US 2238784 A
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Patented Apr. 15, 1941 suac'rmc msonancn DEVICE William J. Scott, James T. Anderson, and Robert S. Wells, Rugby, England, assignors to General Electric Company, a corporation of New York No Drawing. Application December 19, 1939, Se-
rial No. 310,038. In Great Britain January 16,
l 3 Claims.
Our invention relates in general to electric discharge devices, and more particularly to electric discharge devices of the type in which a luminescent material is employed to convert the non-visible radiations of the discharge into visible radiations.
It is well known that when fluorescent powders are used inside electric discharge lamps (especially those operating with a, low-pressure mercury discharge), these powders tend to lose their efliciency. To counteract this loss in efliciency, it has been proposed to shield the fluorescent material'fromthe discharge by means of a layer or admixture of a substance capable of transmitting the exciting ultra-violet radiation from the discharge but preventing ionized mercury from impinging on the fluorescent material. Thus, it has been. proposed to coat the inside surface of the glass lamp envelope with a layer of the fluorescent material, and to protectsuchfluorescent material by means of a binder, such as silicon ester, or by blowing onto the, fluorescent glass. It has also been proposed to mix the fluorescent powder with a protecting medium when applying it to the glass envelope.
coating a thin non-porous layer of a phosphate Certain fluorescent powders are adversely affected by the gas and water vapor evolved from the underlying glass support (envelope); particularly during the exhausting of the lamp. Hence it is desirable to have the protective layer adequately porous or permeable to gas, at least during the initial stages of outgasslng when the possibility of such attackis greatest. It is then in many cases permissible to hermetically seal this porous protective layer to thereby prevent subsequent access of mercury vapor to the powder.
One object of our invention is to provide a protective coating for the fluorescent screens or coatings of electric discharge devices which is porous or.gas-permeable and which is transvention into effect, the glass tube is first ren-- dered fluorescent by coating the inside surface thereof with a layer of fluorescent powder. This layer may be caused to adhere to the glass in any suitable way by a suitable binder such as an enamel, boric acid, phosphoric acid, silicon ester medium, or \it may adhere to the glass merely by nature of its own flneness' and keyingaction. We prefer to app y the fluorescent powder by the method described in co-pending application Serial No. 226,566, T. W. Frech, filed August 24, 1938. The fluorescent powder, with or with out a binder, is applied as a paint-like suspension in a suitable medium, an excess of which is applied to the tube, wetting the inner surface thereof, and then drained out with the tube upright. The fluorescent coating is then baked on and at least part of the suspension medium evaporated or otherwise removed.
' After the baking of' thefluorescent material onto the lamp envelope, one or more layers of a. similar suspension of the protective material is applied in the same manner and the tube then baked at a high temperature to remove all remaining organic components of the suspension medium and cause the coating particles to knit or frit or iuse together, thus forming a porous layer.- It is to be understood. that this layer may be fused together forming a glaze with gas permeable pores at close intervals. This is done, for example, by including a small propor-v tion of particles of different melting point and different co-eflicient of expansion from the main glaze so that numerous small cracks will interlace the glaze when it cools down;
The glaze is preferably made by grinding down one or more ultra-vlolet-transmitting mercuryresistant glasses or transparent. media of the many known types and not by forming the glass in situ from its components as the former method involves less heating or the fluorescent layer.
What we claim as new and desire to secure by Letters Patent of the United States is:
1. An electric discharge device comprising a lglass envelope for the arc discharge, a layer of luminescent materia1 on said glassenvelope, and a protective coating of an ultra-violet tran'sm'it-;
ting gas-permeable glaze on said luminescent material.
2. An electric discharge device comprising a glass envelope for the arc discharge, a layer of luminescent material on said glass envelope, and
' a protective coating of an ultra-violet transmitting glaze on said luminescent material, said glaze consisting of at least two different glasses having different expansion co-eflicients to thereby render'the glaze gas-permeable.
3. An electric discharge device comprising a glass envelope for the arc discharge, a layer of luminescent material on said glass envelope, and a protective coating of an ultra-violet transmitting glaze on said luminescent 7 material, said glaze being interlaced with numerous small cracks to thereby render the same gas-permeable.
WILLIAM J. SCOTT. JAMES T. ANDERSON. R. S. IWELLB.