Cold cathode field emission devices having selective wavelength radiation

The present invention relates to a field emission device that produces light of selected wavelengths ranging from ultraviolet to infrared.

In recent years, there have been high expectations for the introduction of new types of lamps. Lamps emitting light in specific wavelength ranges are required in several different fields of use. For example, cold white light is desirable for car interior lamps, decorative office lamps, hospital lighting such as surgery lighting, and for Liquid Crystal Display (LCD) backlighting. Infrared (IR) light can be used in security, military and other night vision applications. Ultraviolet light can be used for skin tanning, counterfeit detection, exciting a photo-catalytic material, polymer curing, purification, and for sanitation.

White light lamps can emit a broad range of light with a wavelength from 400 nm to 700 nm. Although several lighting techniques have been developed to produce white light, such as fluorescent illumination, incandescent lighting, electro-luminescence and Light-Emitting Diodes (LED), those techniques have disadvantages.

Incandescent lamps generate visible light using a hot filament. The efficiency of incandescent lamps using filaments has not increased significantly for many years. When light is generated from an incandescent lamp, the filament generates significant heat through infrared radiation. Therefore, incandescent lamps require high power dissipation, are relatively inefficient and rapidly increase the temperature of the surrounding environment. Another problem of incandescent lamps is non-uniformity of illumination. It is hard to produce planar luminescence light.

Fluorescent lamps are operated by connecting a high voltage to two electrodes. Plasma, which is generated from the open discharge, creates Ultra-Violet (UV) radiation from mercury vapor in the lamp. Visible light is excited by the UV radiation from a phosphor coating on the inner surface of the lamp. Because of the mercury, fluorescent lamps cause pollution problems during manufacturing and recycling. It is also difficult to produce planar luminescence light.

Electro-Luminescence (EL) lamps suffer from low brightness and a short lifetime. Compared with incandescent lamps and fluorescent lamps, EL lamps have markedly lower brightness. Moreover, the brightness of EL lamps decreases over time. Typically, the brightness of an EL lamp decays about 50% after 1500 hours.

Light-Emitting Diodes (LED) cannot be flat. Additionally, the power consumption of LED's is comparable to or greater than electro-luminescence lamps and fluorescent lamps. Accordingly, it would be desirable to produce a cold white light lamp with low power dissipation, high brightness, and low heat generation to remedy the defects of the prior art.

For security applications, a near infrared spotlight is an ideal light source for a Charge Coupled Device (CCD) camera. Normally, the CCD camera has a sensitive range from 400-1000 nm. With a near-IR spotlight glow, near infrared light (from 750 to 930 nm) is generated. While near-IR light is a suitable light source for a CCD camera, near IR-light is not detectable by human eyes. Thus, such a lamp has broad application in security, military and other night vision fields.

A conventional near-IR spotlight has an incandescent lamp and an IR pass filter to create near-infrared light. However, the large amount of heat generated by the hot filament in the incandescent lamp leads to a short lifetime of the filter. There is a demand for lamps that produce infrared radiation without visible light or heat.

Conventional fluorescent lamps, incandescent lamps, and LED lamps can generate UV light, but have the drawbacks discussed above. Therefore, there exists a need for improved lamps that can produce desired UV light without visible light or IR light.

Field emission lamps are known. An exemplary structure of a field emission lamp is disclosed in U.S. Pat. No. 6,111,354, the contents of which are hereby incorporated herein by reference. A phosphor-coated anode and a field emission cathode are enclosed together in a panel. A voltage source is connected to the cathode and anode in a diode structure; or connected to the cathode, gate and anode in a triode structure. The field emission electrons are accelerated from cathode to anode. When electrons impact a phosphor layer on the anode, the phosphor is excited to luminescence. Field emission lamps are lightweight and have a high luminous efficiency, low power consumption, long lifetime, low working temperature, large panel size and planar structure.

However, a light source with a selective wavelength is desirable to improve upon the prior art.

In an embodiment, the present invention is directed to a lamp comprising a cold cathode field emission plate, an anode plate, and a voltage source connected across the cathode and anode plate. The anode plate has a transparent substrate having a first side and a second side; a transparent conductive coating formed on the first side of the transparent substrate; a phosphor layer formed on the transparent conductive coating, and a filter formed on the second side of the transparent substrate.

In a first embodiment of the present invention, the filter allows only white light to pass. In a second embodiment of the present invention, the filter allows only infrared light to pass. In a third embodiment of the present invention, the filter allows only ultraviolet light to pass.

The present invention is also directed to a method for making an anode plate having a transparent substrate for a field emission device. In an embodiment, the method comprises coating a transparent conductive coating on a first side of the transparent substrate; coating a phosphor layer on the transparent conductive coating; and coating an optical filter onto a second side of the transparent substrate.

