Illumination – Plural light sources – Particular wavelength
Reexamination Certificate
1999-03-26
2001-03-13
Sember, Thomas M. (Department: 2875)
Illumination
Plural light sources
Particular wavelength
C362S235000, C362S296040
Reexamination Certificate
active
06200002
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a luminaire having a reflector which mixes light from a multi-color array of LEDs, and more particularly to a spotlight which generates white light from such an array.
The standard light source for small to moderate size narrow beam lighting for accent lighting and general illumination is the incandescent/halogen bulb, such as a PAR (parabolic aluminized reflector) lamp. These sources are compact and versatile, but they are not very efficient. A given lamp operates at a given color temperature for a fixed power, and while they are dimmable, the color temperature shifts with applied power according to the blackbody law, which may or may not be the variation that the user desires.
An array of LEDs in a each of a plurality of colors offers the possibility of creating a luminaire in which the color temperature may be controlled at any power level, thereby enabling lamp which is dimmable and emits a uniformly white light at any power level.
The English language abstract of JP-A-06 237 017 discloses a polychromatic light emitting diode lamp having a 3×3 array of light emitting diodes of two types, a first type having elements for emitting red light and blue light, and a second type having elements for emitting red light and green light. The stated object is to mix colors so that the mixed color would be recognized as the same color in any direction, but there are no optical provisions to facilitate mixing. It is simply a two-dimensional array of LEDs in a lamp case filled with resin, which would do little more than provide some diffusion.
WO 98/16777 discloses a signal lamp having an array of LEDs in a single color for the purpose of signaling, e.g. a traffic light. A rotationally symmetric housing surrounds the array, but diverges so that the light from the LEDs is transmitted to a collimating Fresnel lens without reflection. If the single-color LEDs were replaced by a multi-colored array, the lens would image the individual LEDs without mixing the colors.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an LED light source that will provide all of the desirable features of PAR lamps, plus the ability to vary and control color temperature, at full power and dimmed, all at greater luminous efficacy.
It is a further object of the invention to provide good color mixing for an extended size of array of LEDs.
It is a still further object to provide a collimated beam of mixed light emerging from the light source.
The light source according to the invention utilizes an array of LEDs, including at least one LED in each of a plurality of colors, for emitting light in each of the plurality of colors. The array is arranged in the entrance aperture of a reflecting tube having an opposed exit aperture from which light is emitted after being reflected and mixed by a circumferential wall extending between the apertures. The light source has an optic axis extending between said apertures centrally of the circumferential wall, and a cross-section transverse to the axis.
According to a preferred embodiment, the cross-section is non-round along at least a part of the optic axis. The cross-section is preferably polygonal along the entire length of the axis. Square and octagonal cross-sections have been found to be suitable for mixing light from the various colors.
It is also preferable for the circumferential wall to diverge from the entrance aperture to the exit aperture, whereby the exit aperture is larger than the entrance aperture. The circumferential wall, seen from the optic axis, preferably has a convex shape, and preferably flares outward toward the exit aperture. That is, the radius of curvature of the wall decreases toward the exit aperture, making the reflector somewhat horn-shaped.
The invention is based on three basic principles. First, that mixing is the opposite of imaging; second, that mixing can be relatively efficient; and third, that it is easier to mix a wide beam than a narrow one.
In an ideal imaging system, each point in the object plane is mapped to a separate point in the image plane. Ideally, no mixing occurs, and in practice this is true on a scale down to the resolution limit of the system. A parabolic reflector retains a certain amount of correlation between input and output, because both its longitudinal shape and its cross-sectional shape contribute to imaging.
A parabolic longitudinal shape converges incident parallel rays upon reflection, and renders diverging incident rays less divergent, rays from a single point being reflected in parallel. Any concave shape looks like a parabola (locally). Conversely, if the longitudinal shape is convex (e.g. horn-shaped) then the incident rays will tend to diverge upon reflection, and nearby input points will tend to be mapped to more distant output points.
A circular cross-section preserves the azimuthal identity of the incident rays. That is, the difference in azimuthal angle between incident and reflected rays is the same, independent of the incident azimuth (in a given longitudinal plane). With a polygonal cross-section, on the other hand, the azimuthal difference varies strongly with the incident azimuth.
To illustrate these principles, four sets of simulations represented in
FIGS. 1
a
to
4
b
were performed using the ray-tracing program ASAP.
FIGS. 1 and 2
represent light distributions from reflectors having parabolic longitudinal shapes with the z-axis as the optic axis, focal points (0,0,0), and a 10 mm aperture in the plane z=0.
FIGS. 3 and 4
represent light distributions from reflectors having convex longitudinal shapes similar to the preferred horn design with a 10 mm entrance aperture in the plane z=0.
FIGS. 1 and 3
represent round cross sections, while
FIGS. 2 and 4
represent square cross sections. In
FIGS. 1
a
to
4
a
the light source is at x=0, and in
FIGS. 1
b
to
4
b
the light source is at x=3 mm. The source emitted a wide Lambertian cone (cone angle 80°) parallel to the z-axis. The light distribution in each figure is simulated in a plane 0.5 m from a point source at z=0, a dot in each field marking the center of the optic axis.
The round parabola of
FIG. 1
shows the strongest imaging, the distribution becoming progressively worse as the source moves off axis (in each case simulations were run at x=0, 1, 2, 3, and 4 mm, the figures representing only x=0 and 3 mm). The square parabola of
FIG. 2
produces broader images that are qualitatively more similar to each other than is the case in FIG.
1
. Nevertheless there are strong features (lines) and abrupt intensity changes (checkerboards) that do not line up with each other in the progression as the point source moves off-axis.
The round horn of
FIG. 3
shows the strong angular imaging of a circular cross-section, but the more slowly varying broad background of the horn shape. The square horn of
FIG. 4
still exhibits residual structure, but the relative intensity differences are minimal, at first glance giving the impression of a featureless distribution. For the stated goal of color mixing from spatially discrete sources, the square horn geometry gives the best results.
The principle that mixing can be relatively efficient must be considered against the notion that the degree of mixing is proportional to the number of reflections N. For a long reflecting tube mixing is good but efficiency is low, because if the reflectivity of the circumferential wall is R, then the transmitted intensity for the ray is R
N
. The reflector shapes according to the invention minimize the number of reflections while maximizing mixing due to the convex shape and the azimuthal shift of reflected rays afforded by the polygonal cross-section.
For the principle that mixing is better for a wide beam than for a narrow one, consider a straight walled tube of length L and side length A. At a given aspect ratio (L/A), the design mixes best for wide angle sources, since the average N is larger. If the size of the exit aperture increases, keeping
Herman Stephen
Marshall Thomas M.
Pashley Michael D.
Shimizu Jeffrey A.
Faller F. Brice
Philips Electronics North America Corp.
Sember Thomas M.
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