具体实施方式：
[0039]The schematic drawings presented herein are not necessarily to scale; however, graphs are assumed to have accurate scales unless otherwise indicated. Like reference numerals used in the figures refer to like elements.
DETAILED DESCRIPTION
[0040]An optical system 100 capable of utilizing the unique properties of the disclosed dual-sided optical films is shown in FIG. 1A. The optical system 100 may be part of a display system, but other devices and applications, including ambient lighting devices such as luminaires, task lights, and static backlit signs, are also contemplated. The system 100 is shown in relation to a Cartesian x-y-z coordinate system so that directions and orientations of selected features can be more easily discussed. The system 100 includes one or more light guides 150, one or more first light sources 134, and one or more second light sources 132. The system 100 also includes a dual-sided optical film 140, further details of which are discussed below. The x-y plane of the coordinate system is assumed to lie parallel to the plane of the film 140, which is also typically parallel to the plane of the light guide 150.
[0041]The light sources 132, 134 are disposed on opposite ends of the light guide, and inject light into the light guide from opposite directions. Each of the light sources may emit light that is nominally white and of a desired hue or color temperature. Alternatively, each light source may emit colored light, e.g., light perceived to be red, green, blue, or another known non-white color, and/or may emit ultraviolet and/or infrared (including near infrared) light. The light sources may also be or comprise clusters of individual light emitting devices, some or all of which may emit non-white colored light, but the combination of light from the individual devices may produce nominally white light, e.g. from the summation of red, green, and blue light. Light sources on opposite ends of the light guide may emit light of different white or non-white colors, or they may emit light of the same colors. The light sources 132, 134 can be of any known design or type, e.g., one or both may be or comprise cold cathode fluorescent lamps (CCFLs), and one or both may be or comprise one or more inorganic solid state light sources such as light emitting diodes (LEDs) or laser diodes, and one or both may be or comprise one or more organic solid state light sources such as organic light emitting diodes (OLEDs). The round shapes used to represent the light sources in the drawings are merely schematic, and should not be construed to exclude LED(s), or any other suitable type of light source. The light sources 132, 134 are preferably electronically controllable such that either one can be energized to an ON state (producing maximum or otherwise significant light output) while keeping the other one in an OFF state (producing little or no light output), or both can be in the ON state at the same time if desired, and both may be turned OFF during non-use. In many cases, the light sources 132, 134 do not need to satisfy any particular requirement with regard to switching speed. For example, although either or both light sources 132, 134 may be capable of repetitively transitioning between the OFF state and the ON state at a rate that is imperceptible to the human eye (e.g., at least 30 or 60 Hz), such a capability is not necessary in many embodiments. (For flicker-free operation, transition rates may be in a range from 50 to 70 Hz, or more; for two-sided operation, transition rates may be in a range from 100 to 140 Hz (or more) for the display panel (if any) and the light sources.) Thus, light sources that have much slower characteristic transition times between the ON and OFF states can also be used.
[0042]The light guide 150 includes a first light input side 150c adjacent to the first light source 134 and an opposing second light input side 150d adjacent to the second light source 132. A first light guide major surface 150b extends between the first side 150c and second side 150d. A second light guide major surface 150a, opposite the first major surface 150b, also extends between the first side 150c and the second side 150d. The major surfaces 150b, 150a of the light guide 150 may be substantially parallel to each other, or they may be non-parallel such that the light guide 150 is wedge-shaped. Light may be reflected or emitted from either surface 150b, 150a of the light guide 150, but in general light is emitted from surface 150a and is reflected from surface 150b. In some cases, a highly reflective surface may be provided on or adjacent to the first surface 150b to assist in re-directing light out through the second surface 150a. Light extraction features such as shallow prism structures 152, or other light extraction features such as lenticular features, white dots, haze coatings, and/or other features, may be disposed on one or both major surfaces 150b, 150a of the light guide 150. Exemplary light extraction features for the light guide are discussed below in connection with FIG. 2. The light extraction features are typically selected so that light emitted from the major surface 150a propagates preferentially at highly oblique angles in air as measured in the x-z plane, rather than propagating at normal or near-normal propagation directions that are parallel to, or deviate only slightly from, the z-axis (again as measured in the x-z plane). For example, the light emitted from the surface 150a into air may have a peak intensity direction that makes an angle relative to the surface normal (z-axis) of 60 degrees or more, or 70 degrees or more, or 80 degrees or more, where the peak intensity direction refers to the direction along which the intensity distribution of the output beam in the x-z plane is a maximum.
