Multilayer optical films are known. Such films typically incorporate a large number of very thin layers of different light transmissive materials, the layers being referred to as microlayers because they are thin enough so that the reflection and transmission characteristics of the optical film are determined in large part by constructive and destructive interference of light reflected from the layer interfaces. Depending on the amount of birefringence (if any) exhibited by the individual microlayers, and the relative refractive index differences for adjacent microlayers, and also on other design characteristics, the multilayer optical films can be made to have reflection and transmission properties that may be characterized as a reflective polarizer in some cases, and as a mirror in other cases, for example.
Reflective polarizers composed of a plurality of microlayers whose in-plane refractive indices are selected to provide a substantial refractive index mismatch between adjacent microlayers along an in-plane block axis and a substantial refractive index match between adjacent microlayers along an in-plane pass axis, with a sufficient number of layers to ensure high reflectivity for normally incident light polarized along one principal direction, referred to as the block axis, while maintaining low reflectivity and high transmission for normally incident light polarized along an orthogonal principal direction, referred to as the pass axis, have been known for some time. See, e.g., U.S. Pat. No. 3,610,729 (Rogers), U.S. Pat. No. 4,446,305 (Rogers et al.), and U.S. Pat. No. 5,486,949 (Schrenk et al.).
More recently, researchers from 3M Company have pointed out the significance of layer-to-layer refractive index characteristics of such films along the direction perpendicular to the film, i.e. the z-axis, and shown how these characteristics play an important role in the reflectivity and transmission of the films at oblique angles of incidence. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza et al. teach, among other things, how a z-axis mismatch in refractive index between adjacent microlayers, more briefly termed the z-index mismatch or Δnz, can be tailored to allow the construction of multilayer stacks for which the Brewster angle—the angle at which reflectance of p-polarized light at an interface goes to zero—is very large or is nonexistent. This in turn allows for the construction of multilayer mirrors and polarizers whose interfacial reflectivity for p-polarized light decreases slowly with increasing angle of incidence, or is independent of angle of incidence, or increases with angle of incidence away from the normal direction. As a result, multilayer films having high reflectivity for both s- and p-polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers, over a wide bandwidth, can be achieved.
Some multilayer optical films are designed for narrow band operation, i.e., over a narrow range of wavelengths, while others are designed for use over a broad wavelength range such as substantially the entire visible or photopic spectrum, or the visible or photopic wavelength range together with near infrared wavelengths, for example.
Some reflective films are designed to reflect light specularly, such that a collimated incident beam is reflected as a collimated or substantially collimated (e.g., having a full-width-at-half-maximum power of no more than 1.0 degrees, or no more than 0.3 degrees) reflected beam. A conventional household or automotive mirror is an example of a specularly reflective film. Other reflective films are designed to reflect light diffusely, such that a collimated incident beam is reflected into a large cone, such as an entire hemisphere, of different scattering directions—for example, the reflected light may have a full-width-at-half-maximum power of at least 15 degrees, or at least 45 degrees). “Flat white” paint is an example of a diffusely reflective film.
In some cases, it is desirable for a reflective film to provide a mixture or suitable balance of specular reflection and diffuse reflection. We refer to such films as “semi-specular” reflective films. One application for such a film may be an edge-lit optical cavity that emits light over an extended area, which may be useable as a backlight, for example. Three such cavities are depicted in FIGS. 1a, 1b, and 1c. An edge-mounted light source may be mounted at the left end of each cavity, but is omitted from the drawings for generality.
A pure specular reflector performs according to the optical rule that “the angle of reflection equals the angle of incidence.” This is seen in the hollow cavity 116a of FIG. 1a. There, front and back reflectors, 112a, 114a are both purely specular. A small portion of an initially launched oblique light ray 150a is transmitted through the front reflector 112a, but the remainder is reflected at an equal angle to the back reflector 114a, and reflected again at an equal angle to the front reflector 112a, and so on as illustrated. This arrangement provides maximum lateral transport of the light across the cavity 116a, since the recycled ray is unimpeded in its lateral transit of the cavity 116a. However, no angular mixing occurs in the cavity, since there is no mechanism to convert light propagating at a given incidence angle to other incidence angles.
A purely Lambertian (diffuse) reflector, on the other hand, redirects light rays equally in all directions. This is seen in the hollow cavity 116b of FIG. 1b, where the front and back reflectors 112b, 114b are both purely Lambertian. The same initially launched oblique light ray 150b is immediately scattered in all directions by the front reflector 112b, most of the scattered light being reflected back into the cavity 116b but some being transmitted through the front reflector 112b. Some of the reflected light travels “forward” (generally to the right as seen in the figure), but an equal amount travels “backward” (generally to the left). By forward scattering, we refer to the lateral or in-plane (in a plane parallel to the scattering surface in question) propagation components of the reflected light. When repeated, this process greatly diminishes the forward-directed component of a light ray after several reflections. The beam is rapidly dispersed, producing minimal lateral transport.
A semi-specular reflector provides a balance of specular and diffusive properties. In the hollow cavity 116c of FIG. 1c, the front reflector 112c is purely specular but the back reflector 114c is semi-specular. The reflected portion of the same initially launched oblique light ray 150c strikes the back reflector 114c, and is substantially forward-scattered in a controlled amount. The reflected cone of light is then partially transmitted but mostly reflected (specularly) back to the back reflector 114c, all while still propagating to a great extent in the “forward” direction.
Semi-specular reflectors can thus be seen to promote the lateral spreading of light across the recycling cavity, while still providing adequate mixing of light ray directions and polarization. Reflectors that are partially diffuse but that have a substantially forward directed component may thus transport more light across a greater distance with fewer total reflections of the light rays. Reference is made to PCT publication WO 2008/144644 (Weber et al.).
Certain design challenges arise when combining a diffusing layer with an MOF. Reference in this regard is made to PCT publication WO 2007/115040 (Weber), “Wide Angle Mirror System”. The design challenges stem from the MOF effectively being optically immersed in a medium of refractive index greater than air, such that light scattered at highly oblique angles by the scattering layer can propagate through the microlayers of the MOF at angles (“supercritical” angles) that are more oblique than the critical angle for the MOF when immersed in air. This effect, combined with the fact that the reflection band of the MOF shifts to shorter wavelengths as the propagation angle increases, and the fact that the spectral width of the reflection band is limited by the number of optical repeat units (ORUs) of microlayers used in the MOF, can result in some of the scattered light, particularly at longer wavelengths, propagating all the way through the MOF to the back or rear major surface thereof. Any dirt or other disturbances such as absorbing materials that are present on such back major surface may cause that light to be absorbed or otherwise lost, detracting from the total reflectivity of the construction. Some solutions to these design challenges are discussed in the '040 PCT publication. However, additional solutions would be of benefit to optical system manufacturers and designers.