisms back through the upper surface of the prismatic panel. Therefore, the panel greatly reduces the amount of light emerging above a predetermined cutoff angle ("direct glare") and produces a relatively uniform illumination level within this cutoff angle. A properly designed and constructed plastic prismatic panel closely approaches the theoretical light control obtainable.

A particularly common prismatic panel is what is known in the art as a "1/8-inch" panel. A 1/8-inch prismatic panel is 0.120.+-.0.005 inches thick, and the term "1/8-inch panel" will be used herein to designate such a panel. 1/8-inch panels are widely used as lay-ins for suspended ceilings and as closures for lay-in fixtures used in suspended ceilings. The standard size of these lay-ins and closures is approximately 2 feet by 4 feet, and it has been found that a 1/8-inch panel is sufficiently rigid to be self-supporting over this span, particularly if the plastic is prestressed. A common prismatic pattern for 1/8-inch panels is one made of recessed cones on 3/16-inch centers. For convenience, a 1/8-inch panel having this pattern will be referred to hereinafter as "the usual" 1/8-inch panel, although other patterns, such as hexagonal prisms, are now used in 1/8-inch panels. This pattern is also used on 3/16-inch thick panels. Cones on more distant centers are used to provide bolder patterns on thicker panels, such as 0.200 -inch panels. Because of the criticality of the apex angle of the cones, the depth of the cones is dependent almost exclusively on the spacing on centers of the cones.

It has long been recognized as desirable to produce a panel which is lighter in weight than the usual 1/8-inch panel, yet has its qualities and its high strength-to-weight ratio. Attempts have been made to produce such a panel, but have resulted in panels having poor optical qualities and insufficient strength to prevent sagging in use and frequent breakage in handling.

One of the objects of this invention is to provide a prismatic lighting panel which is substantially lighter that the usual 1/8-inch lighting panel.

Another object is to provide such a panel which is comparable in strength to the usual 1/8-inch panel, which resists breakage in handling, and which is self-supporting in a 2 foot by 4 foot section.

Another object is to provide such a panel which possesses excellent optical characteristics.

Another object is to provide such a panel which is attractive in appearance, even in relatively small panel areas.

SUMMARY OF THE INVENTION

In accordance with this invention, generally stated, a prismatic panel is provided having a much larger prism size than has heretofore been felt possible in a 1/8-inch panel. A 1/8-inch prismatic lighting panel is provided having recessed conical prisms on 1/4-inch centers. The cones have a depth of 0.100 inches, leaving a nominal thickness of the panel above the apexes of the cones of 0.020 inches. In the preferred embodiment the lines of centers of the cones lie parallel to the edges of the panel. This panel has been found to be lighter than the usual 1/8-inch panel, physically strong and optically efficient. At least in part, the optical properties of the panel are attributable to the unusual contour of the nominally flat upper surface of the panel. Instead of having deeply dished concavities above the intersection points of the conical prisms, it has relatively broad, shallow furrows above the lines of intersection, and shallow concavities over the apexes of the conical prisms.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, FIG. 1 is a bottom plan view of one illustrative embodiment of panel of this invention;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 1;

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 1;

FIG. 5 is a sectional view corresponding to FIG. 2, of a prior art prismatic panel, thinner than a 1/8-inch panel, and having the pattern of the usual 1/8 -inch prismatic panel;

FIG. 6 is a somewhat diagrammatic top plan view of the panel shown in FIG. 1; and

FIG. 7 is a top plan view, corresponding to FIG. 6, of the prior art panel shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, reference numeral 1 indicates one illustrative embodiment of panel of this invention. The panel 1 is made of a transparent plastic, such as an acrylic or polystyrene material. The panel 1 includes a generally plane upper face 2 and a prismatic lower face 3.

The lower face 3 defines a pattern of intersecting recessed cones 5 having apexes 7 equally spaced across the lower surface 3 of the panel 1. The linear distance between adjacent apexes 7a-7b or 7b-7c is nominally one-quarter inch. This dimension should be held to within .+-.0.01 inches. The intersections of linearly adjacent cones 5 form straight edges 9 of square cells 11 on the lower face of the panel. The corners 13 at which the edges 9 meet represent points of tangency of diagonally adjacent cones, such as the cones having apexes 7a and 7c. The points 13 lie in a common plane and represent the lowermost points on the lower face.

The height of each apex 7 from the plane defined by the corners 13 is 0.100 inches .+-.0.005 inches. This leaves a thickness of about 0.020 inches above each apex 7. It will be seen that the apex angle of each recessed cone 5 is about 120.degree.. This is a larger angle than the angle of the usual 1/8-inch panel.

It has been found that the panel 1 possesses a number of unexpected properties. It is considerably lighter than the usual 1/8-inch panel. For example, the usual 1/8-inch panel, when made of polystyrene weighs about 7.3 ounces per square foot, whereas the panel 1 made of the same material weighs about 6.5 ounces per square foot. In acrylic, the usual 1/8-inch panel weighs about 8.1 ounces per square foot whereas the improved panel 1 weighs about 7.3 ounces per square foot. Because the cost of material represents a large part of the cost of prismatic panels, this reduction in weight is of immense importance.

It has also been found that both the strength and the optical properties of the improved panel 1 are comparable to those of the usual 1/8-inch panel, and are far superior to those of panels known heretofore having the same weight as the panel 1. The principal prior attempt to reduce the weight of the usual 1/8-inch panel has been to reduce its thickness. A panel of approximately the same weight as the improved panel 1 may be formed by this method by making the total thickness of the panel about 0.111 inches. Such panels have been made commercially. However, they have proved uniformly unsuitable for most purposes. Their light control is far inferior to the usual 1/8-inch panel, and they sag noticeably in 2 foot by 4 foot enclosures. This type of lighting panel 21 is shown in FIGS. 5 and 7.

