|Publication number||US2115178 A|
|Publication date||Apr 26, 1938|
|Filing date||Jul 28, 1934|
|Priority date||Jul 28, 1934|
|Publication number||US 2115178 A, US 2115178A, US-A-2115178, US2115178 A, US2115178A|
|Inventors||Thomas W Rolph|
|Original Assignee||Holophane Co Inc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (16), Classifications (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
April 1938. "r. w. ROLPH LUMINAIRE HAVING MEANS FOR REDUCING THE 2 Sheets-Sheet l APPARENT SIZE OF THE LIGHT SOURCE Filed July 28, 1934 jPriZArz INVENTOR. Thomas WRoZph BY A TTORNEY.
\x \\;I \Q
I X a 6 v K .-..-W t y y 26, 1938. T. w. RQLPH 2,115,178 LUMINAIRE HAVING MEANS FQR- REDUCING THE APFARENT SIZE OF THE LIGHT SOURCE Filed July 28, 1934 r 2 Sheets-Sheet 2 lNVENT0R' Thomas Mg Ralph ATTORNEK I Patented Apr. 26, 1938 UNITED STATES 2,115,178 I I LUMINAIR-E HAVING MEANS FOR REDUCING THE SOURCE APPARENT SIZE OF THE LIGHT Thomas W. Rolph, Newark, Ohio, assignor Holophane Company, Inc., New York, N. Y., a corporation of Delaware Application July as, 1934, Serial No. 737,375
15 Claims. (01. 24o 1os) This invention relates to luminaires having means for reducing the apparent size of the light source whereby one can obtain an emitted beam of light having a lesser degree of spread than would be obtained with the usual refractors or reflectors. The invention is'applicable to luminaires from which a considerable concentration of light is-e'xpected. With such luminaires, the application of this method of refracting the light will produce more concentrated beams and sharper cut-back from the center of the beam than could otherwise be obtained. In effect, the apparent size of the light-source is reduced.
According to the present invention the light is caused to pass through a succession of refracting elements so disposed that the refraction at the inner (orincident) surface of each refracting element is greater than that at the outer surface of the corresponding element. This brings about a cumulation of the condensing effect of each retracting element.
In light projectors or luminaires in which a beam of light is produced, the minimum spread of the light beam is ordinarily dependent upon the size of the light source and the distance of the light redirecting medium from the source. With past equipment designed for maximum concentration of light, these two factors have been, for practical purposes, the only two affecting the degree of spread obtained from the beam.
In general, the size of .the light source is determined by the characteristics of .the type of lamp used. Lamp designers, in certain cases, try to obtain the most concentrated source possible but there are certain limits beyond which they cannot go and even to go to these limits mas involve sacrifices in efliciency, life or other characteristics of the lamp. In the case of electric incandescent filament lamps, sources which are quite small can be obtained, but even these are not as small as sometimes is desired. In the case of electric incandescent vapor lamps, the .size of the light giving body is necessarily much larger than it is in fllamentlamps. The increased size of the light source greatly increases .the -divergence of light beyond what it would be with concentrated sources, and hence the same concentration is not ordinarily available. The present invention, by providing a new arrangement of light controlling means, overcomes the objectionable spread of light from these large sources and brings it into angles comparable with those where more concentrated light sources are employed. I Q
The accompanying drawings show, for purposes of illustrating the present invention, several embodiments in which the invention may take form,
it being understood that the drawings are illustrative of the invention rather than limiting the same In these drawings:
Fig. 1 shows alight source with two reflectors of difierent size illustrating the efiect of size of reflector upon spread of emerging light beams;
Fig. 2 shows a light source with two refractors of different size illustrating the efiect of size of refractor upon spread of emerging light beams;
Fig. 3 shows, in cross-section, a vapor-tube light source with elements of a reiractor, illustrating the reduction of spread when the direction of the beam is altered;
Fig. 4 is a fragmentary cross-sectional view of a luminaire for a horizontal tubular light source with retracting side plates employing the refracting elements of Figure 3 to reduce the apparent size of the source;
Fig. 5 illustrates a projector designed to reduce the spread of the emitted beam;
Fig. 6 shows a construction similar to that illustrated in Fig. 3 but with the two retracting elements spaced a substantial distance apart; and
Fig. 7 shows how the spread of the emitted light may be reduced even when there is no net change in the direction of the light passing through the refracting elements.
