WO2015085061A1

WO2015085061A1 - Light guiding film with low glare for daylighting

Info

Publication number: WO2015085061A1
Application number: PCT/US2014/068549
Authority: WO
Inventors: Weijun Zhou; Xinyu Gu
Original assignee: Dow Global Technologies Llc
Priority date: 2013-12-06
Filing date: 2014-12-04
Publication date: 2015-06-11

Abstract

A light redirecting member including a base layer (12), a reference plane (20) generally perpendicular to a first side of the base layer; and a plurality of optical elements (18) extending from the first side of the base layer, wherein at least one of the optical elements has a first surface (22) comprising a top inclusion angle between 18 degrees and 20 degrees relative to the reference plane and a second surface (24) comprising a bottom inclusion angle between 18 degrees and 20 degrees relative to the reference plane, wherein the top and bottom inclusion angles are selected to redirect incoming sunlight (50) upwardly (54) from a range of angular orientations during movement of the sun along a solar movement path.

Description

LIGHT GUIDING FILM WITH LOW GLARE FOR DAYLIGHTING

Cross-Reference to Related Applications

[0001] This application claims priority to U.S. Provisional Patent Application No. 61/912,664, filed December 6, 2013 the entire contents of which are incorporated herein by reference in its entirety.

Technical Field

[0002] The present invention relates to a light guiding film, and more particularly relates to a film used on windows for daylighting applications in which sunlight entering a room is redirected in a way that utilizes sunlight more effectively without causing undesirable occurrence of glare.

Background

[0003] Reducing energy usage in buildings can result in significant cost savings for businesses, consumers, and homeowners. One source of energy reduction is to use sunlight more effectively during daylight hours to illuminate the inner areas of a building, including areas that are not immediately adjacent to a window. This can decrease the amount of light required from artificial light sources. Daylighting can be used for such an approach, which involves admitting light from the sun and sky through windows, and redirecting that light towards the ceiling in areas of the room that are spaced far away from the windows.

[0004] A number of different daylighting films are available, which are typically applied to the interior surface of an existing window and/or are applied to the glass prior to installation of a windowpane. In currently available daylighting films, the path of the sun is assumed to move in a generally vertical direction, with the horizontal angle of the sun fixed at a zero degree angle.

Alternatively, currently available films use arbitrary light intensity without accounting for the real solar intensity at different times of the day. Such films can be relatively effective when the sun is within a certain range of angles, but can rapidly lose their effectiveness during times of the day outside of that range. Thus, there is a need to provide an optical film for daylighting purposes that takes the solar movement path into account to more effectively utilize the

l maximum amount of sunlight throughout the day at different seasons and different geographic locations.

Summary

[0005] In one aspect of this invention, an optical film is provided that includes a film base and at least one microstructure that is able to redirect sunlight onto the ceiling of a room through a window or roof opening for interior room illumination. This light can be the primary source of illumination during certain times of the day, although it can also be supplemented with artificial lighting sources. This use of natural sunlight for indoor illumination reduces the amount of energy needed from artificial light sources.

[0006] The light guiding film of the invention includes a plurality of prismatic structures, each of which includes a top apex or inclusion angle and a bottom apex or inclusion angle, wherein the angles have a predetermined relationship to each other. These angles are selected such that the light guiding film redirects sunlight from a variety of different angular directions defined by the solar movement path to the ceiling of a building throughout the day. In an embodiment, the light guiding film will provide a figure of merit (FOM) light harvesting efficiency of at least 50%. It is further a feature of embodiments of the invention that the light harvesting efficiency is high in all fouri seasons, from sunrise to noon to sunset, and in a variety of geographical locations that receive varying amounts of sun from different angles.

[0007] In an aspect of the invention, a light redirecting member is provided that includes a base layer, a reference plane generally perpendicular to a first side of the base layer, and

[0008] a plurality of optical elements extending from the first side of the base layer. At least one of the optical elements includes a first surface having a top inclusion angle between 18 degrees and 20 degrees relative to the reference plane, and a second surface having a bottom inclusion angle between 18 degrees and 20 degrees relative to the reference plane. In this embodiment, the top and bottom inclusion angles are selected to redirect incoming sunlight upwardly from a range of angular orientations during movement of the sun along a solar movement path. The top inclusion angle may be identical to the bottom inclusion angle. In addition, the selection of top and bottom inclusion angles can provide a figure of merit light harvesting efficiency of at least 50% throughout the solar movement path.