The much greater strength-to-weight ratio of the improved panel 1 is due to the greater thickness for a given weight provided by its geometry.

The outstanding optical qualities of the panel 1 are less easily understood. In the production of prismatic panels, it has long been recognized that straight edges and sharp corners on the prisms are prerequisites to good light control. It has been known that degradation of the conical surfaces of the prisms of the usual 1/8-inch panel becomes progressively greater as the thickness of the panel decreases. The sides of the prisms become wavy, and the apexes and intersection lines and points of the cones become rounded. These changes in the prisms decrease their efficiency considerably. Overlooked heretofore has been the effect of deformation of the nominally plane upper face of the panel. When the usual 1/8-inch panel is cooled after being formed, it shrinks somewhat, particularly at points of mass concentration. Thus, dished depressions are formed in the upper face of the panel above the lowermost corners on the lower face. These concavities spread light refracted by the upper surface beyond the critical angle for a plane surface of the same material, and thereby cause a considerable amount of direct glare in accordance with the theory of prismatic panels. The concavities also increase internal reflection in the panel, thereby increasing high angle direct glare. As the thickness of the usual 1/8-inch panel is reduced to that shown in FIG. 5, the depth of these concavities increases greatly, as shown at 23 on the panel 21 of FIG. 5. It is believed that the increase in the depth of the concavities 23, hence their decreased radius of curvature, contributes significantly to the loss of light control in these panels. Furthermore, the apexes of the conical prisms "fall-in" when the usual 1/8-inch panel is reduced to the thickness of the panel 21. This causes a second set of depressions 25 in the upper face of the panel 21. These concavities 25 cause further scattering of incident light within the panel 21.

The concavities 23 and 25 of the panel 21 are noticeably absent from the improved panel 1. Even had depressions of the same depth as those of the panel 21 appeared, their effect would be less than those of the panel 21, because their radius of curvature would be considerably greater. However, it has been found that the upper face 2 of the panel 1 is practically smooth, and is in fact smoother than that of the usual 1/8-inch panel. Furthermore, the pattern of ripples on its face is quite different from that of the usual 1/8-inch panel or the panel 21. The upper face of the usual 1/8-inch panel is evenly marked solely by depressions having the same diameter as the depressions 23. As shown in FIG. 7, the upper face of the panel 21 is evenly marked by the large circular depressions 23 and the smaller circular depressions 25. As shown in FIG. 6, however, the upper face 2 of the panel 1 is marked by a grid of shallow furrows 15 directly above the sides 9 of the cell 11. It is also marked by shallow concavities 17 directly above the apexes 7 of its recessed prisms 5. The sides of the prisms 5 are likewise much flatter than those of the panel 21.

The geometry of the upper surface also unexpectedly complements the prismatic elements. The approximately 120.degree. apex angle of the cones 5 creates a somewhat greater cutoff angle than the approximately 116.degree. apex angle of the cones of the usual 1/8-inch panel. However, because the furrows 15 do not have as much of a light dispersing effect as the concavities 23 and because the concavities 17 are positioned to refract most of the light reflected internally from the prisms 15 rather than reflecting much of it internally, the problem of internal reflection, which is otherwise a major problem with prismatic panels having overly large apex angles, is reduced. The usefulness of the small concavities 17 in preventing high angle direct glare is largely attributable to their being much closer, vertically, to the apexes 7 of the cones 5 than is the upper surface of previously known panels.

The reasons for the greatly enhanced upper and lower faces of the panel 1 are not fully understood. As has been noted, because of the greater size of each cell 11, the same vertical shrinkage causes less degradation of both surfaces. This difference does not seem to explain the major part of the enhancement, however. In large part, the enhancement appears to stem from the geometry and proportions of the panel, that is, the distribution of material within the panel. This distribution, at least with the preferred production method, produces an apparent linear shrinkage along the edges of the cones 5 and along the upper face 2. This explanation is supported by the observation that the apex angles of the prisms 5, besides being sharper than those of the panel 21, increase in size somewhat more than would be expected beyond the angles of the mold prisms.

The panel of the present invention may be produced in a conventional manner by casting or extrusion. The preferred method is an extrusion process in which a cylindrical rotating mold impresses the prismatic pattern on semimolten material under heat and pressure. The cooling process is begun while the material is still in contact with the mold, and the pressure is maintained while the cooling begins. This method assures sharp reproduction of the prismatic surface and a strong, stress-free panel.

Numerous variations in the prismatic panel of this invention, within the scope of the appended claims, will occur to those skilled in the art in the light of the foregoing disclosure. Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

Claims

We claim:

1. A 1/8-inch plastic prismatic lighting panel comprising a generally plane upper face and a prismatic lower face, said lower face defining a plurality of intersecting recessed cones, the apexes of said cones being equally spaced from each other whereby the intersections of linearly adjacent cones form sides of square cells and diagonally adjacent cones are tangent to each other at lowermost points of said lower face, the distance between linearly adjacent apexes, hence between the sides of said cells, being substantially one-quarter inch, the vertical distance from each said apex to said upper face being substantially 0.02 inches, and the vertical distance from each said apex to said lowermost points being substantially 0.10 inches.

2. The lighting panel of claim 1 wherein said sides of said cells are parallel to the edges of said panel.

3. The lighting panel of claim 1 wherein said upper face defines shallow furrows directly above said sides of said cells, parallel with said sides, and said upper face also defines shallow circular depressions directly above said apexes.