779,627. Discharge lamps; cathode ray tubes; electrode materials; luminescent screens. VIERKOTTER, P. June 25, 1953 [June 28, 1952 ; June 12, 1953], No. 17685/53. Class 39(1). Light source comprises at least one energy source which radiates upon heating visible light and X-rays and a fluorescent element comprising fluorescent particles dispersed in a medium allowing visible light to pass, the particles having substantially the same refractive index as the medium in which they are dispersed relatively to the visible light emitted by the fluorescent particles under the action of the non-visible radiation from the energy source. The principle of the invention is shown in Fig. 1 in which the energy source 1 is in a gaseous atmosphere or vacuum enclosed by a transparent or translucent envelope 3 on the inner face of which is applied a layer of luminescent material 4 embedded in a medium which can be separated from the enclosed space 2 by a lining 5. The components of the luminescent material and medium can be chosen so that the combination has a clear glass quality. The source of energy 1 can be a conductor or semiconductor through which current passes directly or indirectly or can constitute directly or indirectly the non-conducting envelope of a highly heated carrier of metal for example. The heating of the source 1 is preferably produced by the impact of electrons or ions or by high-frequency oscillations. The source 1 can be heated as an anode by the impact of electrons, a conducting or non-conducting layer of no more than molecular thickness sufficing for the radiation of light, &c. A thin airtight metal spherical or hemi-spherical envelope can be rendered incandescent in a vacuum by electron bombardment to heat an external coating of the envelope made of radiating material in a gas or vapour atmosphere in a glass flask. The source 1 can be a fine-mesh net of wire coated with radiating material or of wire containing the radiating material or of the radiating material alone of large surface and small thickness heated by electron bombardment as an anode. The source 1 can also form a thin layer between two flat or curved fine-mesh nets or can constitute the core or coating of a fine wire coil or spiral or in the form of a tangled knot of wire or foil can surround a cathode. The surface between the layers 5 and 4 can be formed, e.g. as a semi-permeable metal layer, so as to enable the excitation energy to enter the luminescent layer 4 without loss while totally reflecting the light emitted by the luminescent material. A mixture of luminescent material can be embedded in a medium the refractive index of which is approximately the same as that of each of the luminescent materials for the visible light emitted therefrom. In the construction shown in Fig. 2, which is similar to the structure shown in Fig. 3 of Specification 779,628, the envelope 3 which carries the luminescent material 4 encloses the cathode 24 consisting of a peg of material of high melting point provided with tips 25 and thin portions 36. The envelope 3 is evacuated through the pump connection 27. The cathode 24 is connected by a terminal 28 to a transformer 29 which is earthed. The electrons from the cathode 24 impinge on the fine mesh inner surface of the hollow, spherical, thinwalled radiator 32 made of tungsten, carbon, zirconium oxide, cerium oxide or thorium oxide and raise it to incandescence to emit visible light, ultra-violet light and X-rays. The radiator 32 is connected to supports 33 fastened to a metal ring 34 which is earthed together with the luminescent material 4. Reflecting fluorescent screens 35 and a reflecting surface 37 which may contain luminescent material are provided. The screen 37 and its support 31 can be transparent and the reverse side of the support 31 can be coated with luminescent material. The whole support can consist of fluorescent glass.' Several surfaces 37 can be used either parallel or at a slant or can be arranged around the envelope in a circle or polygon. The supply circuit can be arranged so that when the radiator 32 is incandescent the positive halfwave is connected to the cathode 24, the radiator 32 emitting electrons which impinge on the cathode which can be constructed as the radiator, the tips 25 being omitted or serving for momentarily instituting the discharge.