US2392305A - High efficiency fluorescent lamp - Google Patents

Description

HIGH EFFICIENCY F'LUORESCENT LAMP Filed April 13, 1943 www-,MM

ATTORNEY Patented Jan. 8, 1946 HIGH EFFICIENCY FLUORESCENT LAMP Norman C. Beese, Verona, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh.

Pa., a corporation of Pennsylvania Application April 13, 1943, Serial No. 482,845

4 Claims.

The present invention relates to gaseous electric discharge lamps 1 and more particularly to what are known to the art as iluorescent lamps.

It has heretofore been customary to employ mercury vapor as the discharge supporting medium in lamps of this type and in order to convert the invisible ultra-violet resonance radiations of low pressure lamps into visible light, a coating of fluorescent material, commonly called phosphors, is applied to the surface of the lamp envelope. In cases where the lamp operates at high pressures and temperatures, such coating is applied to the wall of the outer container.

In the present commercial fluorescent lamps phosphors that are utilized are excited chieily by the invisible mercury resonance radiations at 2537 Angstrom units. The color of the light emitted by such lamps is controlled by the mixture of phosphors applied as a coating to the wall of the envelope. There is, however, a certain amount of monochromatic yellow, green and blue visible light approximating five lumens per Watt which comes through the coating directly from the mercury arc discharge and causes certain objects to appear somewhat unnatural.

Also, since present type iluorescent lamps operate most eiiiciently with a bulb temperature of about 45 C.envelop'es of considerable surface area must be employed which precludes a source of high intrinsic brightness and makes such lamps difficult to adapt to general home lighting. Experimental data also indicates that under certain conditions there is practically a 100% quantum transfer eiiiciency in converting ultra-violet radiation into visible light by excitation of a phosphor. However, one quantum of mercury resonant radiation at 2537 Angstrom units converted into visible light at 5074 Angtsrom units involves a 50% energy loss, since the energy associated with each quantum is inversely proportional toits wavelength. Consequently, the same quantum of visible light at 5074 Angstrom units, if produced by one quantum of ultra-violet energy at 3650 Angstrom units, involves a loss of only about 25%. Hence an efficient source of visible light can be produced by the proper selection of a phosphor whose excitation response is as close to the near ultra-violet at about 4000 Angstrom units as possible. In contrast to the light emitted by a mercury vapor lamp, it has been found that the radiation from a lamp utilizing arsenic as the discharge supporting medium is exceptionally poor in visible light and substantially devoid of the objectionable and pronounced yellow, green lamp.

Moreover, a high pressure arsenic lamp mounted inside a fluorescent glass outer envelope or Within a clear or opal outer bulb coated with a suitable phosphor would not be as temperaturedependent as the present type commercial uorescent lamps and can therefore be used over a much wider ambient temperature range. In addition, an arsenic lamp produces a continuous spectrum with its maximum radiant energy in the near ultra-violet region at about 3650 Angstrom units. Consequently, to utilize the high quantum transfer eiilciency of the near ultraviolet arsenic spectrum, it is necessary only to select a suitable phosphor that is capable of absorbing these radiations and converting them efllciently into visible radiations.

The conventional phosphors used in present type commercial fluorescentlamps, such as silicates, tungstates, or borates, do not respond to the. near ultra-violet radiations and hence are inert. However, the group of sulphide phosphors, such as zinc, cadmium, calcium and strontium, are highly activated by these near ultra-violet radiations. Certain minerals like Wernerite (comercial composition being principally a calcium-aluminum-silicate) are also very luminous when excited by near ultra-violet radiations. Morever, made synthetically, such material has all the advantages of the sulphides with the addition of much greater chemical stability and hence renders it readily adaptable for lamp phosphors. l

Accordingly, while the utilization of an arsenic lamp is the primary source of invisible radiations in combination with a phosphor whose excitation response is in the region of maximum radiant energy from the lamp, a highly eilicient source of visible light is produced since the visible light is almost entirely dependent upon the excitation of the phosphor coating and since, as above noted, the visible light, and particularly the monochromatic yellow, green and blue colors, is not present. On the other hand, a high'pressure mercury lamp emits principally the monochromatic yellow, green and blue colors from the mercury arc. together with a relatively small contribution from the phosphor, which merely corrects slightly the light from the mercury lamp and amounts to only a few per cent of the total visible radiations.

It is accordingly the object of the present invention to provide a fluorescent lamp which is viearly 1atiiicient and of comparatively long commerc e' l Another object of the present invention is the .provision of a iluorescent lamp wherein the discharge generates a continuous spectrum and a coating material is employed whichhas the peak of its response near the wavelength band from the discharge, thus producing a highly emcient lamp. l

facilitate starting of the discharge.

