Lighting fixtures such as emergency exit lighting fixtures have employed incandescent and/or flourescent illumination devices for many years. Although incandescent and flourescent bulbs are inexpensive, their service life is limited. Because of the fact that they emit essentially white light, however, they may be conveniently placed behind red or green translucent screens for bright and uniform illumination of indicia such as the word "exit" or the figure of a human in motion. The omnidirectional radiation characteristic of incandescent and flourescent devices also enhances bright and uniform illumination of the indicia. However, incandescent devices, and to a lesser extent, fluorescent devices, emit heat, suffer efficiency losses and require heavier power supplies and battery backups.
Light emitting diodes ("LEDs") have in recent times enjoyed greater use in lighting fixtures such as exit lighting primarily because of their longer service life. They also generally require lighter duty power supplies and battery backups. Maintenance costs are greatly reduced because LEDs last about a million hours before degrading to half power.
Although LEDs were first commercialized in the early 1960s, they then featured a luminous efficiency on the order of 0.15 lumen ("lm") per watt ("lm/W") as compared to 3.5 lm/W of a red filtered incandescent lamp. Not until the development of isoelectronically doped indirect semiconductors in the mid-1970s did diodes become brighter and more varied in color. Red, yellow, and green diodes so manufactured could produce a luminous efficiency in the range of 1 lm/W; single heterostructure diodes raised the efficiency to a range of 2 lm/W. Exit lighting use of LEDs increased dramatically, however, after 1989, when double-heterostructure diodes began to exceed the luminous efficiency of red filtered incandescent lamps.
Although such red LEDs have been used in braking lights mounted in the center of automobile rear windows (so called center high-mounted stoplights or "CHMSL's"), the highly directional light pattern emitted by LEDs presents problems in exit lighting. Various approaches have been developed in an effort to diffuse LED light patterns in order to illuminate exit lighting indicia uniformly and to present sufficient brightness. Interior portions of the exit lighting housing have been coated with reflective material, various lighting baffles and reflective surfaces have been provided, and the LEDs have been located within miniature luminaires in an effort to diffuse the lighting sufficiently to illuminate the exit lighting indicia in an acceptable fashion.
One recently developed approach to this problem has been the use of red or green LED's and red or green screens, respectively, to scatter light in a relatively diffuse, more omnidirectional manner. Accordingly, exit lighting manufacturers have employed banks of LEDs, among other structures, behind red screens of materials that are employed to diffuse the light, which screens may in turn be located behind stencils in the exit lighting faceplates forming the indicia to create a bright and evenly illuminated message. Although some of these diffusing materials could also function in a transformational manner as defined below to absorb light in a first frequency band and reemit at a second if the proper sorts of LED's had been employed, to the inventors' knowledge red LED's have been used only with red screens and green LED's with green screens so that only the diffusing properties of such materials are exploited.
The green LED exit lighting fixtures have generally enjoyed less success than the reds. Efforts to position green emitting LEDs behind green screens result in dim, if evenly lighted, indicia. Green LEDs behind green diffusing screens which are simply translucent and do not employ transformation material feature hot spots in which individual LEDs may be discerned as they form dots on the indicia. The tradeoff between hot spots and dimness has plagued the green exit lighting field.