Electric lamp and discharge devices: systems – Pulsating or a.c. supply
Reexamination Certificate
1999-06-17
2001-02-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
C315S291000
Reexamination Certificate
active
06184633
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a ballast for high-intensity gas is charge lamps. More specifically, it relates to the reduction of vertical color segregation in a high intensity gas discharge lamp by the use of sequential excitation of the input power or the use of frequency sweep in combination with amplitude modulation.
Discharge lamps have been operated in pulsed mode, as illustrated in U.S. Pat. No. 4,904,903.This patent teaches methods of operating fluorescent mercury vapor, sodium and metal halide (MH) lamps, so that the input is electronically, periodically gated for a portion of the wave period. This pulsed operation is effective in eliminating undesirable electromagnetic and radio interference emissions.
Color control of high-intensity discharge (HID) lamps by pulsing techniques is also well known as shown by U.S. Pat. Nos. 4,137,484, 4,839,565, and 4,963,796. Japanese Patent No. 432153 teaches the use of exterior temperature regulation to control the color of the discharge lamp. Other color-controlling methods include interior temperature regulating techniques and varying the salts within the discharge tube.
One of the major problems in the operation of discharge lamps is the deformation of the arc within the discharge tube by convective gas flow. Techniques for stabilizing and centering this arc have been developed. U.S. Pat. No. 5,134,345 illustrates a method of avoiding acoustic frequencies that cause destabilizing phenomena. The method of this patent teaches the detection of arc instabilities, and changing the drive frequencies that cause them.
In U.S. Pat. No. 5,306,987, an arc stabilization technique is illustrated in which the frequency of the drive signal is modulated. A similar method of controlling the arc in discharge lamps is illustrated in U.S. Pat. No. 5,198,727. With this method, the arc is centered by the “acoustic perturbations” induced by the frequency of the drive signals. The acoustic perturbations compel the gas or vapor movement patterns to counter the gravity-induced convection.
U.S. Pat. No. 5,684,367 discloses a method of controlling arc destabilization in HID lamps by amplitude modulation of a high frequency signal and pulsing the lamp, which can be used to change the color characteristics of the lamp.
A new class of high intensity discharge lamps has been produced in the past five years with ceramic (polycrystalline alumina) envelopes. Philips Lighting Company of Somerset, N.J., for example, sells such ceramic discharge metal halide (CDM) lamps under the trademark MASTERCOLOR™. The discharge envelope is cylindrical in shape, and the aspect ratio, i.e., the inner length (IL) divided by the inner diameter (ID) is close to one. At least one new type of cylindrical lamp is being developed which has a much larger aspect ratio. Such new lamps have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs. Projecting an image of the arc onto a screen shows that the bottom part of the arc appears pink while the top part looks green. This is caused by the absence of complete mixing of the atomic metal additives in the discharge. In the upper part of the discharge there is too much thallium emission and insufficient sodium emission. The result of vertical segregation is a lamp with higher color temperature compared to horizontal operation and possibly decreased efficacy.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of the invention to provide an improved ballast for a high-intensity discharge lamp.
It is also an object of the invention to reduce vertical color segregation in a high-intensity discharge lamp.
These and other objects are attained by a first aspect of the invention wherein a high intensity discharge lamp is operated through a current frequency sweep within a sweep time, in combination with amplitude modulation. The typical parameters for such operation are a current frequency sweep from 45 to 55 kHz within a sweep time of 10 milliseconds, a constant amplitude modulation frequency of 24.5 kHz and a 0.24 of modulation index. The modulation index is defined as (V
max
−V
min
)/(V
max
+V
min
), where V
max
is the maximum peak to peak voltage of the amplitude modulated envelope and V
min
is the minimum peak to peak voltage of the amplitude modulated envelope. This frequency range of 45 to 55 kHz is between the first azimuthal acoustic resonance mode and the first radial acoustic resonance mode. Definitions of the acoustic resonances—For a cylindrical lamp the power frequency of the first azimuthal mode is equal to 1.84*C
r
/&pgr;*D where C
r
is the average speed of sound in the radial plane and D is the inner diameter of the lamp.
The power frequency of the first radial acoustic resonance mode is equal to 3.83*C
r
/&pgr;*D where C
r
is the average speed of sound in the radial plane and D is the inner diameter of the lamp.
These and other objects are likewise achieved by a second aspect of the invention wherein a swept high frequency (current) sine wave is followed in time by a fixed current frequency sine wave, corresponding approximately to one half of the power frequency of a longitudinal acoustic mode, this sequence being continuously repeated.
REFERENCES:
patent: 4137484 (1979-01-01), Osteen
patent: 4373146 (1983-02-01), Bonazoli et al.
patent: 4740729 (1988-04-01), Chow
patent: 4839565 (1989-06-01), Osteen
patent: 4904903 (1990-02-01), Pacholok
patent: 4963796 (1990-10-01), Gottschling et al.
patent: 5134345 (1992-07-01), El-Hamamsy et al.
patent: 5198727 (1993-03-01), Allen et al.
patent: 5306987 (1994-04-01), Dakin et al.
patent: 5365151 (1994-11-01), Spiegel et al.
patent: 5508592 (1996-04-01), Lapatovich et al.
patent: 5684367 (1997-11-01), Moskowitz et al.
patent: 5990633 (1999-11-01), Hirschmann et al.
patent: 0615278A1 (1994-09-01), None
patent: 0785702A2 (1997-07-01), None
Dinh Trinh Vo
Franzblau Bernard
Philips Electronics North America Corporation
Wong Don
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