By reference now to Fig. 3, Ait will be noted that a lamp utilizing arsenic for supporting the discharge results in the production of a band or continuous spectrum extending over a range from about l2800 Angstrom u nits through ythe longer A further object of the present invention is the provision of a fluorescent lamp employing a discharge supporting medium generating a continuous spectrum and wherein a coating material is employed which exhibits no fatigue when excited by suchcontinuous spectrum. thus prolonging the commercially useful life of the-lamp.

Still further objects of the present invention will become obvious to those skilled -in the art by reference to the accompanying drawing wherein:

Fig. 1 is an elevational view of an electric energy translation device in the form of vai high pressure discharge lamp constructed in accord'- ance with the' present invention;

, Fig. 2 is a fragmentary view similar to Fig. 1

, but showing a modiiication which the lamp of the present invention may take, and

Fig. 3 is an illustration of the line y spectrum produced by mercury vapor in comparison with the continuous spectrum produced by arsenic and also showing the spectral response of sulphide phosphors.

Referring now to the drawing in detail, the lamp as shown'in Fig. 1 comprises an enclosing envelope 5 of suitable vitreous material, such as hard or soft glass pervious to visible light. Appropriately secured to the envelope is a base 6 of the usual type to enable the lamp to be screwed into a socket.

The envelope 5 is provided with a reentrant stem press 'I to which is hermetically sealed a pair oi leading-in conductors 8 and 9 of suitable material and as shown are o1' rod-like form widening out immediately above the stem press and extending parallel to each other longitudinally of the enclosing envelope 5.

A pair of bridges I and I2 of suitable insulating material tie the leading-in conductors together for the purpose of forming a rigid mount for a discharge lamp I3. 'I'his lamp as shown is of the high pressure type and comprises an en` velope, I4 of vitreous material capable of `Withstanding high pressure and temperature and permeable to ultra-violet radiations such, for example, as quartz. Interiorly of the envelope I4, the customary electrodes I and I6 are provided for sustaining a discharge through the gaseous medium within the envelope. In addition, a pair of metallic shields I 1 and I8 are carried by the bridges adjacent the supported ends of the lamp I3 for the purpose of conserving heat and preventing condensation of the gaseous medium within the envelope I4. v

'I'he lamp electrodes are connected to the leading-in conductors 8 and 9 by iiexible connections I9 and 20 and for starting purposes an impedance device 22 is connected between the leadingin conductor 8 and the customary starting electrode 23, as shown. In order to make the lamp generate a continuous spectrum, arsenic is utilized as the ionizable medium for sustaining the discharge between the electrodes I5 and I6, which medium attains a pressure of approximately 3 atmospheres during operation. In addition, an inert gas, such as argon, neon or the like, or a mixture of such gases, at several milliwavelength near ultra-violet and visible region beyond 4350 Angstrom units. Acomparison with 'the spectral distribution from a mercury vapor lamp, as shown' immediately above the arsenic spectrogram of Fig. 3, indicates that throughout this same spectral range there are but relatively few radiation lines, whereas with arsenic the entire range is encompassed by the continuous spectrum with maximum radiantenergy being in the xenatrsultra-violet region at V about 3650 Angstrom An arsenic lamp isknown to be a very poor generator of visible light even at high pressure. For example, a lamp operating at watts gave only 250 lumens or about 2:8 lumens per watt.

For the purpose of converting the near ultraviolet into visible light, the interior surface of the envelope 5 is providedwith a coating 24. Experiments have shown that the sulphide group of phosphors are most suitable for this coating because theyhave a. spectral response in the near ultra-violet just at theregion where the arsenic lamp gives out its maximum radiant energy. Moreover, I have discovered that such sulphide group of phosphors exhibitv no appreciable fatigue after a comparatively long useful life.

Forthese reasons the coating 24 comprises zinc sulphide, zinc cadmium sulphide, or a mixture of these or other sulphides.' depending upon the color of visible light desired, which, as shown by the lower portion of Fig. 3, responds to wavelengths ranging from about A2500 Angstrom units the eiciency of the lamp does not become Vim-` paired even after many hundred hours of operation. For best results the phosphor coating must be kept relatively cool, i. e., below about C., and since the lamp I3 operates at a relatively high temperature, the outer envelope is provided as an isothermal vsurface sumciently large as to maintain the coating within the above noted temperature range. This, for example, requires an outer envelope area of about 250 sq. cm. for a 10G-watt lamp which is equivalent to a spherical envelope 3.5 inches in diameter or a prolate ellipsoid of 8 cm. minor axis and 12.5 cm. major axis. The outer envelope 5 may be highly evacuated or iilled with nitrogen gas, care being taken to exclude moisture.

