Incandescent lamp with selective color filter

#The conventional incandescent lamps for producing light of a particular color, for example, red, blue or green, are generally of two types. The first uses a so-called absorptive filter in which the desired color is produced by filters placed external to the lamp or by a finish applied directly to the lamp envelope, usually on the outside. The filters have an absorptive action, that is, they absorb light energy in the unwanted portion of the spectrum which is transformed into heat for reradiation. Energy of the desired wavelength (color) is transmitted through the filter. These types of filters generally are of the organic type, e.g. paints, or possibly can be a silicon coating.

#Another type of lamp for producing a selected color utilizes a multi-layer filter coating of a number of non-metallic films of low and high refractive indicies which are vaporized onto the glass envelope. Each layer of the coating is one-quarter (1/4) wavelength thick, resulting in high reflectance at that particular wavelength. Combinations of these materials and their thicknesses produce the desired spectral distribution of transmitted light. In general, such coatings are called "dichroic filters" and have as many as fifteen to twenty-one layers. Such lamps are disclosed, for example, in an article by Beesley entitled "New High-efficiency Color For PAR Lamps Using Multi-layer Interference Coatings" appearing in Illuminating Engineering, March 1964 (pages 208-212).

#The present invention relates to an incandescent lamp for producing a desired color of visible light. The lamp utilizes a coating of a so-called etalon type in which a thin film layer of an insulating material is located between two thin film layers of a metal, the coating being called a composite metal-insulator-metal coating. The thin films of the coating are formed on the wall of the incandescent lamp envelope with the thickness of the individual films of the coating and their inter-relationships selected so as to maximize the coating transmission characteristics to energy produced by the filament for a wavelength of a particular color in the visible range. Also, the coating can be formed to maximize the reflecting properties to energy in the infrared range and, in conjunction with the envelope, to reflect the energy back to the filament to increase the efficiency of the lamp. The latter arrangement is described in the aforementioned copending patent application in which the coating transmits the major, if not the entire, portion of the visible spectrum produced by the incandescent filament. In addition, the present invention is capable of producing "white" light by the use of an etalon coating in which several transmission regions are selected.

#Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which:

#FIG. 1 is a side elevational view in section showing an etalon coating in accordance with the present invention;

#FIG. 2 is a diagram illustrating the response characteristics of etalon coating;

#FIG. 1 shows a fragment of a substrate 22, for example, an incandescent lamp envelope of lime glass, or PYREX, or other suitable glass, on which an etalon coating 15 is laid down. The particular type of glass is not critical. The etalon coating has three discrete thin film layers, which are shown greatly magnified and not to scale. The first of these is a thin film layer 12 of a reflecting, electrically conductive metal, such as silver, which is deposited on one surface of the substrate 22, a thin film layer of an insulating (dielectric) material 11, described below, which is deposited on the metal film layer 12 and an outer thin film layer 10 of a reflecting metal, which also can be the same as the first film 12, which is deposited on the insulating material 22. Any conventional and suitable techniques can be used for depositing the three film layers, some of these being, for example, chemical deposition, vapor deposition, sputtering, RF sputtering, etc.

#The three film layers are preferably made separate and discrete from each other. That is, it is preferred that there be no interdiffusion of the materials of the layers. However, as described below, the film layers cooperate and are inter-related as a composite coating to produce the desired transmission and reflection characteristics and they are designed as a composite.

#Incident radiation, shown by the arrow I, assumed to have components in the visible portion of the spectrum as well as energy in the infrared portion of the spectrum, is shown as impinging upon the layer 10 most remote from substrate 22. In accordance with the invention as described below, the coating transmits a maximum amount of energy in a particular region of the visible portion of the spectrum so as to produce a desired color, for example, green, blue, yellow, or "white", etc. In addition, the coating is preferably designed to reflect a maximum amount of energy in the longer wavelength range, including the infrared region.