[0009] Further, the top and bottom inclusion angles can be selected to provide a glare that is preferably less than 5%, more preferably less than 3%, and most preferably less than 2%.

Brief Description of the Drawings

[0010] The present invention will be further explained with reference to the appended Figures, wherein like structure is referred to by like numerals throughout the several views, and wherein:

[0011] Figure 1 is a cross-sectional side view of a portion of an embodiment of a light guiding film of the invention;

[0012] Figure 2 is a cross-sectional side view of the light guiding film of

Figure 1 with its prisms facing in the direction toward incoming sunlight;

[0013] Figure 3 is a cross-sectional side view of the light guiding film of

Figure 1 with its prisms facing in the direction away from incoming sunlight;

[0014] Figure 4 is a cross-sectional side view of a portion of an embodiment of a light guiding film of the invention; and

[0015] Figure 5 is a cross-sectional side view of a portion of an embodiment of a light guiding film of the invention.

Detailed Description

[0016] Referring now to the Figures, wherein the components are labeled with like numerals throughout the several Figures, and initially to Figure 1 , an embodiment of a light guiding film 10 is illustrated. Film 10 generally includes a base layer 12 having a first surface 14, a second surface 16, and a plurality of extending optical elements or prisms 18 extending from the first surface 14 of base layer 12. A reference plane 20 extends in a generally perpendicular direction to the first surface 14 of base layer 12. In cross-section, each of the optical elements 18 includes first angled side 22 and a second angled side 24, wherein the first and second sides 22, 24 intersect or meet at a distal point or tip 26 that is spaced from the first surface 14. Each of the optical elements 18 includes a pitch length 30 measured between the points where the first and second sides 22, 24 meet the first surface 14, and a prism height 32, which is measured from the distal tip 26 to the first surface 14 along a reference line 34. Each of the optical elements 18 is further defined as having a top inclusion or apex angle 36 between the first side 22 of element 18 and reference line 34, and a bottom inclusion or apex angle 38 between the second side 24 of element 18 and reference line 34, wherein these angles are measured inside the prisms 18.

[0017] Figure 2 illustrates the portion of a light guiding film 10 illustrated in Figure 1 , with its prisms 18 facing toward incoming sunlight. The film 10 is shown with an exemplary incoming sunlight ray 40 that is positioned at an incoming angle 42 relative to the reference plane 20. This sunlight ray 40 will enter the film 10 through the first angled side 22 of each of the optical elements 18 and exit from the second surface of the base layer 12 (as represented by a ray 44) at an exit angle 46 relative to the reference plane 20.

[0018] Similarly, Figure 3 illustrates the portion of a light guiding film

10 illustrated in Figure 1, with its prisms 18 facing away from incoming sunlight so that the incoming light will first enter the film 10 through the second surface 1 of base layer 12. The film 10 is shown with an exemplary incoming sunlight ray 50 that is positioned at an incoming angle 52 relative to the reference plane 20. This sunlight ray 50 will enter the film 10 through the second surface 16 and exit from the optical elements 18 (as represented by ray 54) at an exit angle 56 relative to the reference plane 20.

[0019] With continued reference to any of Figures 1-3, the prisms 18 of the light guiding film 10 are provided with particular configurations in accordance with the invention. These configurations are designed to offer highly efficient light directing performance with the figure of merit (FOM) being at least 50%, wherein calculation of this value is discussed below. In other words, the top and bottom inclusion angles 36, 38 are selected in such a way that the optical film can redirect incoming sunlight from a variety of different angular directions, which are defined by the solar movement path. The light is directed toward the ceiling of a building throughout the day, with an FOM light harvesting efficiency of at least 50%, along with mimmal glare. To achieve both a FOM of at least 50% and low glare, the top inclusion angle 36 is selected to be between 18 and 20 degrees, and the bottom inclusion angle 38 is also selected to be between 18 and 20 degrees. The angles may be different from each other; however, in an embodiment of the invention, these angles are identical to each other for each prism of a particular light guiding film.