[0043]The light guide 150 may have a solid form, i.e., it may have an entirely solid interior between the first and second major surfaces 150a, 150b. The solid material may be or comprise any suitable light-transmissive material, such as glass, acrylic, polyester, or other suitable polymer or non-polymer materials. Alternatively, the light guide 150 may be hollow, i.e., its interior may be air or another gas, or vacuum. If hollow, the light guide 150 is provided with optical films or similar components on opposite sides thereof to provide the first and second major surfaces 150a, 150b. Hollow light guides may also be partitioned or subdivided into multiple light guides. Whether solid or hollow, the light guide 150 may be substantially planar, or it may be non-planar, e.g., undulating or curved, and the curvature may be slight (close to planar) or great, including cases where the light guide curves in on itself to form a complete or partial tube. Such tubes may have any desired cross-sectional shape, including curved shapes such as a circle or ellipse, or polygonal shapes such as a square, rectangle, or triangle, or combinations of any such shapes, A hollow tubular light guide may in this regard be made from a single piece of optical film or similar component(s) that turns in on itself to form a hollow tube, in which case the first and second major surfaces of the light guide may both be construed to be provided by such optical film or component(s). The curvature may be only in the x-z plane, or only in the y-z plane, or in both planes. Although the light guide and dual-sided film may be non-planar, for simplicity they are shown in the figures as being planar; in the former case one may interpret the figures as showing a small enough portion of the light guide and/or optical film such that it appears to be planar. Whether solid or hollow, depending on the material(s) of construction and their respective thicknesses, the light guide may be physically rigid, or it may be flexible. A flexible light guide or optical film may be flexed or otherwise manipulated to change its shape from planar to curved or vice versa, or from curved in one plane to curved in an orthogonal plane.
[0044]The dual-sided optical film 140, which is assumed to lie in or define a film plane generally parallel to the x-y plane, is disposed to receive obliquely-emitted light from the light guide 150. The film 140 has a first structured surface 140a, and a second structured surface 140b opposite the first structured surface. Elongated lenslets 144 are formed in the structured surface 140a, which is oriented generally away from the light guide 150.
[0045]Elongated prisms (shown better in figures that follow) are formed in the second structured surface 140b, which is oriented generally towards the light guide 150. In this orientation, light emitted from the major surface 150a of the light guide 150 is incident on the prisms, which help to deviate the incident light. The incident light is deviated by and passes through the film 140 to provide a film light output that emerges from the film 140. As described further below, the properties of the film light output can be influenced by which of the light sources 132, 134 is in an ON state, as well as by the spatial relationships between the lenslets and the prisms. When one light source is ON, a first film light output may comprise a first group of N angularly separated light beams. When the opposite light source is ON, a second film light output may comprise a second group of N angularly separated light beams, which beams may be substantially aligned with, or not aligned with, the first group of light beams. As shown better in other figures below, the prisms are grouped into clusters of adjacent prisms, the clusters being separated from each other, and each prism cluster being associated with a corresponding one of the lenslets. These prisms have sharp apexes so as to provide beam edges, measured e.g. from a plot of intensity versus angle, that are sharp.
[0046]Both the prisms and the lenslets 144 are typically linear, or, in cases where one or both are not precisely linear (e.g. not straight), they are otherwise extended or elongated along a particular in-plane axis. Thus, the lenslets 144 may extend along lenslet axes that are parallel to each other. One such axis is shown in FIG. 1B as axis 145, which is assumed to be parallel to the y-axis. The prisms may extend along respective prism axes that are parallel to each other. The lenslet axes of elongation are typically parallel to the prism axes of elongation. Perfect parallelism is not required, and axes that deviate slightly from perfect parallelism may also be considered to be parallel; however, misalignment results in different amounts of registration between a given lenslet/prism cluster pair at different places along their length on the working surface of the dual-sided film—and such differences in the degree of registration (regardless of whether the degree of registration is tailored to have precise alignment, or intentional misalignment, of the relevant vertices or other reference points, as discussed below) are desirably about 1 micron or less. In some cases, extraction features such as prism structures 152 on the major surface 150b of the light guide may be linear or elongated along axes that are parallel to the elongation axes of the lenslets and prisms of the film 140; alternatively, such extraction features of the light guide 150 may be oriented at other angles.