It has been stated above that the degree of concentration obtained is partially a function of the distance of the light controlling medium from the light-source. This is illustrated in Fig. 1, which shows a lamp I with light-source 2, small reflector 3 and larger reflector 4. Light ray 5 strikes thesmall reflector at point 6 and is reflected as shown. Ifv the small reflector were not used, the same light ray which may then be called 1 would strike the large reflector at point 8 and be reflected as shown. The direction of the reflected light is the same in the two cases since the reflectors are alike except for size and the slope at 8 is the same as at 6. With the small reflector, the angle included between the point of reflection 6 and the extremities of the light source 9 and ID is 10 in the example illustrated. Therefore, the spread of the reflected light is also 10. In the case of the large reflector, it is ob vious from the construction that the angle of light included between the point of reflection 8 and the extremities of the light-source 9 and i0 is smaller than in the case of the small reflector. Actually this angle is 6 in the figure shown. Therefore, the spread of the reflected light from a given point on the reflector is only 6 with the large reflector as compared with 10 for the corresponding point on the small reflector.
The action is similar in the case of refraction of light. Fig.2 shows a lamp I I with light source l2 and small refracto-r i 3 and large refractor l4. 7
Light ray l5 strikes the small refractor l3. at point i6 and is refracted into the direction indicated. If the small refractor were not used, the large refractor would intercept the same light ray now numbered I! at point 18 and this light '9 ray would be refracted in direction shown. This] 1 direction is the same as refracted ray 15 from the small re-fractor, the two refractors being identical in design. The angle of light included from point Hi to the extremities of the filament is in this particular illustration Therefore, with ordinary methods of refraction, the spread of the light beam emerging from point [6 is also 10 or 5 each side of the light ray [5 shown emerging from the small refractor. In the case of the large refractor the spread of light from point 18 to the extremities of the light source is less than in the case of the small refractor. This is obvious from. the geometrical construction in the figure. The actual spread in this case is 6. Hence, the light emerging from point l8 on the large refractor will have a spread of 6 or 3 each side of the central emerging light my ll. This shows how the increase in size of a re-v fractor reduces the spread of the emerging light beam.
However, it is expensive and often impossible to make the size of a reflector or a refractorfor concentrated light sources so large that the spread of the emerging beam is as small as de-. sired. The difliculty increases enormously with large sources such as vapor tubes. Instead .of occupying an angle of less than 10, (which is easily obtained with concentrated sources) it may occupy angles in the neighborhood-of 30, or more. If ordinary light control is employed, the minimum spread of the emergent beam would be the same high angle, resulting in poor concentration and absence of sharp cut-oiI.
In Fig. 3 the light source I9 is large in size representing in'cross-section a tube of incandescent vapor. A section of a refractor toredirect the light from this source is shown at 20 and 2|, 2i) being an inner piece and 2| an outer piece. A typical point 22 on the inside of the inner piece receives light from the center-of the source represented by light ray 23 and from the outer edges of the source represented by light rays 24 and 25. In the particular figure shown the spread of each of the extreme light rays from the central light .ray is Since light ray 25 strikes the surface a at 22 at a greater-angle of incidence than light ray 23, it'will receive greater deviation in the refracting medium'than light ray 23. Similarly light ray 23 will receive greater deviation than light ray 24. Therefore, the three light rays in the glass will be brought closer together and the spread of 15 each side-of the central light ray will be reduced. In the arrangement illustrated, the spread is reduced to 8.1 above the light ray 23 and 9.3 belowthe light ray 23, or a total spread of 1'7.4 in the glass 29. In order to obtain the maximum advantage from this reduction in spread of light,'the outer surface 2% of the inner refracting piece is given a slope which will produce as little refraction as is possible. The slope selected is such that central light ray 23 when emerging from the outer surface of the inner piece will suffer no refraction at all. Light rays 24 and 25 are refracted slightly away from light ray 23 but this refraction is not as large as that obtained at the point 22 on theinner surface and therefore, the net spread of the light emerging from the inner piece of glass is less than when it entered the glass. In the arrangement shown, the spread of light in the air between the two pieces of glass is 12.3" above the central light ray and 14 below the central light ray or a total of 26.3".