[0020] As discussed herein, the figure of merit (FOM) is calculated and defined as one measurement of the efficiency of a particular daylighting film configuration. First, the total amount of light transmittance into a room interior is determined. This light has an exit angle of at least 5 degrees above the horizontal plane averaged over the azimuth angle of 0 degrees to 80 degrees throughout one day for each season, represented by the Spring Equinox, Summer Solstice, Fall Equinox, and Winter Solstice. At an azimuth angle of 0 degrees, the sunlight propagation plane will be vertical to the window, wherein at an azimuth angle of -90 degrees or 90 degrees, it corresponds respectively to sunlight rising from the east and the sun setting to the west, with the sunlight propagation plane being parallel to the window. The redirected light

transmittance is obtained for each design, with TS/E, TS,¹ TF/E, TW, representing the daily average light transmittance for the Spring Equinox, the Summer Solstice, the Fall Equinox, and the Winter Solstice, respectively. The FOM is then calculated using the following equation: FOM = (TS/E + TS + TF/E + TW)/4. This calculation represents the average percentage of redirected light over the four seasons.

[0021] As shown in Table 1 below, multiple tests were performed using a number of light guided films generally of the configuration described and shown relative to Figures 1-3. Each of the designs includes optical elements with different combinations of inclusion angles, wherein the angle shown as (20, 20) of the table is in reference to a film including optical elements that have a top inclusion angle of 20 degrees and a bottom inclusion angle of 20 degrees, wherein the angle shown as (25, 12) of the table is in reference to a film including optical elements that have a top inclusion angle of 25 degrees and a bottom inclusion angle of 12 degrees, and so on throughout the various "Design Name" entries of the table. The glare for each of the optical element configurations was theoretically analyzed with Desktop Radiance and Building Design Advisor (BDA) available from Lawrence Berkeley National Lab to determine how the material would perform in the two representative geographic locations of Chicago and Houston, and with the light guiding film being positioned so that it faces south and east in both cities. A calculated glare occurrence of greater than 5% is generally considered to be unacceptable in the daylighting industry, however, a glare in the range of 3% to 5% is considered to be moderately acceptable, and a glare of less than 2% is most desirable. In addition, the energy savings were calculated for each of the optical elements, with an annual energy savings equal to or below 30% being least desirable, between 30% and 35% being moderately desirable, and energy savings of greater than 35% being most desirable. It is noted that the daylighting industry generally looks for glare performance that is similar to that produced by light entering a room through Venetian blinds (i.e., no glare occurrence, along with energy savings performance that is better than that of light louver products that currently are commercially available, or approximately 30%).

Table 1 [0022] As is shown by these results, a light guiding film having optical elements with top and bottom inclusion angles of 20 degrees provide the most desirable results for glare in all locations and orientations, along with good energy savings. That is, while a variety of different optical designs can be used to achieve relatively good energy savings as compared to using artificial lighting for the interior areas of buildings, many of these configurations fall short of delivering a desired low glare performance throughout the year at different geographical locations when the top and/or bottom inclusion angles are different from the most preferable optical designs. In addition, when a light guiding film admits too much light (e.g., see Designs la-Y and la-N in Table 1), glare may occur at a significantly larger percentage of daytime than in a more balanced design (e.g., see Design 13Y in Table 1).

[0023] With continued reference to Table 1 , the Figure of Merit (FOM) for various light guiding film designs were calculated for the representative locations of Chicago and Houston. In general, a larger percentage of light will be admitted into a room with a higher FOM. However, depending on the angular distribution of the re-directed light, some of the energy savings may not be realized, such as in a situation in which the re-directed light does not penetrate deep into the room. Therefore, the FOM values do not necessarily correlate exactly with the anticipated annual energy savings for each optical design.

