Metal halide reflector lamp with beam color homogenizer

#The instant invention pertains to metal halide lamps, and, more particularly to metal halide lamps enclosed in a reflective optic. Such applications include, but are not limited to spot and flood illumination, highlighting objects de art, merchandise and facade illumination, and other general illumination applications.

#Low wattage quartz metal halide and miniature ceramic metal halide (HCl) lamps have been on the market for some time. These lamps are designed to be small concentrated sources of light for inclusion into reflectors for down-lighting and concentrated illumination (spots or floods). A key advantage offered by these lamps is the potential replacement of tungsten-halogen PAR or AR reflector lamps with more energy efficient metal halide lamps while preserving good color rendition, and uniform beam color. Examples of these types of lamps are described in U.S. Patent Publication Nos. 2003/0193280 and 2005/0184632.

#However, metal halide lamps in reflector applications tend to exhibit strong color variations in the far field beam which are undesirable and essentially absent in tungsten-halogen PAR lamps. These color variations occur because of segregation in the electric arc of the radiating species, absorption of the salts on the burner interior surface and radiation escaping from the burner which does not impinge on the primary optical control surface. This color separation is somewhat mitigated by the use of dappled glass lenses over the output aperture of the reflector and swirl lines on the interior of the reflector. Still, it would be an advantage to improve the homogenization of the color of the emitted light across the beam pattern of the lamp.

#FIG. 1 is a plot of the measured distribution of illuminance on a target screen placed at 1.6 m from a 70 W HCl burner in a PAR 38 reflector lamp.

#FIG. 2 is a plot of the measured spatial color temperature distribution of the light emitted from a horizontally burning 70 W HCl burner in a PAR 38 reflector lamp.

#FIGS. 3a and 3b are illustrations of a prior art ceramic metal halide reflector lamp (FIG. 3a) and an enlarged view of its jacketed ceramic burner (FIG. 3b).

#For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

#FIG. 1 shows the isolux lines measured for a 70 W HCl PAR38 lamp burning horizontally and projected onto a screen 1.6 m away. As shown, the luminous intensity should decrease uniformly outward from the center (>17500 lx (lumens/m2)) of the beam pattern. However, existing metal halide reflector lamps exhibit a color non-uniformity over this field, particularly when operated in other than a vertical, base-up orientation. The non-uniformity in the correlated color temperature (CCT) for a horizontally operated 70 W HCl PAR38 lamp is shown in FIG. 2. The CCT metric displayed in FIG. 2 is a common metric used to describe the color of the light emitted by a lamp. Another less commonly used metric is to map the CIE chromaticity coordinates (x,y) using the 1931 or 1976 systems.

#The non-uniformity of the metal halide reflector lamps has its roots in the color separation mechanisms described above and may be understood by reference to FIGS. 3a and 3b. In particular, FIG. 3a illustrates how the irregular and uncontrollable positioning of the salt melt pool 5 can affect the light emanating from the discharge volume 2 of burner 7 especially in the isolated solid angle, dΩ=2π(1−cos θ), as defined by polar angle, θ. Unlike the light emitted in directions 13, 15, light emitted from the burner 7 in the solid angle (shown delimited by dashed arrows 10, 11) does not impinge on the primary optical control surface, viz. the reflector 20. Any color variation within this uncontrolled solid angle cannot be easily mixed with the light from the rest of the burner prior to exiting the open end 17 of reflector 20. In fact, when the arc radiation passes through the salt pool (as shown by arrow 3 in FIG. 3b), the radiation is strongly filtered, as the salts absorb preferentially in the near UV and blue. Consequently light from the isolated solid angle dΩ can be reddish yellow. This is illustrated in FIG. 4 which shows the absorption of the salt pool for a typical 3000K rare earth salt blend. In particular, FIG. 4 shows a ratio of spectral radiance of light passing through a salt droplet to light passing through the wall of a polycrystalline alumina burner (as indicated by arrow 4 in FIG. 3b). This preferential wavelength absorption may have the effect of making objects in the periphery appear reddish on one side and bluish on the other.