[0047]In the film 140 or pertinent portion thereof, there is a one-to-one correspondence of lenslets 144 to prism clusters. Thus, for each prism cluster there is a unique lenslet 144 with which the given prism cluster primarily interacts, and vice versa. One, some, or all of the lenslets 144 may be in substantial registration with their respective prism clusters. Alternatively, the film 140 may be designed to incorporate a deliberate misalignment or mis-registration of some or all of the lenslets relative to their respective prism clusters. Related to alignment or misalignment of the lenslets and prism clusters is the center-to-center spacings or pitches of these elements. In the case of a display system, the pitch of the lenslets 144 and the pitch of the prism clusters (as well as the pitch of the individual prisms in the prism clusters) may be selected to reduce or eliminate Moire patterns with respect to periodic features in the display panel. These various pitch dimensions can also be determined or selected based upon manufacturability. Useful pitch ranges for the lenslets 144 and the prism clusters on the respective structured surfaces of the optical film 140 is about 10 microns to about 140 microns, for example, but this should not be interpreted in an unduly limiting way.
[0048]The system 100 can have any useful shape or configuration. In many embodiments, the light guide 150, and/or the dual-sided optical film 140 can have a square or rectangular shape. In some embodiments, however, any or all of these elements may have more than four sides and/or a curved shape.
[0049]A switchable driving element 160 is electrically connected to the first and second light sources 132, 134. This element may contain a suitable electrical power supply, e.g. one or more voltage sources and/or current sources, capable of energizing one or both of the light sources 132, 134. The power supply may be a single power supply module or element, or a group or network of power supply elements, e.g., one power supply element for each light source. The driving element 160 may also contain a switch that is coupled to the power supply and to the electrical supply lines that connect to the light sources. The switch may be a single transistor or other switching element, or a group or network of switching modules or elements. The switch and power supply within the driving element 160 may be configured to have several operational modes. These modes may include two, three, or all of: a mode in which only the first light source 134 is ON; a mode in which only the second light source 132 is ON; a mode in which both the first and second light sources are ON; and a mode in which neither of the first and second light sources are ON (i.e., both are OFF).
[0050]We describe in more detail below how the dual-sided optical film 140, when provided with separated clusters of adjacent prisms, can provide the optical system with the capability to produce a light output characterized by a group of light beams that are closely spaced but separated from each other in output angle. The group of beams has sharp edges at two opposite boundaries of the beams, and the individual beams may also have sharp edges. The characteristics and features of the light output are controlled by design details of the lenslets and prism clusters, as explained further below.
[0051]FIG. 1B is a schematic perspective view of the optical system 100 showing the light guide 150, the optical film 140, and the second light sources 132. Like elements between FIGS. 1A and 1B have like reference numerals, and need not be further discussed. The optical film 140 includes lenslets 144 oriented away from the light guide 150 and prisms with prism peaks oriented toward the light guide 150. The axis of elongation 145 of the lenslets, which may also correspond to the axis of elongation of the prisms, is shown to be parallel to the y-axis. In the case of the prisms of the structured surface 140b, the elongation axis runs parallel to the vertex of the prism. The film 140 is shown to be adjacent the light guide 150 but spaced slightly apart. The film 140 may also be mounted or held so that it is in contact with the light guide 150, e.g. the film 140 may rest upon the light guide 150, while still substantially maintaining an air/polymer interface at the facets or inclined side surfaces of the prisms (with a physically thin but optically thick layer of air) so that their refractive characteristics can be preserved. Alternatively, a low refractive index bonding material may be used between the prisms and the light guide 150 to bond the film 140 to the light guide. In this regard, nanovoided materials having an ultra low index (ULI) of refraction are known that can come somewhat close in refractive index to air, and that can be used for this purpose. See e.g. patent application publications WO 2010/120864 (Hao et al.) and WO 2011/088161 (Wolk et al.), which discuss ULI materials whose refractive index (n) is in a range from about n≈1.15 to n≈1.35. See also patent application publications WO 2010/120422 (Kolb et al.), WO 2010/120468 (Kolb et al.), WO 2012/054320 (Coggio et al.), and US 2010/0208349 (Beer et al.). Air gap spacing techniques, e.g. wherein an array of microreplicated posts is used to bond the two components together while substantially maintaining an air gap between them, may also be used. See e.g. patent application publication US 2013/0039077 (Edmonds et al.).
