Color-corrected hollow prismatic light guide luminaire

Details Color-corrected hollow prismatic light guide luminaire Color-corrected hollow prismatic light guide luminaire

2002-10-04

2004-09-28

Cariaso, Alan (Department: 2875)

Illumination

Light source and modifier

Including selected wavelength modifier

C362S583000, C362S340000

Reexamination Certificate

active

06796686

ABSTRACT:

TECHNICAL FIELD
The color temperature of light emitted by a hollow prismatic light guide luminaire is varied by yellow hue filtering flare light near the light guide's light input end to counteract aesthetically undesirable far-end reddening gradient of the emitted light.
BACKGROUND
FIGS. 1A-1C
depict a prior art hollow prism light guide luminaire
10
generally representative of those described in U.S. Pat. Nos. 4,615,579; 4,750,798; and, 4,787,708 (Whitehead). As explained in U.S. Pat. No. 5,339,382 (Whitehead) light rays emitted by light source
12
are guided along and confined within light guide
14
by means of total internal reflection. Light guide
14
is housed within an opaque cover
16
having a reflective inner surface
18
and a light emitting aperture
20
. One or more white-colored diffuse light extractors
22
are provided within guide
14
. Light rays guided along guide
14
occasionally strike extractor
22
, causing a random change in the direction of such rays; usually into a direction which guide
14
is unable to confine by total internal reflection, thus allowing such rays to escape from guide
14
. For example, a light ray originating at point
24
is reflected by extractor
22
and strikes guide
14
at an angle which results in further reflection of the ray such that it escapes through the wall of guide
14
and is emitted through aperture
20
in a direction
26
. Similarly, a light ray originating at point
28
strikes extractor
22
, escapes through guide
14
and is emitted through aperture
20
in direction
30
.
Light rays emitted through aperture
20
can be used for interior space illumination. In such case, luminaire
10
is preferably configured to emit substantially uniformly bright light through all points along aperture
20
as is for example explained in U.S. Pat. No. 4,850,665 (Whitehead). But, because the refractive index and light transmissivity characteristics of the dielectric material used to form light guide
14
vary as a function of wavelength, an aesthetically undesirable color gradient is observed along luminaire
10
. More particularly, light guide
14
absorbs some blue light rays, so as distance from light source
12
increases, progressively fewer blue light rays are guided along light guide
14
. Consequently, light emitted through aperture
20
at distances farther from light source
12
is perceived as more “red” than light emitted through aperture
20
at distances closer to light source
12
, even if the emitted light is uniformly bright at all points on aperture
20
. This color temperature drop or far-end “reddening” of light guide
14
as a function of distance from light source
12
is on the order of 600 degrees Kelvin relative to a nominal correlated color temperature for a typical light guide having a length L
G
greater than 20 times the light guide's diameter D
G
.
FIG. 2A
graphically illustrates the color temperature drop along light guide
14
and also shows that luminaire
10
's luminance characteristic is reduced (typically by as much as 30%) as a function of distance along light guide
14
due to light absorption losses.
Preferably, no visually perceptible color gradient is observable along luminaire
10
. A typical prior art technique for reducing the observable color gradient is to vary the color of extractor
22
as a function of distance from light source
12
along light guide
14
. Typically, extractor
22
is located at the end of light guide
14
farthest from light source
12
and has a light transmissivity characteristic which varies as a function of distance from light source
12
along light guide
14
, to achieve the desired uniformly bright light emission through all points along aperture
20
. If extractor
22
has a blue color and varies in width as a function of distance from light source
12
along light guide
14
(less blue extractor material at the end of extractor
22
closest to light source
12
and progressively more blue extractor material toward the end of extractor
22
farthest from light source
12
) reddening of light guide
14
is offset since extractor
22
preferentially passes blue light while absorbing red light as a function of distance from light source
12
along light guide
14
. As
FIG. 2B
graphically illustrates, the red absorption required to effectively offset far-end reddening of light guide
14
reduces luminaire
10
's luminance characteristic by as much as 40% by absorbing a significant fraction (as much as 10%) of the red light rays guided along light guide
14
. Such absorption losses are unacceptable in many lighting situations since they may require a more expensive light source having greater light output capability to achieve a desired minimum output luminance. Moreover, because extractor
22
's shape typically varies along light guide
14
as a function of distance from light source
12
(see
FIGS. 1A and 1C
) luminaire
10
's perceived color depends on the observer's viewing angle relative to luminaire
10
, which is aesthetically undesirable.
This invention offsets far-end reddening in a hollow prismatic light guide luminaire without significantly reducing the luminaire's luminance characteristic.
SUMMARY OF INVENTION
The invention facilitates color temperature variation of light emitted by a luminaire having a hollow prismatic light guide formed of a material which absorbs blue light rays such that more red light rays are guided along the light guide at distances father from a light input end of the light guide than at distances closer to the light input end. The desired color temperature variation is achieved by color filtering flare light rays which escape through the light guide at points close to the light input end and reflecting the color filtered light rays back into the light guide. The filter color lies within a CIE-1931 chromaticity diagram {4800° Kelvin; 570 nm; 600 nm} color gamut triangle.
The amount of color filtering is advantageously varied as a function of distance along the light guide and in inverse proportion to the absorption of blue light rays by the light guide material. This can be achieved by varying the filter's intensity (color saturation) as a function of distance along the light guide, or by varying the filter's width as a function of distance along the light guide, or by varying both the filter's color intensity and width as a function of distance along the light guide.


REFERENCES:
patent: 4615579 (1986-10-01), Whitehead
patent: 4750798 (1988-06-01), Whitehead
patent: 4787708 (1988-11-01), Whitehead
patent: 4850665 (1989-07-01), Whitehead
patent: 5219217 (1993-06-01), Aikens
patent: 5258896 (1993-11-01), Dreyer, Jr.
patent: 5339382 (1994-08-01), Whitehead
“About Light Guides,” web site publication of TIR Systems Ltd., Vancouver, B.C., Canada at http://www.tirsys.com/gl-technology/b_tech.html.
“Colour,” website publication of TIR Systems Ltd., Vancouver, B.C., Canada at http://www.tirsys.com/gl-technologY/b_color.html.
“Colour Change Design Information,” web site publication of TIR Systems Ltd., Vancouver, B.C., Canada at http://www.tirsys.com/gl-application/b_colorchng02.html.

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