US 3840767 A
Description (OCR text may contain errors)
nited States Patent [191 Lake SELECTIVE SPECTRAL OUTPUT METAL HALIDE LAMP  Inventor: William H. Lake, Novelty, Ohio  Assignee: General Electric Company,
- Schenectady, NY.
 Filed: Aug. 23, 1973  Appl. No.: 390,768
 U.S. Cl 313/184, 313/225, 313/229  Int. Cl. H01j 61/18  Field of Search 313/184, 225, 226, 227,
 References Cited UNITED STATES PATENTS 3,234,42l 2/1966 Reiling .L 313/25 Oct. 8, 1974 Primary Examiner-Eli Lieberman Attorney, Agent, or FirmErnest W. Legree; Lawrence R. Kempton; Frank L. Neuhauser [5 7] ABSTRACT 7 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION The invention relates to high pressure metal vapor lamps using an arc dischargein metal halide vapors to produce light or radiation. It is an improvement over the lamp of copending application Ser. No. 218,491, filed Jan. 17, 1972, by applicant jointly with Delmar D. Kershaw and John M. Sato, titled Selective Spectral Output Metal Halide Lamp, and assigned to the same assignee as the present invention.
In lamps intended for general lighting, the goal is gen erally to achieve the highest efficieney possible to gether with balanced white light output. However, there are other applications for electric lamps wherein emission scattered throughout the visible spectrum is undesirable. For instance in reprographie applications for making colored copies, radiation concentrated in the three primary colors, blue, green and red is desired. Likewise in some photochemical applications, high energy emission in specific regionsor bandsis required in order to promote a chemical process, and emission in other bands must be suppressed because it may inhibit the process or produce undesirable reactions.
In the above-mentioned copending application there is described and claimed a lamp sometimes referred to as a ZLT lamp containing zinc iodide, lithium iodide, and thallium iodide as the primary fill speciesfThis lamp fulfilled the needs of a reprographic process utilizing inks with photoelectric responses in three spectral energy ranges, from 440 nm to 480 nm, from 525 nm to 540 nm, and from 630 to 680 nanometers. In the ZLT lamp, the energy in the blue band was provided by a lithium line at 460 nm. Subsequent improvements in the reprographic process called for a blue band located at slightly shorter wavelengths from 400 nm to 420 nm.
' in a red band from 630 to 680 nm.
SUMMARY OF THE INVENTION In accordance with my invention, a source of radiation concentrated in the blue, green, and red bands constituting the three primary colors is a high pressure metal halide vapor are lamp having a filling comprising a limited quantity of zinc iodide serving as a buffer species which produces substantially no radiation and lithiumiodide, thallium iodide and gallium iodide serving as emitter species.
In a preferred embodiment of the invention sometimes known a's'a GZLT lamp wherein the spectral output is concentrated in the blue from 400 to 420 nm in the green from 525 to 540 nm, and in the redfrom 630 to 680 nm, the quantity of Lil is at least 0.2 milligrams per cubic centimeter of envelope volume, the quantity of Znl is from .02 to 1.5 mg/cc. the quantity of Tll is from .02 to 3 rug/cc, and the quantity 01 Gal; is from 0.1 to 1.0 mg/cc. Optionally there may be provided Hg in a quantity less than I milligram per cubic centimeter.
DESCRIPTION OF DRAWING FIG. 1 illustrates a high pressure tubular lamp embodying the invention.
FIG. 2 shows the spectral output of a preferred embodiment of the invention known as the GZLT lamp.
FIG. 3 shows the spectral output of the prior ZLT lamp to provide a basis of comparison.
DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1, a lamp embodying the invention in preferred form comprises an arc tube 1 of quartz or fused silica about 8 millimeters inside diameter and I0 millimeters outside diameter having sealed therein at opposite ends a pair of arcing electrodes 2, 2' defining an arc gap of about 34 centimeters. The lamp is shown with a central portion cut out for ease of illustration; its internal volume is about 17 cc and its overall length is about 41 centimeters. The electrode inleads 3, 3 have intermediate thin molybdenum foil sections 4 hermetically sealed through pinch seals 5 of known l-beam cross section at the ends of the tube. The electrodes each comprise a double layer tungsten wire helix 6 wrapped around a tungsten core wire 7, and may be conventionally activated by thorium oxide applied as a coating on the turns of the helix or filling the interstices between turns. However the same electrodes may also be used without activating material of any kind in order to reduce segregation of species and variation in output along the length of the lamp, as taughtin the earlier mentioned copending application.