This process is now continued with the outer piece of glass or refracting medium. The angle at which the light strikes the surface 21a of this outer piece is such that a considerable degree of refractionis obtained for the central light ray 23. The limiting light rays are also refracted, being obviously refracted more than 23, since the angleof incidence is greater for light ray 25 than for light ray .23. Similarly light ray 24 is refracted. less than light ray 23 because its angle of incidence is less than the angle of in.- cidence of light ray 23. Consequently within the medium 2|, the spread of light is still further reduced and in the case illustrated, it becomes 4.6" above light ray 23 and 6.9" below light ray 23, or-a-total of 11.5. The outer surface 25b of this medium is placed normal to the emerging light ray 23, Consequently, there is no refraction as this ray emerges from the medium. There is inevitably some spreading action for the two extreme rays 24 and 25 but this-spreading action is much less'than the condensing action at the inner surface because the angles of incidence aremuch lower. As a result, the spread of .the emerging light is much less than the spread of the entering light. In the case shown, the actual spread of .light ray 25 above light ray 23 is 7 and the actual spread of light ray 24 below light ray 23 is 10.5". Therefore, this particular combinationof refracting prisms has reduced the spread of light from the original source from to 175. This amounts to a reduction in the apparent size of the light source To obtainthe same angle of spread by former structures would mean a great increase in distance of refractor from source-or a great increase in size of refractorsimilar to that shown in Fig. 2.
It is not necessary to have the central emerging light ray always leave the outer surface at the normal to that surface. Some useful redirection of light will often be desired at the outer surface of one or both pieces. This will lessen the condensing action but some condensation will be .obtained','aslong as the refraction at the inner surface.
Reduced spread of" the emerging light has several definite advantages. It increases the concentration of light and this increase in concentration is'addedto whatever concentration may be obtained by the lenticular effect of the prism de- It also'makes possible what is known as a sharper'cut-oflz In many lighting problems, it is desirable to -have the luminaire give this sharp cut-off in order to project as much light as possible in 'a given direction with as little light as possible in a direction a few degrees away from the first. For example, in street lighting, the spread of lightup and down the street must be at high angles because of the considerable distance between luminaires; at the same time, if
surface ofeach piece is greater than at the outer the beams of light are directed at too high an angle, they enter the eyes of the users of the street and cause annoying glare. It is desirable to direct the beams at a high angle and to have the cut-off sharp so that a few degrees above the beam angle the candlepower has dropped to a low figure which will not produce-such glare.
Fig. 4 is an illustration of the application of this principle to design for-a street lighting luminaire. In this figure, 26 represents in crosssection a tubular light source of appreciable. size. 21 and 28 represent side plates each-comprising two pieces of glass or. other retracting medium provided with prisms as illustrated in Figure 3. The result is that a high degree'ot concentration is obtained at the particular angle desired. In this case, the angle of maximum candlepower is 75 to nadir or 15 below the horizontal. :From the discussion of Fig. 3, it will be obvious that the concentration of light obtained at 75:will be considerably greater with this systemot refraction than with systems ordinarily used. It will further be obvious that-the candlepower will decrease very rapidly above 75 becoming negligible at the angles close to the horizontal which are likely to be directed toward the eyes of users of the street. Fig. shows a projector designed'to obtain a single circular beam of light with reduced spread.
In this case, an incandescent filament lamp 32 is shown with light source 33 in the form of a small straight filament. Even though the lamp manufacturer may make thefilament as small as possible, there is some size to it and this size tends to give a spread to the emerging beam. However, by means of the condensing action of the refractors described herein, this spread can be greatly reduced. In the structure shown in Fig. 5, three successive retractors are used, 34, 35, and 3B. A light ray 3? near the extreme edge of the structure is retracted successively by the three retracting pieces to emerge in the beam direction. The retraction in each piece, however, is obtained at the surface at which the light enters. The surface at which the light leaves the article is set to be normal to the direction of the emerging light ray. There is, therefore, no retraction of light ray 3! at the surface at which'it emerges from each piece of glass. The extreme light rays 38 and 39 from the edges of the filament striking the same point as light ray 3'! on the inside of the inner piece are retracted as described under Fig.
