US 3288990 A
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Nov. 29, 1966 L. G. STAHLHUT PANEL Filed Sept. 25, 1964 FIGB FIG2
gyra/2 Mex United States Patent 3,288,990 PANEL Leo G. Stahlhut, Kirkwood, Mo., assignor to K-S-H Plastics, Inc., Kirkwood, Mo., a corporation of Missouri Filed Sept. 25, 1964, Ser. No. 399,237 7 Claims. (Cl. 240-106) This invention relates to prismatic lighting panels of the type generally used in fluorescent light fixtures.
The advantages of the use of the prismatic panels in lighting fixtures are well known in the art. Prismatic panels accept light and bend it downwardly, by virtue of their refraction of the light, out of what is known as the direct glare zone. The direct glare zone encompasses the angular area from the horizontal to about S-45 below lthe horizontal from a panel mounted above eye level. The refracting of the light out of the direct glare zone not only ensures that persons walking or sitting are not likely to look directly into the glare of light from a fixture, but also concentrates the light downwardly where it is most needed.
This control of light out of the direct glare zone has been accomplished satisfactorily both by well made prismatic panels and by louvered panels. However, this has not resolved the entire problem of providing satisfactory lighting.
Prismatic panels can be excessively bright directly beneath them, causing a reflected glare problem, and lamp images are very apparent from below. The use of fewer lamps will provide lower brightness beneath the panel but the lamp images are still as bright, and Ithe use of fewer lamps per panel entails the use of more fixtures and panels to provide a given intensity of light.
White opalescent or translucent panels can also be used to reduce the brightness. Such panels substantially eliminate the reflected glare problem but restore the problem of the direct glare zone because the translucence or opalescence of the panels destroys their effectiveness as prismatic panels. That is to say, the diffusion of the light extends to the very surface of the panel along the prisms, so that the light is no longer directed downwardly by the prismatic configuration of the surface.
One -of the objects of this invention is to provide a lighting panel with low brightness, indistinct lamp images, and excellent light control out of the direct glare zone, as compared with lighting panels now in use.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.
In accordance with this invention, generally stated, a lighting panel is provided which is made up of a transparent prismatic plate, with light-controlling prisms on the light-emergent side and a planar surface on the lightreceiving side, and, on the planar side, a light diffusing film spaced slightly from the planar surface of the plate through a major portion of its area and preferably bonded in patterned lareas to that surface. The film can be pigmented, coated with a pigmented layer of material which may `but need not necessarily be a bonding agent, printed, frosted or colored.
In the preferred embodiment, the diffusing film is bonded to the prismatic plate in a regular grid of narrow lines which not only bonds the film over the entire reach of the plate but provides an attractive pattern, since the brightness along the bond lines is greater than in the areas bounded by those lines. The absolute area of the bond lines is so small relative to the total area, as not appreciably to decrease the effectiveness of the panel.
The resultant lighting panel directs the light out of the zone of direct glare, reduces the -brightness directly beneath the panel and obscures the lamp images, thus pro- 3,288,990' Patented Nov. 29, 1966 viding the advantages of the various different types of lighting panels known heretofore, while largely eliminating their several disadvantages.
While the theory of its operation does not form a part of this invention, it is postulated that light from a source such as a fiuorescent tube, is first scattered internally of the diffusing film, by the particles of pigment, by which the excessive brightness and lamp image are largely eliminated, so that `a nearly uniform distribution of light is provided between the film and the transmitting plate. This light is accepted from each point of its emergence from the diffusing film as if the film were not there, i.e. as if the light were arriving from the primary source. Accordingly, the light entering the transparent plate is refracted by the prisms on the light-emergent side of the panel in the same way in which it would have been had the light impinged directly from the primary source upon the plane side of the transparent panel.
This theory is supported by an observation of the superiority of the panel of this invention over a panel in which the upper surface of a transparent prismatic plate is frosted or roughened, in which a layer of pigment is incorporated into the upper surface or in which a pigmented film is bonded directly to the plate over its entire area. Under these circumstances, the light entering the homogeneous medium of the transparent plate is scattered in every direction, instead of being confined to a cone. shaped sector, as is light which impinges on the fiat, optically clear surface of a transparent prismatic panel the planar surface of which has not :been frosted, roughened, or pigmented. When the light is thus scattered, the prismatic elements are not able properly to control the emergent light.
A discussion of the use of prisms to produce the desired cut-off angle, i.e. the lower limit of the direct glare zone which it is desired to maintain, and illustrations of the application of the standard optical formulae to be applied to accomplish the desired result, are set out in McPhail, United States Patent No. 2,474,317.
In the drawing:
FIGURE l is a somewhat schematic view of a lighting fixture containing one 'embodiment of panel of this invention;
FIGURE 2 is a bottom plan view of a fixture containing the panel shown in FIGURE 1 and a conventional prismatic panel;
FIGURE 3 is an enlarged sectional view taken along the line 3 3 of FIGURE 2;
FIGURE 4 is a fragmentary sectional view corresponding to the view shown in FIGURE 3, showing another embodiment of panel of this invention; rand FIGURE 5 is a fragmentary sectional view corresponding to the view shown in FIGURE 3, showing still another embodiment of pan'el of this invention.
Referring now to the drawing for one illustrative embodiment of lighting panel of this invention, reference numeral 1 indicates a completed panel of this invention, which, in FIGURE 1, is shown as mounted in a troffer 2 in a ceiling 3. Within the troffer 2, and between a reflecting surface of the troffer and the upper side of the lpanel 1, are fluorescent lighting tubes 4. FIGURE 1 merely illustrates the control of the light by the panel 1, which directs the light downwardly out of the direct glare zone.
