One of the most common light fixtures is the recessed can downlight (RCD), which is an open-bottom can that contains a lightbulb, most commonly an incandescent bulb or a fluorescent bulb. The fixture is typically connected to the power mains at 120 to 277 volts, 50/60 Hz. RCDs are generally installed during the construction of a building before the ceiling material (such as plaster or gypsum board) is applied. Therefore, they are not easily removed or substantially reconfigured during their lifetime.
RCDs generally also accommodate lightbulbs having various sizes, different overall dimensions (i.e., length, width, and diameter), and varied light-distribution capabilities. For example, various bulbs have narrow, medium, or wide (flood) light distributions. Therefore, the internal features of the RCD are constructed to accommodate many (if not all) different bulb types. Such features include mechanisms to adjust the vertical position of the bulb socket, as well as reflectors that channel and distribute the light. Because there are so many different lightbulbs and finishes, a very large number of trim rings and optics combinations may be utilized in RCDs, in addition to the various spacers that accommodate the bulbs. Thus a complex arrangement of parts is needed for each RCD that is produced.
Because LEDs have very high efficiency (e.g., 100 lumens per watt compared to 10-15 lumens per watt for incandescent or halogen lights) and a long lifetime (e.g., 10,000-100,000 hours), they are attractive for virtually all lighting applications. LED retrofit fixtures have been designed to replace existing, installed RCD fixtures. U.S. Ser. No. 14/660,159, filed on Mar. 17, 2015, for example, describes a retrofit kit that enables retrofitting of a wide variety of different RCDs (e.g., RCDs incorporating fluorescent bulbs) with a single “universal” LED-based fixture that is quickly and efficiently installable. Within the retrofit kit, the LED light sources and control electronics are modularized for ease of assembly and installation. In addition, the retrofit kit may be utilized substantially independently of the specific lightbulb being replaced yet conforms to the volume and desired level of illumination of the existing RCD.
A retrofit kit as described in the '801 application may include a discrete driver module featuring circuitry for supplying power to and controlling the LED light source(s), as well as, in various embodiments, circuitry for controlling the LEDs based on sensed temperature (for example, the temperature of the LEDs themselves or of one or more temperature sensors such as thermistors in close proximity to the LEDs). The driver module is electrically connected to a discrete lighting module featuring one or more LEDs (for example, several LEDs arranged in a rectilinear array) via a flexible conduit that contains and protects one or more wires carrying electrical signals between the two modules. The lighting module may incorporate one or more temperature sensors for sensing the temperature of the LED(s) and/or the ambient temperature, and the driver module may incorporate thermal-feedback circuitry for controlling power supply to the LED(s) based on the sensed temperature. The lighting module also typically incorporates an integral or removable heat sink.
Unfortunately, the high power levels often required to drive an LED retrofit solution to maintain previous lighting levels may generate so much heat that merely heat-sinking the LEDs can prove insufficient. LED lifetime can be substantially shortened by excessive operating temperatures; in general, it is advisable to maintain the LED below 100° C. during operation. Indeed, even where such passive measures as finned heat sinks are sufficient from a performance perspective, they may be incompatible with the physical restrictions of a retrofit; the volume within a light source such as an RCD is limited, and the airflow needed for effective heat sinking may be impossible within the fixture space. Even when there is adequate room for a large heat sink, it may displace the light source so as to create glare and ultimately impose a cap on light output. For example, it may be necessary to change the configuration to position the LEDs lower in the can, resulting in an out-of-focus condition for the LEDs and/or considerable visual glare, which is highly undesirable. Placing the heat sink outside the can (reflector) also is usually not possible due to the mounting and support structure of the light fixture. Moreover, the region above the can may be filled with insulation and the building's structural elements, such as rafters and beams, may either restrict airflow or make the use of the space impossible.