3 and when emerging from the final piece of glass are spread from light ray 3'! to a much lesser degree than when st1'iking,the inner surface of the inside piece 34. With'light ray 40, the refraction is less than with light ray3! because 40,
on emerging from the light source, is nearer to the direction of the final beam otlightfl l lence, the condensing efiect of this system of refraction is less with the light from the source striking the inner piece at the point at which light ray 4B particular type of design, it is very valuable to have a light source in the form of a short line coinciding with the axis of the system? With this type of light source, the condensing action is greatest where it is needed the most, represented in the figure by light ray 31. It is lesser in degree where it is needed less, represented by light ray 40 and least of all where it is not needed, represented by light ray 4|. The general scheme of a projector utilizing this system of refraction may, however, be applied to any light source.
This type of projector may advantageously be supplemented by a spherical reflector 42. This will return the light rays striking it back through the source to be acted upon by the retracting pieces in the same way as the light striking them directly. 1 l
Since several retractions are frequently necessary to get a desirable condensation of light, it is possible to turn light into the desired direction even though it emerges from the light source at a large angle away from this direction. In Figure 5, a-large angle of light is included by the retracting system and when this is further supplemented. by the spherical reflector, an optical system of very high efficiency is obtained considered from the standpoint of the amount of the original light acted upon.
t will be understood that the invention is not confined to the use of two or three successive retracting plates or articles. Any number may be used. Two or three will probably satisfy most requirements but certain problems requiring the most extreme concentration may require more than three successive retracting articles.
Fig. 6 illustrates a retracting arrangement similar to-that shown in Fig. 3 but with the two retracting elements spaced apart a distance greater than is necessary for mechanical clearance. It is sometimes desirable to use a construction of this character and it frequently carries with it certain optical advantages. In this figure, 48 is a light source of considerable size. A light ray from the center of the source is shown at 49 striking the point 50 on the inner retracting element.- A light ray 5| proceeding from the center of the sourceat a higher angle than 49 strikes the inner retracting element at a point above 50. Similarly a light ray 52 also proceeding from the center of the-source in a direction lower than ray 49 strikes a point on the inner retracting element lower than point 50. *In a construction of this kind, it is common to have a large amount of the light which emerges from the structure concentrated at the same angle. In street lighting, for example, this angle usually is 15 below the horizontal. Therefore, light ray 5| must receive a greater degree of retraction than light ray 49. Similarly light ray 52 will receive a lesser degree of refraction than light ray 49. Consequently the prisms will grow progressively deeper from bottom to top of the structure. This prismatic change maybe in both pieces or in only one piece, but in the present instance, it is desirable to make as much progressive change as can be conveniently made in the prisms of the outer prismatic piece. Therefore, light ray 49 striking the outer piece at point 53 receivesa certain degree'ot deviation at this point whereas light ray 5| striking the outer piece at point 54 above 53 mustreceive a greater deviation and light ray 52 striking the outer piece at point 55 must receive a lesser degree .of deviation. The prism at 54, therefore, gives greater refraction than the prism at 53 and the prism at 53 gives greater refraction than the prism at 55. Considering ow the light rays 55 and 51 proceeding from the outer limits of the light source and striking point 50, it will be clear that light ray 56 striking the outer retracting piece at point 55 suffers less refraction at this point than light ray 49 striking the outer piece at point 53. This tends to reduce the spread between light rays 49 and 56. Similarly light ray 5'! striking the inner piece at point'58 strikes the outer piece at point 54 and suffers a greater degree of refraction than light ray 49. Thus, the greater the space between the two refracting elements, the greater will be the additional condensing effect obtained by the progressive change in prisms on the outer piece. 1
- It is not necessary to have the inner and outer pieces parallel. Sometimes, for mechanical or optical reasons, the two pieces will slope or curve away from each other, coming close together at certain points and being far apart at others. In any part of the structure where the spacing between the two parts exceeds that necessary for mechanical clearance advantage may be taken of this additional condensation obtained by the progressive change in the prisms. Frequently it is possible to use this to obtain additional condensation of light where it is most needed and yet retain normal spacing between the two pieces wherever that is mechanically advantageous.