The modification as shown in Fig. 2 diiers from that previously described merely in the conguration of the lamp and its structural support. As shown in this gure, the lamp 33 is of U-shaped conguration with its ends being supported by heat shields 34 and 35, the latter of which are secured to a single bridge 36. The'reentrant press 1 in this modication is provided with a support rod 31 sealed thereto which forms a support for the bridge 36 and at the same time extends above the bridge, being provided with a bifurcated end 38 to form a cradle for the base portion of the lamp 33, thus increasing the rigidity of the support assembly. The leading-in conductors and 9, although much shorter in length, connect to the respective electrodes I and I6 in the same manner as previously described in Fig. 1, except that no necessity exists for a ilexible connection such as at I9 and 20 of` Fig. 1. The impedance 22 is again connected to the starting electrode 23v and in all other respects this modification is identical to the structure of Fig. 1.

The advantage in this design is the elimination of supports in the path of the ultra-violet between the lamp 33 and the outer envelope 5, which otherwise shadows a small percentage of the emitted ultraviolet radiations as in Fig. l. In addition, since the lamp 33 is U-shaped, its axis of curvature closely approximates that of the outer envelope 5, thereby tending to dispose the greater area of the coating 24 equi-distant from the emitting surface of the lamp 33 and increasing slightly the uniformity of excitation of the coating.-

It thus becomes obvious to those skilled in the art that a highly eilicient iluorescent lamp is herein provided wherein a continuous spectrum is produced by the discharge having its maximum radiant energy in the near ultra-violent region of the spectrum. Moreover, a coating is disposed in the path of the emitted near ultraviolet radiations which has a spectral response in the precise region where the lamp emits its maximum radiant energy, thereby producing a highly efficient iluorescent lamp.A

Although several embodiments of the present invention have been shown and described, it is to be understood that still further modifications may be made without departing from the spirit and scope of the appended claims.

I claim: l

1. A gaseous electric discharge lamp comprising a vitreous envelope pervious to ultra-violet radiations, electrodes therein for initiating and sustaining a discharge upon the application of a potential thereto, an ionizable medium in said envelope including arsenic for generating a continuous spectrum with its maximum radiant energy in the near ultra-violet region at approximately 3650 A. U.. a container surrounding said discharge lamp, and a fluorescent coating on the surface of said container comprising a sulphide phosphor having its spectral response in thesame region as the maximum radiant energy of the continuous arsenic spectrum from said lamp at approximately 4,000 A. U. and excited-by the latter to convert the invisible radiations and form a highly etiicient source ot visible radiations,

2. A gaseous electric discharge lamp compris' ing a vitreous envelope pervious to ultra-violet radiations, electrodes therein for initiating and sustaining a discharge upon the application of a potential thereto, an ionizable medium in said envelope including arsenic for generating a continuous spectrum with its maximum radiant energy in the near ultra-violet region at approximately 3650 A. U., a container surrounding said discharge lamp, a fluorescent coating on the sur- `face of said container comprising a sulphide phosphor having its spectral'response in the same region as the maximum radiant energy of the continuous arsenic spectrum from said lamp at approximating 4,000 A. U. and excited by the latter to convert the invisible radiations and form a highly eflicient source of visible radiations, and

said container constituting an isothermal surface of suilicient area to maintain the temperature of the uorescent coating below about 150 C.

3. VA gaseous electric discharge lamp comprising a vitreous envelope pervious to ultra--violet radiations, electrodes therein for initiating and sustaining a. discharge upon the application of a potential thereto, an ionizable medium in said envelope including arsenic for generating a continuous spectrum with its maximum radiant en-` ergy in the near ultra-violet region at approximately 3650 A. U., a container surrounding said discharge lamp, and a fluorescent coating on the surface of said container comprising zinc sulphide having its spectral response in the same region as the maximum radiant energy of the continuous arsenic spectrum from said lamp at approximately 4,000 A. U. and excited by the latter to convert the invisible radiations and form` a highly eiilcient source of visible radiations.

4. A gaseous electric discharge lamp comprising a vitreous envelope pervious to ultra-violet radiations, electrodes therein for initiating and sustaining a discharge upon the application of a potential thereto, an ionizable medium in said envelope including arsenic for generating a continuous spectrum with its maximum radiant energy in the near ultra-violet region at approximately 3650 A. U., a container surrounding said discharge lamp, and a fluorescent coating on the f at approximately 4,000 A. U. and excited by the latter to convert the invisible radiations and form;

a highly efiicient source of visible radiations.

NORMAN C. BEESE.

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