In FIGURE 2, a panel 1 is represented as occupying half the length of a louver, with a conventional prismatic transparent panel 101 occupying the other half. Lamp images 102 are shown in much the way in which they normally appear through the conventional fixture when viewed directly i.e. in plan, and as they appear through the panel 1, where they are dintinguishaible only by an area of slightly decreased brightness midway between the fluorescent tubes.
Referring now to FIGURE 3, the panel 1 is shown 4as 'being made up -of a polystyrene transparent plate 10, with a planar surface 11 and a prismatic surface 12. A diffusing film overlies the planer face 11 and is bonded to it at regular, spaced intervals :along narrow bond lines 22. The bond lines 22 form 4a grid 23, as shown particularly in FIGURE 2. In this embodiment, the film 20 is bonded by means of a clear adhesive 24, and the film itself is pigmented with suicient minute particles 21 to disperse light yentering the film 20, but not enough to interfere seriously with the transmission of light through lthe film.
An air space 25 is present within the span between successive bond lines. The film, in practice, may appear to the unaided eye t-o be in contact w-ith the plate throughout the overlain area, but this is not the case. Unless there is a discontinuity (air space 25), the panel is little improved over the ordinary -prismatic panel.
The film 20 is preferably la .polyvinyl fluoride film, which may be of the type sold under the trademark Tedlar by the E. I. du Pont de Nemours & Company. Suitable means of bonding the film to the transparent plate are described in a ico-pending application of Edelmann and Hawes, Serial No. 370,887. The film is preferably on the order of .0005 to .005 inch thick. Much thinner film cannot produce sufiicient diffusion; much thicker film tends to absorb too much light.
Suitable pigment for producing diffusion of light, without interfering impractically with its transmission, is well known to the art. It can take the form of platelets of very finely divided silica, pigment grade zinc oxide and the like. The incorp-oration into the hlm of an ultraviolet screening agent will help protect the film, pigment, and plate, from the effec-ts of ultraviolet radiation from a light source `on the film side of the panel.
It can be -seen that other materials such Ias acrylics, polyvinyl chloride, or polyethylene can be rused for the transparent plate, as well as for the film.
The patterned bonding can, of course, take any desired form, such as small islands for dots, various configurations of continuons lines andthe like. If a pigmented adhesive is used .as a coating, the bonding can be accomplished by selective heating, pressure, Iresist or activating chemical means. The film can also be frosted, colored by dye or by colored pigment, or coated with a pigmented coating which is not adhesive -as respects the prismatic plate. It can be seen that coatings and patterns yon a thin film lend themselves to printing techni-ques as well as other means of application.
In FIGURE 4 Aa 'coating 45 is provided on the inside surface of a :film 40. The coating has pigment particles 41 in it, 'and in the embodiment shown is adhesive in character. Bonding areas 48 are indicated as having been defined by heat and pressure, as indicated by tthe shallow `depressions 49 in the film 40.
The side of the plate to which the diffusing film is applied has Ibeen described as planar. While this is the preferred form, it can be seen that ribs or bosses may be provided on the surface to which the film is bonded so as to provide patterned spacing members to the top of which the lfilm m-ay lbe bonded. Such `an `arrangement is illustrated in FIGURE 5, in which a film 50 is shown,
for illustration, with ya pigmented coating 55 on its outer side, and is bon-ded, on its inner side, to bosses 57 on the 'upper surface of a transparent plate 51.
It can be seen that the prismatic plates can be made of .any other transparent materials which are now oommonly in use, or which may be developed, since the criteria for their use are optical and not chemical.
Numerous other variati-ons in the construction of lighting panels of this invention, within the scope of the appended claims, will become apparent to those skilled in the art in the light of the lforegoing disclosure.
Having thus zdescribed the invention, what is claimed and desired to be secured by Letters Patent is:
1. A lighting panel comprising a transparent prismatic plate with one prismatic surface and an opposite smooth planar surface and la diffusing film overlying said planar surface, bonded thereto in spaced bon-ded areas and spaced therefrom intermedi-ate said bonded areas, said bonded areas `constituting a minor part of the total area of the plate over-lain by said film.
2. The lighting panel -Of claim 1 wherein the film is a pigmented polyvinyl fluo-ride.
3. The lighting panel of claim 1 wherein the film is on the vorder of .0005 to .005 inch thick.
4. A lighting panel comprising .a transparent prismatic plate with a prismatic surface and an opposite surface and a diffusing film overlying said opposite surface, bonded thereto in spaced bonded areas .and spaced therefrom intermediate said bonded areas, said bonded areas consituting a minor part of the total area of the plate overlain 'by said diffusing film,
5. A lighting panel comprising a transparent prismatic plate with a prismatic surface .and an opposite surface, a diffusing film overlying said opposite surface and a pigmented coating on said film, said film being bonded to the said opposite surface in spaced bonded `areas and spaced therefrom intermediate said bonded areas, said bonded areas constituting a minor part `of the total area of the plate overlain by said diffusing film.
y6. A lighting panel comprising a transparent prism-atic plate with a prismatic surface and an opposite surface, 4a diffusing film overlying said opposite surface and a pigmented coating on the side of the iilm immediately adjacent the said plate, said coating, hence said film, being bonded to the said opposite surface in spaced bonded .areas and spaced therefrom intermediate said Ibon-ded areas, said bonded areas constituting a minor gint ofthe total area of the plate overlain by said diffusing 7. The lighting panel of claim 6 wherein the pigmented coating is an adhesive bondable in the discrete bonding areas to said plate.
References Cited by the Examiner UNITED STATES PATENTS 3,069,974 12/1962 Marks etal 88-65 3,222,515 12/1965 Orr 240-106 FOREIGN PATENTS 1,067,322 10/1959 Germany.
NORTON ANSHER, Primary Examiner'.
C. R. RHODES, Assistant Examiner.