Further, if FOM light transmittance for a design is too high, such as equal to or greater than 65%, the design can lead to higher glare occurrence and is therefore not as desirable. Thus, a preferred range of FOM is between 50% and 65%.

[0024] Figure 4 is another exemplary embodiment of a light guiding film 110 of the invention. Film 110 generally includes a base layer 112 having a first surface 114, a second surface 116, and a plurality of extending optical elements or prisms 118 extending from the first surface 114 of base layer 112. The shape of these prisms 118 may be referred to as a truncated V-shaped prism. A reference plane 120 extends in a generally perpendicular direction relative to the first surface 114 of base layer 112. In cross-section, each of the optical elements 118 includes first angled side 122, a second angled side 124, and a prism apex 126 extending between the distal ends of the first and second angled sides 122, 124. Each of the optical elements 118 includes a pitch length 130 measured between the points where the first sides 122 of two adjacent optical elements 118 meet the first surface 114, and a prism height 132, which is measured from the prism apex 126 to the first surface 114 along a first reference line 134. The prism apex 126 has a length 140, and adjacent prisms 118 are spaced from each other at their bases by a distance 142.

[0025] Each of the optical elements 118 is further defined as having a top inclusion or apex angle 136 between the first side 122 of element 118 and a second reference line 135, and a bottom inclusion or apex angle 138 between the second side 124 of element 118 and a third reference line 137, wherein these inclusion angles are measured inside the prisms 118. In this arrangement, the pitch length 130 is calculated using the length of the prism apex (La), the space between adjacent prisms (Lv), the height of the prisms (H), the first inclusion angle (Al) and the second inclusion angle (A2) in the following equation:

P=(La) + (Lv) + H[tan(Al) + tan(A2)]. This design provides the feature of semi-transparency in addition to light redirecting performance. The most preferred inclusion angle is between 18 and 20 degrees for both the top and bottom apex.

[0026] Light guiding film 110 of Figure 4 is positioned with its prisms 118 facing away from incoming sunlight so that the incoming light will first enter the film 110 through the second surface 116 of base layer 112. The film 110 is shown with an exemplary incoming sunlight ray 150 that is positioned at an incoming angle 152 relative to the reference plane 120. This sunlight ray 150 will enter the film 110 through the second surface 116 and exit from the film 110 (as represented by ray 154) at an exit angle 156 relative to the reference plane 120.

[0027] Figure 5 is another exemplary embodiment of a light guiding film

210 of the invention. Film 210 generally includes a base layer 212 having a first surface 214, a second surface 216, and a plurality of extending optical elements or shaped prisms 218 extending from the first surface 214 of base layer 212. A reference plane 220 extends in a generally perpendicular direction relative to the first surface 214 of base layer 212. In cross-section, each of the optical elements 218 includes first curved side 222 and a second angled side 224, wherein the first and second sides 222, 224 intersect or meet at a distal point or tip 226 that is spaced from the first surface 214. Adjacent optical elements 218 are spaced from each other at their bases by a distance 242. Each of the optical elements 218 includes a pitch length 230 measured between the points where the first sides 222 of two adjacent optical elements 218 meet the first surface 214, and a prism height 232, which is measured from the prism tip 226 to the first surface 214 along a reference line 234. Each of the optical elements 218 is further defined as having a bottom inclusion or apex angle 238 between the second side 224 of element 218 and reference line 234, wherein these inclusion angles are measured inside the prisms 218. This prism design may allow for less sensitivity to solar movement while still providing the desired light redirecting performance. While there is no specific restriction for the top inclusion angle, the bottom inclusion angle is preferably in the range of between 18 and 20 degrees.

[0028] Light guiding film 210 of Figure 5 is positioned with its prisms

218 facing away from incoming sunlight so that the incoming light will first enter the film 210 through the second surface 216 of base layer 212. The film 210 is shown with an exemplary incoming sunlight ray 250 that is positioned at an incoming angle 252 relative to the reference plane 220. This sunlight ray 250 will enter the film 210 through the second surface 216 and exit from the film 210 (as represented by ray 254) at an exit angle 256 relative to the reference plane 220.