In the ZLT lamp which was designed to radiate in the blue from 440 to 480 nm, in the green from 525 to 540 nm and in the red from 630 to 680 'nm and with energy levels within these bands in the approximate ratio 122:2, the following filling was used:
Znl, 0.02 to [.5 mg/cc Lil 0.2 to 2 mg/cc Tll 0.02 to 3 mg/cc Hg Not over 1.0 mg/cc Znl as buffer, the power which may be delivered to the lamp before envelope limiting. temperature is reached is almost twice as much as when Hg is used for the buffer. The thallium iodide content also reduces the arc temperature and is a factor in permitting higher input power. Mercury in an amount not exceeding about 0.25 mg/cc, may be added to reduce the reignition voltage by a large factor, as much as threefold, without introducing any substantial characteristic mercury radiation. A somewhat higher proportion of mercury, up to about I mg/ec. may be added to decrease thermal losses to the wall and increase the lifetime of charge carriers in order to improve lamp power factor. reduce current, and lower electrode losses at the same power input.
Inmodifying the ZLT lamp by adding other metal iodides to provide specific spectral responses, the prob lem is to avoid upsetting the balance and high efficiency already present. A suitable emitter must be selected, but the ultimate choice of materials for buffer or emitter species must take into account the ionization potentials and the lowest significant excitation potential (LSEP). Unless the LSEP of the buffer exceeds by at least approximately 2 volts the excitation level of the desired radiation, the buffer radiation will be substantial and if it is outside the desired region, it will reduce lamp efficiency. Only a limited number of metals have primary radiation in the desired region. Of these many have so-called picket fence radiation, that is spectral lines scattered throughout the visible range and are not suitable. The iodides of others have too low a vapor pressure to provide sufficient radiation.
l have discovered that gallium iodide may be used as an effective emitter in combination with zinc iodide as buffer and lithium iodide and thallium iodide as emitters. This means that gallium iodide may be added to the ZLT lamp filling to shift its blue band radiation to a shorter wavelength while leaving the green band and red band radiation substantially unaffected. The principal line radiation of gallium occurs at 417.2 nm and it has another line of about 60% of the amplitude of the principal line at 403.3 nm, as may be seen in FIG. 2. The addition of gallium has the beneficial effect of adding these two lines to the spectrum without appreciably affecting the thallium line at 535 nm in the green band or the lithium line at 670 nm in the red band, as may be seen by comparing the spectral curve of FIG. 2 with that of FIG. 3.
The quantity of gallium iodide added to the lamp should be from 0.1 to 1.0 milligrams per cubic centimeter of lamp volume. Below 0.1 mg/cc, the radiation from the gallium is insignificant. As the quantity of gallium is increased, the radiation output at 417.2 nm, which is its principal line, saturates rather quickly and at a level of about 0.5 mg per cc of lamp volume, the continuum radiation begins to rise. This sets a practical limit of about 1 mg per cc on the quantity of gallium desirable.
By way of example of a GZLT lamp constituting a preferred example of the invention, an envelope such as illustrated in FIG. 1 and previously described was provided with the following fill and gave the spectral curve of FIG. 2.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A high intensity lamp providing radiation concentrated in selected blue, green, and red spectral bands comprising:
a hermetically sealed light-transmissive envelope;
a pair of arc electrodes sealed therein and defining an arc gap;
an inert starting gas at a pressure of a few torr therein;
and a charge therein comprising per cubic centimeter of envelope volume from about .02 to 1.5 milligrams of Znl from about 0.2 to 2.0 milligrams of Lil, from about .02 to 3.0 milligrams of Tll, and from about 0.1 to 1.0 milligrams of GaI 2. A lamp as in claim 1 comprising in addition not over about 0.25 milligrams of mercury per cubic centimeter of envelope volume 3. A lamp as in claim 1 containing in addition not over about 1 milligram of mercury per cubic centimeterofenvelope volume. i
4. A high intensity lamp providing radiation concentrated in the blue spectral band from 400 to 420 nm, in the green spectral band from 525 to 540 nm and in the red spectral band from 630 to 680 nm comprising:
a tubular elongated sealed light-transmissive envelope; a pair of arc electrodes sealed therein and defining an arc gap; an inert starting gas at a pressure of a few torr therein; and a charge therein comprising per cubic centimeter of envelope volume from about .02 to'l.5 milligrams of Znl from about 0.2 to 2.0 milligrams of Lil, from about .02 to 3.0 milligrams of Tll, and m t .-.Q.!. l @ms9 G aa 5. A lamp as in claim 4 containing in addition not over about 0.25 milligrams of mercury per cubic centimeter of envelope volume.
6. A lamp as in claim 4 containing in addition not over about 1 milligram of mercury per cubic centimeter of envelope volume.
7. A lamp as in claim 4 containing per cubic centimeter of envelope volume about 0.29 mg of Znl about 0.42 mg of Lil, about 0.42 mg of Tll, about 0.33 mg of Gal and about 0.08 mg of Hg.