[0052]The disclosed dual-sided optical films and associated components may be provided in a variety of forms and configurations. In some cases, the dual-sided optical film may be packaged, sold, or used by itself, e.g. in piece, sheet, or roll form. In other cases, the dual-sided optical film may be packaged, sold, or used with a light guide whose output beam characteristics are tailored for use with the dual-sided film. In such cases, the dual-sided film may be bonded to the light guide as discussed above, or they may not be bonded to each other. In some cases, the dual-sided optical film may be packaged, sold, or used with both a light guide that is tailored for use with the dual-sided film, and one or more LED(s) or other light source(s) that are adapted to inject light into the light guide, e.g., from opposite ends thereof as shown generally in FIG. 1A. The dual-sided film, the light guide, and the light source(s) may be bonded, attached, or otherwise held in proximity to each other to form a lighting module, which may be large or small, rigid or flexible, and substantially flat/planar or non-flat/non-planar, and which may be used by itself or in combination with other components. A lighting system that includes a dual-sided optical film, a light guide, and one or more light source(s) may be adapted for any desired end use, e.g., a display, a backlight, a luminaire, a task light, static backlit signs, or a general-purpose lighting module.
[0053]FIG. 2 shows a schematic perspective view of an exemplary light guide 250 that may be suitable for use with some or all of the disclosed dual-sided optical films. The light guide 250 may be substituted for the light guide 150 in FIG. 1A, and the properties, options, and alternatives discussed in connection with the light guide 150 will be understood to apply equally to the light guide 250. Cartesian x-y-z coordinates are provided in FIG. 2 in a manner consistent with the coordinates of FIGS. 1A and 1B. FIG. 2 shows in exaggerated fashion exemplary surface structure on the two major surfaces of the light guide 250, but other orientations of the structured surface(s) relative to the edges or boundaries of the light guide can be used. The light guide 250 includes a first major surface 250a from which light is extracted towards a dual-sided optical film, a second major surface 250b opposite the first major surface, and side surfaces 250d, 250c which may serve as light injection surfaces for the first and second light sources as discussed elsewhere herein. For example, one light source may be positioned along the side surface 250c to provide a first oblique light beam emitted from the light guide 250, and a similar light source can be positioned along the side surface 250d to provide a second oblique light beam emitted from the light guide 250. An oblique light beam in this regard refers to a light beam whose intensity distribution in the x-z plane has a peak intensity direction of 60 degrees or more, or 70 degrees or more, or 80 degrees or more relative to the surface normal (z-axis), as discussed above.
[0054]The rear major surface 250b of the light guide is preferably machined, molded, or otherwise formed to provide a linear array of shallow prism structures 252. These prism structures are elongated along axes parallel to the y-axis, and are designed to reflect an appropriate portion of the light propagating along the length of the light guide (along the x-axis) so that the reflected light can refract out of the front major surface 250a into air (or a tangible material of suitably low refractive index) at a suitably oblique angle, and onward to the dual-sided optical film. In many cases, it is desirable for the reflected light to be extracted from the front major surface 250a relatively uniformly along the length of the light guide 250. The surface 250b may be coated with a reflective film such as aluminum, or it may have no such reflective coating. In the absence of any such reflective coating, a separate back reflector may be provided proximate the surface 250b to reflect any downward-propagating light that passes through the light guide so that such light is reflected back into and through the light guide. The prism structures 252 typically have a depth that is shallow relative to the overall thickness of the light guide, and a width or pitch that is small relative to the length of the light guide. The prism structures 252 have apex angles that are typically much greater than the apex angles of prisms used in the disclosed dual-sided optical films. The light guide may be made of any transparent optical material, typically with low scattering such as polycarbonate, or an acrylic polymer such as Spartech Polycast material. In one exemplary embodiment, the light guide may be made of acrylic material, such as cell-cast acrylic, and may have an overall thickness of 1.4 mm and a length of 140 mm along the x-axis, and the prisms may have a depth of 2.9 micrometers and a width of 81.6 micrometers, corresponding to a prism apex angle of about 172 degrees. The reader will understand that these values are merely exemplary, and should not be construed as unduly limiting.
