Backlights can be considered to fall into one of two categories depending on where the internal light sources are positioned relative to the output area of the backlight, where the backlight “output area” corresponds to the viewable area or region of a luminaire or backlit display device. The “output area” of a backlight is sometimes referred to herein as an “output region” or “output surface” to distinguish between the region or surface itself and the area (the numerical quantity having units of square meters, square millimeters, square inches, or the like) of that region or surface.
A first category is “edge-lit” backlights. In an edge-lit backlight, one or more light sources are disposed—from a plan-view perspective—along an outer border or periphery of the backlight construction, generally outside the area or zone corresponding to the output area. Often, the light source(s) are shielded from view by a frame or bezel that borders the output area of the backlight. The light source(s) emit light into a component referred to as a “light guide,” particularly in cases where a very thin profile backlight is desired, as in laptop computer displays. The light guide is a clear, solid, and relatively thin plate whose length and width dimensions are on the order of the backlight output area. The light guide uses total internal reflection (TIR) to transport or guide light from the edge-mounted lamps across the entire length or width of the light guide to the opposite edge of the backlight, and a non-uniform pattern of localized extraction structures is provided on a surface of the light guide to redirect some of this guided light out of the light guide toward the output area of the backlight. Such backlights can also include light management films, such as a reflective material disposed behind or below the light guide, diffusive films, reflective polarizing film and prismatic BEF film(s) disposed in front of or above the light guide to increase on-axis brightness.
The second category is “direct-lit” backlights. In a direct-lit backlight, one or more light sources are disposed—from a plan-view perspective—substantially within the area or zone corresponding to the output area, normally in a regular array or pattern within the zone. Alternatively, one can say that the light source(s) in a direct-lit backlight are disposed directly behind the output area of the backlight. A strongly diffusing plate is mounted above the light sources to spread light over the output area. Again, light management films, such as diffuser films, a reflective polarizer film and prismatic BEF film(s), can also be placed atop the diffuser plate for improved on-axis brightness and efficiency.
Backlights of one type or another are often used with liquid crystal (LC)-based displays. Liquid crystal display (LCD) panels, because of their method of operation, utilize only one polarization state of light, and hence for LCD applications it may be important to know the backlight's brightness and uniformity for light of the correct or useable polarization state, rather than simply the brightness and uniformity of light that may be unpolarized. In that regard, with all other factors being equal, a backlight that emits light predominantly or exclusively in the useable polarization state is more efficient in an LCD application than a backlight that emits unpolarized light. Nevertheless, backlights that emit light that is not exclusively in the useable polarization state, even to the extent of emitting randomly polarized light, are still fully useable in LCD applications, since the non-useable polarization state can be easily eliminated by an absorbing polarizer provided at the back of the LCD panel. Such backlights that emit randomly polarized light are often preferred for backlighting of graphic display and signage, which do not have a preferred polarization of illumination light. Drawbacks or limitations of existing direct-lit and edge-lit backlights can include inefficiencies associated with the light management film stacks required for spatial uniformity of brightness on the output area (surface), for light collimation, and in some instances, for output light polarization control. The backlight system described herein may provide one or more advantages over prior backlight systems.