In Fig. 5 it was notedthat the direction of light ray 4! was not changed by the retracting system. No condensing effect was obtained by the refracting system at this point. It is possible, however, to utilize this scheme of refraction and obtain a condensing effect at an angle at which no redirection is to be obtained. This is illustrated in Fig. '7. A light source 44 emits light ray 45 which is to go through the refracting system without any net. change of angle. However, the inner surface of the first refracting piece is set to give a considerable degree of refraction to light ray 45. The outer surface of this inner piece is set to give little or no refraction to light ray 45. Then the inner surface of the second piece is set to refract light ray 45 back to its original direction and the outer surface of the outer piece is set to allow this light ray to emerge without refraction. Therefore, the light ray from the center of the light source is refracted by the inner piece away from desired direction and is refracted by the outer piece back to the desired direction. Light rays 46 and 47 represent the light rays striking the inner piece at the same point as 45, but coming from the extreme limits of the light source. These undergo the same refracting effeet as illustrated in Fig. 3 and emerge from the inner refracting piece at a lesser angle of spread from the central ray than when they enter. In passing through the outer medium, the same condensing eflect occurs even though the direction of refraction is opposite from that obtained with the inner piece. Consequently the light rays 45 and 41 emerge from the refracting system at angles from the'central ray 45 which are less than the corresponding angles from 45 on the entering side, yet light ray 45 has suffered no not change in direction. I
' Thus, this system of refraction for obtaining a condensing effect may be utilized by means of refraction in the same direction from two successive pieces of'glass or refraction in opposite directions from the. two successive pieces. In either case, the desired effect is obtained.
I It will be understood that this system of refraction may be utilized in luminaires of varied character. Whenever concentration of light is desired, this system is a possible means of ob-' taining 21 better effect than would otherwise be obtained. The system is not limited to any particular type of light source or'to any particular fleld'of lighting or to any character of luminaire.
It is obvious that the invention may be embodied in many forms and constructions within the scope of the claims, and I wish it to be understood that the particular forms shown are but a few of the many forms. Various modifications and changes being possible, I do not otherwise limit myself in any way with respect thereto What'is claimed is: i
1.- A device for concentrating light rays from a source of substantial size, comprising a succession of refra'cting elements traversed by said rays, 9. point in the inner surface of the first element receiving a beam of convergent incident light at oblique angles and transmitting the light through said first element at a reduced angle of divergence,:the--outer surface of said first element being substantially normal to the axis of the light beam-in :the refracting medium so as to transmit the light to the air with substantially less divergence than the convergence of the original light rays, the inner surface of the second element receiving said divergent light and refractively transmitting it at further reduction in angle of divergence to the outer surface thereof, said second outer surface being substantially normal to the axis of the light beam in the second refracting medium and transmitting the light to the air with still less divergence than it had between the refracting elements, whereby the angle of spread of the beam is reduced.
2. Ina light controlling device, a refractor, a light source having a substantial dimension in a plane normal to the plane of the adjacent surface of the refractor, the refractor surface being oblique to incident rays from the light source whereby rays originating at the extremities of the light source in said plane and converging on a point on said surface are refractively transmitted at less angle of divergence than the original angle of convergence, the opposite surface of the refractor being substantially normal to the light ray in the glass originating midway between the extremities, and a second refractor whose adjacent surface has prisms onto which light rays fromthe first refractor fall, and whose outer surface is substantially normal to emergent light.
3. A luminaire comprising a long light source of substantial transverse dimension, a double walled refracting plate having prisms parallel with the axis of the light source, the prisms havingan externalangle of incidence greater than the internal angle of incidence to collect the light emitted through a wide transverse angle and concentrate it into a beam having less angle of divergence in transverse planes than the angle of convergence of rays in transverse planes onto corresponding points on the plate.
4. A luminaire comprisinga long light source of substantial transverse dimension, a double walled refracting plate parallel with the axis of the light source and having smooth exposed surfaces and'adjacent prismatic surfaces, the prisms having an external angle of incidence greater than the internal angle of incidence to collect the light emitted through a wide transverse angle and concentrate it into a beam normal to the outer surface of the plate and'having less angle of divergence in transverse planes than the angle of convergence of rays in transverse planes onto corresponding points on the plate.
5. A luminaire comprising a horizontal light source of substantial vertical dimension, a slop ing' double'walledrefracting plate parallel with the axis of the light source and having smooth exposed surfaces and adjacent prismatic surfaces, the prisms having an external angle of incidence greater than the internal angle of incidence to collect the light emitted through a wide vertical angle and concentrate it into a downwardly slanting beam having less angle of divergence in transverse planes than the angle of convergence of rays from the top and bottom of the light source onto corresponding points on the plate.