[0055]The front major surface 250a of the light guide may be machined, molded, or otherwise formed to provide a linear array of lenticular structures or features 254 that are parallel to each other and to a lenticular elongation axis. In contrast to the elongation axis of the prism structures 252, the lenticular elongation axis is typically parallel to the x-axis. The lenticular structures 254 may be shaped and oriented to enhance angular spreading in the y-z plane for light that passes out of the light guide through the front major surface, and, if desired, to limit spatial spreading along the y-axis for light that remains in the light guide by reflection from the front major surface. In some cases, the lenticular structures 254 may have a depth that is shallow relative to the overall thickness of the light guide, and a width or pitch that is small relative to the width of the light guide. In some cases, the lenticular structures may be relatively strongly curved, while in other cases they may be more weakly curved. In one embodiment, the light guide may be made of cell-cast acrylic and may have an overall thickness of 0.76 mm, a length of 141 mm along the x-axis, and a width of 66 mm along the y-axis, and the lenticular structures 254 may each have a radius of 35.6 micrometers, a depth of 32.8 micrometers, and a width 323 of 72.6 mm, for example. In this embodiment, the prism structures 252 may have a depth of 2.9 micrometers, a width of 81.6 micrometers, and a prism apex angle of about 172 degrees. Again, the reader will understand that these embodiments are merely exemplary, and should not be construed as unduly limiting; for example, structures other than lenticular structures may be used on the front major surface of the light guide.
[0056]As mentioned above, the lenticular structures 254 may be shaped and oriented to limit spatial spreading along the y-axis for light that remains in the light guide by reflection from the front major surface. Limited spatial spreading along the y-axis can also be achieved, or enhanced, with light sources that are collimated (including substantially collimated) in the plane of the light guide, i.e., the x-y plane. Such a light source may be a relatively small area LED die or dies in combination with one or more collimating lenses, mirrors, or the like. FIG. 2A shows the light guide 250 of FIG. 2 in combination with light sources 232a, 232b, 232c arranged along side surface 250d, and light sources 234a, 234b, 234c arranged along side surface 250c. These light sources may be substantially collimated, or the lenticular structures 254 may be shaped to limit spatial spreading of light along the y-axis, or both. In the figure, the light sources 232a, 232b, 232c are shown as being ON, and the other light sources are OFF. Due to the collimation of the light sources, the shape of the lenticular structures 254, or both, the light sources 232a, 232b, 232c illuminate respective stripes or bands 250-1, 250-2, 250-3 of the light guide 250. The bands may be distinct, with little or no overlap as shown in the figure, or they may overlap to some extent. Each of the light sources may be independently addressable, such that the light guide can be effectively subdivided or partitioned as a function of which light sources on each side of the light guide are turned ON. For example, only one of the bands 250-1, 250-2, 250-3 may be illuminated, or only two may be illuminated, or all of the bands may be illuminated. Light sources 234a, 234b, 234c, which are located on the opposite side of the light guide, may be aligned with their counterpart light sources at side surface 250d such that they illuminate the same respective bands 250-1, 250-2, 250-3; alternately, the light sources 234a, 234b, 234c may be shifted or staggered along the y-direction relative to the light sources at side surface 250d, such that they illuminate other bands which may or may not overlap with each other in similar fashion to bands 250-1, 250-2, 250-3. The light sources 232a, 232b, 232c, 234a, 234b, 234c may all emit white light, or light of a non-white color or wavelength, or the light sources may emit different colors. A given portion of the light guide 250, such as any of the bands 250-1, 250-2, 250-3, may thus function as an independent light guide, and may emit at least two different output beams as a function of whether only its associated light source(s) at one side surface (e.g. surface 250d) is ON, or whether only its associated light source(s) at the opposite side surface (e.g. surface 250c) is ON, or whether both such light sources are ON. When a dual-sided optical film is used with such a light guide, the spatially banded or striped output capability of the light guide is substantially transferred to the dual-sided optical film, such that, by energizing the appropriate light source(s), the disclosed light outputs (including e.g. groups of angularly separated light beams) can emerge from the dual-sided optical film over all (all stripes or bands), or only a portion (at least one but less than all stripes or bands), or none (no stripes or bands) of its output surface.