[0029] The materials proved herein can further include one or more additional layers on one or both sides of the light guiding films or members of the invention to provide additional desired performance when in use. For example, one or more materials can be coated or otherwise added to either or both sides of the light guiding film, such as a hard coating layer, an anti-dazzling layer, or the like.

[0030] The embodiments described herein are generally provided as film materials that can be manufactured and attached to a planar member, such as a window. The window films of the invention can be manufactured using an extrusion roll molding process, thermal embossing process, UV cast and curing processes, or the like, for large scale manufacturing of daylighting films. For an extrusion roll molding process, polymenthylmethacrylate (PMMA), cyclic olefin polymers and copolymers, cyclic block copolymers, and polycarbonate (PC) are exemplary optical resins that can be used. It is also contemplated that the prism structures and configurations described herein may instead be formed directly onto a window surface by various techniques, such as printing, etching, laminating, patterning, and the like, such that the window itself acts generally as a base layer for the extending prism structures.

{0031] The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.

Claims

Claims:

1. A light redirecting member comprising:

a base layer;

a reference plane generally perpendicular to a first side of the base layer; and a plurality of optical elements extending from the first side of the base layer, wherein at least one of the optical elements comprises:

a first surface comprising a top inclusion angle between 18 degrees and 20 degrees relative to the reference plane;

a second surface comprising a bottom inclusion angle between 18 degrees and 20 degrees relative to the reference plane;

wherein the top and bottom inclusion angles are selected to redirect incoming sunlight upwardly from a range of angular orientations during movement of the sun along a solar movement path.

2. The light redirecting member of claim 1 , wherein the top inclusion angle is identical to the bottom inclusion angle.

3. The light redirecting member according to either of claims 1 or 2, wherein the plurality of optical elements comprises an array of extending V-shaped elements, wherein at least one of the V-shaped elements comprises an apex at an intersection point of its first and second surfaces.

4. The light redirecting member of claim 3, wherein the reference plane intersects with the apex of the at least one V-shaped element.

5. The light redirecting member according to any of claims 1-4, wherein the selection of top and bottom inclusion angles provides a figure of merit light harvesting efficiency of at least 50% throughout the solar movement path.

6. The light redirecting member of claim 5, wherein the figure of merit light harvesting efficiency is calculated as (T_{S E} + Ts + T_{F E} + Tw)/4, wherein T_S/E represents a daily average light transmission into a room interior of the Spring Equinox, Ύ$ represents a daily average light transmission into the room interior of the Summer Solstice, TF/E represents a daily average light transmission into the room interior of the Fall Equinox, and Tw represents a daily average light transmission into the room interior of the Winter Solstice, wherein the average light transmissions comprise a portion of transmitted light that is 5 degrees above the horizontal plane.

7. The light redirecting member according to any of claims 1-6, wherein the base layer comprises one of a film, a planar glass panel, and a planar plastic panel.

8. The light redirecting member according to any of claims 1-7, wherein the upwardly redirected incoming sunlight is directed toward a horizontal ceiling surface within a building.

9. The light redirecting member according to any of claims 1 -8, wherein the reference plane comprises a horizontal plane.

10. The light redirecting member according to either of claims 1 or 2, wherein the plurality of optical elements comprises an array of extending truncated V-shaped elements, wherein each truncated V-shaped element comprises a truncated surface extending between distal ends of its first and second surfaces.

11. The light redirecting member of according to any of claims 1-10 wherein the base layer further comprises a second side that is opposite its first side, and an attachment layer for attaching the second side of the base layer to a substrate.

12. The light redirecting member of claim 11 , wherein the substrate is a window.

13. The light redirecting member according to any of claims 1-12, further comprising at least one additional layer adjacent to at least one of the plurality of optical elements and a second side of the base layer, wherein the at least one additional layer comprises at least one of a hard coating layer and an anti-dazzling layer.

14. The light redirecting member of claim 1 , wherein the top and bottom inclusion angles are selected to provide a glare that is preferably less than 5%, more preferably less than 3%, and most preferably less than 2%.

15. The light redirecting member of claim 14, wherein the top inclusion angle is identical to the bottom inclusion angle.