1. Field
Exemplary embodiments of the present invention relate to wide-angle illumination patterns suitable for short-throw lighting, and a general design method for generating their surface profiles.
2. Discussion of the Background
Light emitting diodes (LEDs) may generate light in zones so small (a few mm across) that it is a challenge to spread their flux uniformly over a large target zone, especially one that is much wider than its distance from the LED. So-called short-throw lighting, of close targets, is the polar opposite of spot lighting, which aims at distant targets. Just as LEDs by themselves may not produce a spotlight beam, and so need collimating lenses, they may be equally unsuitable for wide-angle illumination as well, and so need illumination lenses to do the job.
A prime example of short throw lighting is the optical lens for the back light unit (BLU) for a direct-view liquid crystal display (LCD) TVs. Here, the overall thickness of the BLU is usually 26 mm or less and the inter-distance between LEDs is about 200 mm. LCD backlighting may consist of fluorescent tubes arrayed around the edge of a transparent waveguide, that inject their light into the waveguide, which performs the actual backlighting by uniform ejection. While fluorescent tubes may be on the backlight perimeter due to their thickness, light-emitting diodes are so much smaller that they can be placed directly behind the LCD display, (so called “direct-view backlight”), but their punctate nature makes uniformity more difficult, prompting a wide range of prior art over the last twenty years. Not all of this art, however, was suitable for ultra-thin displays.
Another striking application with nearly as restrictive an aspect ratio is that of reach-in refrigerator cabinets. Commercial refrigerator cabinets for retail trade commonly have glass doors with lighting means installed behind the door hinging posts, which in the trade are called mullions. Until recent times, tubular fluorescent lamps have been the only means of shelf lighting, in spite of how cold conditions negatively affect their luminosity and lifetime. Also, fluorescent lamps may produce a very non-uniform lighting pattern on the cabinet shelves. Light-emitting diodes, however, may be favored by cold conditions and are much smaller than fluorescent tubes, which allow for illumination lenses to be employed to provide a much more uniform pattern than fluorescent tubes ever could. Because fluorescent tubes radiate in all directions instead of just upon the shelves, much of their light is wasted. With the proper illumination lenses, however, LEDs can be much more efficient, allowing lower power levels than fluorescent tubes, in spite of the latter's good efficacy.
LED illumination lenses can be classified into three groups, according to how many LEDs are used:
(1) Extruded linear lenses with a line of small closely spaced LEDs, such as described in U.S. Pat. Nos. 7,273,299 and 7,731,395.
(2) Circularly symmetric illumination lenses.
(3) Free-form illumination lenses with rectangular patterns, such as described in U.S. Pat. No. 7,674,019.
The first two approaches may require many LEDs in order to achieve reasonable uniformity, but recent trends in LEDs have produced such high luminosity that fewer LEDs may be needed, allowing significant power savings. This is the advantage of the last approach, but free-form lenses generating rectangular patterns have proved difficult to produce, via injection molding, with sufficient figural accuracy for their overlaps to be caustic-free (caustics are conspicuous small regions of elevated illuminance).
The prior art is even more challenged, moreover, when fewer LEDs are needed due to ongoing year-over-year improvements in LED flux output. After all, backlight thickness is actually relative to the inter-LED spacing, not to the overall width of the entire backlight. For example, in a 1″ thick LCD backlight with 4″ spacing between LEDs, the lens task is proportionally similar to the abovementioned refrigerator cabinet. Because of the smaller size of an LCD as compared to a 2.5 by 5 foot refrigerator door, lower-power LEDs with smaller emission area may be used, typically a Top-LED configuration with no dome-like silicone lens.
Non-specific design methods for addressing this problem may be found in U.S. Pat. Appl. No. 2006/0138437 and U.S. Pat. Nos. 7,348,723; 7,445,370; 7,621,657; and 7,798,679. U.S. Pat. No. 7,798,679, however, contains only generically vague descriptions of that lens profile, and has no specific method of distinguishing the vast number of significantly different shapes fitting its general description.
Experience has shown that illumination lenses may be unforgiving of small shape errors, such as result from unskilled injection molding or subtle design flaws. Very small changes in local slope of a lens can result in highly visible illumination artifacts sufficient to ruin an attempt at a product. Therefore such generic descriptions, such those in U.S. Pat. No. 7,798,679, may be insufficient for practical use, because even the most erroneous and ill-performing lens fulfills them just as well as an accurate, high-performing lens. Further, U.S. Pat. No. 7,798,679 never provides the specific, distinguishing shape-specifications whose precise details are so necessary for modern optical manufacturing.