6. A luminaire comprising a succession of prismatic refractors each having annular prisms opposite a substantially smooth surface, an axially disposed light source of substantial axial dimension, said surfaces and prisms being so adjusted that the external angle of incidence is greater than the internal angle of incidence and so that refraction occurs on the inner face of each refractor to bend the ray toward the axis of the system, the outer face of the refractor being substantially normal to the light rays emergent therefrom.
7. In an optical system, a light source of substantial dimension and a light concentrating device onto the incident surface of which fall converging rays from the source, the light concentrating device having a succession of refracting elements traversed by said rays and in each of which the external angle of incidence is greater than the internal ange of incidence, whereby the refraction obtained at the first surface of each refracting element is greater than that obtained at the second surface.
8. An optical system such as claimed in claim 7, wherein the refracting elements are so disposed that the refractions' of a single ray are all in the same plane.
9. An optical system such as claimed in claim 7, wherein the refracting elements are so disposed that the refractions produced are all in the same direction so that the direction of the emergent light difiers from that of the incident light.
10. An optical system such as claimed in claim 7, wherein the refracting elements are so disposed that the refraction produced by one element is opposite to that produced by the next successive element.
11. An optical system such as claimed in claim 7, wherein the second surface of. at least one of the refracting elements is substantially normal to the light rays emerging therefrom.
12. An optical system such as claimed in claim 7, wherein the second surface of each of the refracting elements is substantially normal to the light rays emerging therefrom.
13. An optical system such as claimed in claim 7, wherein the inner surface of the first refracting element and. the outer surface of the outer refracting element are smooth and the opposed surface of each of said elements is prismatic.
14. An optical system such as claimed in claim 7, wherein the direction of the emergent light is substantially the same as the incident light.
15. An optical system such as claimed in claim 7, wherein the refracting elements are spaced apart a distance greater than necessary for mechanical clearance.
THOMAS W. ROLPI-I.
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US2596049 *||May 7, 1948||May 6, 1952||Hartford Nat Bank & Trust Co||Screen comprising a fresnel surface and projection device comprising such a screen|
|US2897346 *||Jun 28, 1955||Jul 28, 1959||Stevens Richard William||Electric lanterns|
|US4118114 *||Jun 21, 1976||Oct 3, 1978||Minnesota Mining And Manufacturing Company||Low-glare overhead projector|
|US5095415 *||Oct 30, 1989||Mar 10, 1992||Minnesota Mining And Manufacturing Company||Light fixture for externally illuminated sign|
|US6356391||Oct 8, 1999||Mar 12, 2002||3M Innovative Properties Company||Optical film with variable angle prisms|
|US6447135||Oct 8, 1999||Sep 10, 2002||3M Innovative Properties Company||Lightguide having a directly secured reflector and method of making the same|
|US6560026||Jan 16, 2002||May 6, 2003||Mark E. Gardiner||Optical film with variable angle prisms|
|US6707611||Jan 29, 2003||Mar 16, 2004||3M Innovative Properties Company||Optical film with variable angle prisms|
|US6845212||Oct 8, 1999||Jan 18, 2005||3M Innovative Properties Company||Optical element having programmed optical structures|
|US7046905||Jul 11, 2000||May 16, 2006||3M Innovative Properties Company||Blacklight with structured surfaces|
|US7221847||Aug 3, 2004||May 22, 2007||3M Innovative Properties Company||Optical elements having programmed optical structures|
|US7873256||Sep 14, 2005||Jan 18, 2011||3M Innovative Properties Company||Backlight with structured surfaces|
|US8588574||Oct 30, 2007||Nov 19, 2013||3M Innovative Properties Company||Backlight with structured surfaces|
|US20050001043 *||Aug 3, 2004||Jan 6, 2005||3M Innovative Properties Company||Optical elements having programmed optical structures|
|US20060051048 *||Sep 14, 2005||Mar 9, 2006||Gardiner Mark E||Backlight with structured surfaces|
|US20080050088 *||Oct 30, 2007||Feb 28, 2008||3M Innovative Properties Company||Backlight with structured surfaces|