[0057]Turning now to FIG. 3, we see there another schematic side view of the lighting system 100 of FIG. 1A. In FIG. 3, only the light source 134 is energized (ON), and the light source 132 is not energized (OFF). Due to the characteristics of the light guide 150, the characteristics of the optical film 140, and the interaction between the light guide and the optical film, light from the light source 134 produces a first film light output 310 emerging from the dual-sided optical film. The reader will understand that although the light output 310 is drawn above a central portion of the film 140, we assume for this particular embodiment that this same light output is emitted from substantially the entire first structured surface 140a. The light output 310 has an angular distribution in the x-z plane characterized by a group of closely spaced (as a function of angle θ) but angularly separated lobes 310a, 310b, . . . , 310h. The outermost lobes 310a, 310h define sharp transitions at the outer opposite edges or sides of the generally fan-shaped light output 310. Between those outer edges, the brightness of the output 310 fluctuates rapidly and substantially as a function of angle to define the eight distinct lobes 310a, 310b, 310c, etc. Depending upon the amount of fluctuation between the lobe peaks and the relative minima between lobes, some or all of the lobes may be considered to be separate light beams, as discussed below. The number N of distinct lobes or beams, in this case N=8, may be equal to the number of individual prisms in each of the prism clusters on the structured surface 140b, as discussed further below.
[0058]Light from the energized light source 134 enters the light guide 150 through the first side 150c. This light travels along the light guide 150 generally in the positive x-direction, the light reflecting from the major surfaces 150a, 150b to provide a first guided light beam 134-1. As the beam 134-1 propagates, some of the light is refracted or otherwise extracted from the major surface 150a to provide an oblique light beam 134-2, represented by obliquely oriented arrows representing a direction of maximum light intensity in the x-z plane. The oblique light beam 134-2 is typically emitted over substantially the entire surface area of the major surface 150a, i.e., not only in the geometric center of the major surface 150a but also at or near its edges and at intermediate positions in between, as indicated by the multiple oblique arrows. The oblique light beam 134-2 has a direction of maximum light intensity that is most closely aligned with the positive x-direction. The direction of maximum light intensity of the beam 134-2 may deviate from the positive x-direction by, for example, 30 degrees or less, or 20 degrees or less, or 15 degrees or less, or 10 degrees or less.
[0059]Because of the directionality of the oblique light beam 134-2, light from the light source 134 may enter the dual-sided optical film 140 predominantly through only one inclined side surface of each of the prisms on the second structured surface 140b of the film 140. Refraction provided at such inclined surfaces, in cooperation with reflection provided at other inclined surfaces of the prisms, and in cooperation with refraction provided by the lenslets 144, causes light to emerge from the film 140 as the first film light output 310. The first film light output 310 arises from the summation of individual light outputs emitted from each lenslet 144 across the film 140, which individual outputs are referred to as lenslet light outputs. For simplicity, we assume that the film 140 is configured such that the individual lenslet light outputs have angular distributions that are the same as each other, and the same as that of the film light output 310. In other embodiments, the angular distributions of the individual lenslet light outputs may differ from each other, and which would then sum together to provide an overall film light output that has a different angular distribution from that of the individual lenslet light outputs.
[0060]If the first light source 134 is turned OFF and the second light source 132 is turned ON, the system 100 produces a second film light output, which is also characterized by a generally fan-shaped angular distribution in the x-z plane which is or includes a group of closely spaced (as a function of angle θ) but angularly separated lobes, the outermost lobes defining sharp transitions at the outer opposite edges or sides of the light output. Depending upon the amount of fluctuation between the lobe peaks and the relative minima between lobes, some or all of the lobes may be considered to be separate light beams. The second film light output typically covers an angular range that differs from that of the first film light output, but the angular distributions of these two film light outputs typically overlap, whether or not any of their respective individual lobes (or beams) overlap. FIG. 4 shows a typical second film light output 410 that may be produced in a manner consistent with the first film light output 310 of FIG. 3, with the same dual-sided optical film 140.
[0061]Thus, in FIG. 4, the lighting system 100 is shown again, except that the light source 134 is not energized (OFF), and the light source 132 is energized (ON). Due to the characteristics of the light guide 150, the characteristics of the dual-sided optical film 140, and the interaction between the light guide and the optical film, light from the li