Illumination – Supported by vehicle structure – Light modifier
Reexamination Certificate
1999-05-20
2001-08-14
Husar, Stephen (Department: 2875)
Illumination
Supported by vehicle structure
Light modifier
C362S292000, C362S332000, C362S354000
Reexamination Certificate
active
06273596
ABSTRACT:
BACKGROUND OF THE INVENTION
Almost all of the prior art of lens design can be subsumed under the imaging category, the purpose of which is an accurate rendering of the appearance of an object. Lenses have seen little use in the illumination field, where reflectors have predominated because of their lower cost and superior efficiency when used with conventional light sources. Thus, there has been little emphasis on the design of lenses for illumination, where the purpose is the fulfillment of a prescribed pattern of light distribution, and imaging of the light source is undesirable. Many of these prescriptions are for rectangular and other non-circular light patterns.
Because optical lenses have overwhelmingly been formed by grinding and polishing, their surfaces are figures of revolution, such as spheres, tori, and cylinders. In general, figures of revolution are not suitable for forming illumination patterns that are not circularly symmetric.
The present invention embodies a different method of lens design than that of optical imaging lenses. It utilizes shapes that are not figures of revolution, but which can be manufactured by molding of plastic or glass. They are especially suitable for use with light-emitting diodes, the tiny sizes of which allow such lenses to be small and easier to injection mold.
The most important lighting prescriptions addressed by the present invention are for vehicular lamps, by the Society of Automotive Engineers, and for ships, by the United States Coast Guard. Particular embodiments of the present invention address these prescriptions. These are far-field prescriptions for an angular distribution of light, whereas illumination prescriptions are for some nearby surface, such as the walls or ceiling of a room.
The current art of luminaire design utilizes the method of computerized searching through a number of design variations, with ray tracing used to evaluate the closeness of a candidate luminaire's output to the desired light distribution. This technique is not completely satisfactory because the vastness of the design space precludes an exact match of luminaire output to prescribed output, given that the design starting point is only a guess.
An example of traditional design is the lenslet array utilized in automotive signal lamps. Numerous small lens elements, usually spherical, cylindrical, or toric, transform the collimated beam from a reflector into a wide-angle beam shaped to fulfill government standards. Such combinations of reflector and lenslet arrays, however, typically have poor values of efficiency, such as one third. Although the reflector can be blamed for much of this inefficiency, also at fault is the restriction of lenslet shapes to spheres, cylinders, and torics (formed by rotating cutting elements), which greatly limits the designer's ability to match the shape of the output beam to the prescribed pattern. Such a match maximizes efficiency, since every point of the specification can be met with a minimum amount of light.
The general design of rotationally symmetric luminaires uses the method of matching the cumulative distribution of source intensity with that of the desired output. Cumulative intensity runs from 0 to 100%, starting at the optical axis and going outwards to the edge of the desired output pattern. Another cumulative distribution is calculated for the intensity of the light source, over the angular range to be redirected by the luminaire. Then, any angle of a ray from the source, having a particular percentage of cumulative source intensity, is redirected into an output angle having the same percentage of cumulative output intensity. From these two angles is calculated the angle the luminaire surface must have to perform the redirection. Then the actual luminaire surface is derived by radial integration outwards from an initial starting point. The resultant shape has the slope necessary to redirect the light from a rotationally symmetric source into a prescribed rotationally symmetric output pattern.
This method of matching cumulative distributions is not suitable, however, for cases where either the source intensity distribution or the desired output pattern are not figures of revolution, that is, azimuthally constant. (An example of such a source is a light-emitting diode with two bonding pads.) This is because a cumulative distribution is inherently one-dimensional, while the two dimensions of angle space prevent a unique line integral from being used to calculate a meaningful index of the shape of the distribution.
In optical lens design, the conceptually closest design method is that of anamorphic lenses. These, however, are designed for a prescribed distortion pattern, a quite different matter than fulfilling a prescribed variation in luminous intensity.
SUMMARY OF THE INVENTION
The present invention concerns a general class of illumination lenses that can accurately match a source with a particular desired output, when either or both of these are not figures of revolution. No trial and error processes are required for their design. The mathematical discipline of differential geometry is the basis for the generation of the shapes of particular lenses. As with the above-mentioned method of rotational symmetry, there are two basic stages in the design process:
(1) deriving a distribution of surface slope by matching source and output intensity patterns;
(2) generating the luminaire shape by integrating the surface slope.
At each stage, however, surface theory requires completely different design methods than those of the one-dimensional case of rotational symmetry. When surfaces are studied as curved two-dimensional spaces, intrinsic differential geometry is involved, regarding properties unaffected by folding, but altered by stretching.
In the case of the present invention, however, the lens surface operates in our everyday three-dimensional space, so that extrinsic differential geometry is used to design it. For example, a polyhedron, such as a cube, has a three-dimensional shape studied by extrinsic differential geometry; but, it also has such intrinsic properties as those revealed by drawing triangles on it that enclose a corner. These triangles will violate the laws of plane trigonometry (i.e., their interior angles do not add to 180°), so that this surface's cubic nature is an intrinsic aspect, independent of it being in three-dimensional space.
The particular use of extrinsic differential geometry for the present invention is in surface synthesis, whereby the lens surface is integrated from the specification of its tilt at a large number of points. The surface tilt is calculated according to the laws of optics from knowledge of how the light from a source must be redirected in order to fulfill a particular prescription. When either the source light or the prescription has an intensity distribution that is not rotationally symmetric, design methods of the prior art are deficient, as discussed above. The present invention utilizes computer calculations to numerically specify a lens surface given the intensity distributions of the source and the desired output.
When dealing with an intensity distribution that is not rotationally symmetric, the arena of expressing this distribution is the surface of a sphere of unit radius, known mathematically as the Gaussian sphere measured in steradians, with 4&pgr; being the solid angle of the entire Gaussian sphere. One steradian is a circle 65.5° across, or a square 59.3° on a side, in either case a total of (180/)
2
=3282.8 square degrees. Luminous intensity is simply the amount of lumens emitted into a solid angle, with a candela=1 lumen per steradian (this has replaced the old term “candlepower”, which could mean either intensity or a measuring unit of intensity). An intensity pattern can be graphically presented with either a two-dimensional map of contours of constant intensity or a three-dimensional map with height representing intensity.
Central to the design method of the present invention, and an object of the invention, is a
Haefliger William W.
Husar Stephen
Teledyne Lighting and Display Products, Inc.
Ward John Anthony
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