1. Field of the Invention
The present invention is directed to low pressure mercury vapor lamps, more commonly known as fluorescent lamps, having a lamp envelope with phosphor coating, and more particularly, to a coating with three phosphors over an alumina pre-coat.
2. Discussion of the Prior Art
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (UV) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
There is an increase in the use of fluorescent lamps because of reduced consumption of electricity. To further reduce electricity consumption, there is a drive to increase efficiency of fluorescent lamps, referred to as luminous efficacy which is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW).
Thus, more efficient and longer life fluorescent lamps are desired. However, significant excess of mercury is introduced into the lamp to meet desired long lamp lifetime of 20,000 hours or more. This is necessary because different lamp components, such as the glass envelope, phosphor coatings and electrodes use up the mercury in the lamp. Such increased use of mercury is not desirable and is detrimental to the environment. Accordingly, there is a drive to reduce mercury consumption in fluorescent lamps without a reduction in the lamp life.
To increase efficiency and reduce mercury consumption without a reduction in the lamp life, different blends of phosphors are used for the luminescent coating. Further, a metal oxide layer is provided between the luminescent coating and glass envelope. The metal oxide layer reflects the UV radiation back into the phosphor luminescent layer through which it has already passed for further conversion of the UV radiation to visible light. This improves phosphor utilization and enhances light output. The metal oxide layer also reduces mercury consumption by reducing mercury bound at the tubular portion of the lamp.
Desirable fluorescent lamps characteristics include high brightness and high color rendering. Fluorescent lamps referred to as xe2x80x9cColortonexe2x80x9d lamps belong to a family of light sources having high color rendering indices (CRI). These particular fluorescent lamps are used to alleviate seasonal disorders and are used professionally in the color appraisal field and photography. In particular, Colortone 75 lamps are used when visually appraising color uniformity of production sheets.
Colortone 75 lamps have a correlated color temperature of 7500K, with a high CRI being greater than 90. There are organizations that promulgate standards that specify particular minimum values for the CRI and other lamp specification, such at the American National Standard Institute (ANSI) and the International Standard Organization (ISO) standards. For the Colortone 75 lamps, the ANSI and ISO standards require that the CRI must be over 90. The ISO standard further requires that the separate color rendering indices R1-R8 be over 80. Conventional Colortone 75 lamps are made with phosphors that are high consumers of mercury, and cannot pass the TCLP (Toxicity Characteristic Leaching Procedure) test without sacrificing lamp life.
In particular, a conventional fluorescent Colortone 75 lamp is made with a three-phosphor mixture of Strontium Magnesium Phosphor (Sr. Mag), Blue Halophosphor (BH), i.e., Ca10(PO4)6F2:Sb, and Zinc Silicate (ZS), i.e., Zn2SiO4:Mn. The Sr. Mag is very rich in the red region of the spectrum, while the BH and ZS contribute to the blue and green regions of the visible spectrum for the Colortone 75.
The combination of these three-phosphors produces a broad spectrum in the visible region with high color rendering properties being greater than 90. However, these phosphor mixtures are detrimental for mercury consumption. In particular, Sr. Mag is the highest consumer of mercury and its high percentage renders the conventional Colortone 75 lamps non-TCLP compliant.
Accordingly, there is a need for fluorescent Colortone lamps with high CRI and reduced mercury that pass TCLP.
The object of the present invention is to provide fluorescent Colortone lamps with high CRI and reduced mercury consumption.
The present invention accomplishes the above and other objects by providing an electric lamp having an envelope with an inner surface and at least one electrode, such as two electrodes located at both ends of the envelope tube. The electrodes transfer electric power to generate ultraviolet radiation in the envelope which is filled with mercury and a charge sustaining gas. The inner surface of the envelope is pre-coated with a metal oxide layer, such as an aluminum oxide layer, to reflect ultraviolet radiation back into the envelope.
A phosphor layer is formed over the aluminum oxide to convert the ultraviolet radiation to visible light. The phosphor layer for a 7500K Colortone is a mixture of three phosphors, namely, blue luminescing Blue Halophosphor (BH), red-luminescing Cerium Gadolinium Magnesium Borate (CBTM), and 3000K-luminescing Calcium Halophosphor, also referred to as Warm White (WW).