For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross sectional view of a lamp according to a first embodiment of the present invention;

FIG. 2 is a plan view of a large area panel for a field emission lamp according to the present invention;

FIG. 3 is a side view of the lamp of FIG. 2 ;

FIG. 4 shows a spectrum of an IR light producing phosphor usable with the present invention;

FIG. 5 shows a spectrum of an IR pass edge filter usable with the present invention;

FIG. 6 illustrates the spectrum of IR radiation from the phosphor through an IR pass filter; and

FIG. 7 is an illustration of a security system utilizing an infrared lamp according to an embodiment of the present invention.

In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be obvious to those skilled in the art, however, that the present invention may be practiced without such specific details. In other instances, the present invention is based on the basic theory of field emission and the prior art of field emission lamps. The present invention discloses a planar field emission lamp for illuminating at selective light wavelengths from UV to IR. The field emission lamps according to the present invention provide the benefits of lightweight, high luminous efficiency, low power consumption, long lifetime, low working temperature, large panel size and planar structure.

Referring now to the drawings wherein depicted elements are not necessarily shown to scale. FIG. 1 shows the structure of a field emission lamp 10 according to a first embodiment of the present invention. In the lamp 10 , a cathode 12 , a gate 14 and an anode 16 are arranged in a triode structure. A first spacer 18 maintains proper spacing between the cathode 12 and the gate 14 . A second spacer 20 maintains proper spacing between the gate 14 and the anode 16 . A voltage source (not shown) is connected between the cathode 12 and the anode 16 .

A typical triode cathode 12 has sharp metal or silicone tip emitters (not shown) which cause electrons to be drawn off, or emitted. The present invention may utilize sharp metal or silicone tip emitters. Additionally, the present invention can use a cathode 12 with a low work function material. The low work function material may be, for example, Carbon Nano-Tube (CNT), Diamond Like Carbon (DLC), diamond, AlN or AlGaN. LaB 6 , Si, or Mo can also be used.

In order to fabricate a cathode 12 for use with the present invention, a first conductive layer, which can be Ni, Cu, Ag, Co, Fe, or one of the other conductive metals, is deposited on a substrate. Subsequently, a masking material is randomly sprayed onto the first conductive layer. The spraying may be done by such methods as dusting, sprinkling, or smoking. The masking material can be photosensitive material, plastic, glass, metal or ceramic particles which can be removed in a later step. The masking material can be in a form of solid particles or liquid droplets, or a combination.

After the masking material spraying process, an insulating layer and a second conductive layer for the gate are deposited to form the triode field emission emitters. The masking material is then removed, such as by water, solvents, or developers in an ultrasonic bath or other process known in the art, leaving openings in the insulating layer and second conductive layer. The resulting triode field emission emitters are then randomly distributed. Subsequently, an emitter material can be deposited in the openings, in electrical contact with the first conductive layer. The deposition process can be printing (such as inkjet printing or screen-printing), spin-coating, or direct growth, depending on the material of the first conducting layer.

As shown in FIG. 1 , the anode 16 has a transparent substrate 22 , a transparent conductive thin film coating 24 and a phosphor layer 26 . An optical filter 28 is applied to the transparent substrate 22 to obtain precise wavelengths of light from the lamp 10 . The transparent substrate 22 can be made from, for example, plastic, glass, quartz and Al 2 O 3 .

The transparent conductive thin film coating 24 may be made of, for example, Indium Tin Oxide (ITO) or ZnO. The transparent conductive thin film coating 24 provides electrical conductivity and does not decrease the transparency of the substrate. Preferably, the transparent conductive layer is prepared before the phosphor layer 26 is coated. The transparent conductive layer may be coated on the transparent substrate using, for example, a sol-gel method, sputtering coating, or evaporation coating.

The phosphor layer 26 may be coated onto the thin film coating 24 using a common printing technique such as screen printing, ink-jet printing, a sol-gel process, electrophoretic deposition, sedimentation, and spin coating. The quality of the phosphor layer 26 printing is an important factor in the luminescence quality of the phosphor screen and therefore the lamp. The phosphor materials are chosen depending on the desired wavelength range of radiation from the lamp. The deposition of the phosphor layer 26 will be described further in the examples below.

The optical filter 28 is a coating layer that is used to allow a desired wavelength range of light to pass through while blocking undesirable wavelengths of light. The optical filter preferably utilizes a multilayer coating. The optical filter can consist of organic dye and/or inorganic dye. The inorganic dye can be incorporated in the transparent substrate 22 . Organic dye can be coated on the transparent substrate 22 by spin-coating or printing. The multilayer structure may include compounds selected from the group consisting of TiO 2 , Ta 2 O 5 , SiO 2 , ThF 4 , ThO 2 , AlF 3 , LaF 3 , NdF 3 , ZrO 2 and Na 3 AlF 6 .

The gate 14 is formed to surround the cathode and to provide an extraction potential. The electrons are extracted by applying a voltage to the gate 14 . Once electrons are extracted from the emitters on the cathode 12 , the electrons are accelerated toward the anode 16 by a fixed electric field. Light is excited from the phosphor layer 26 by the accelerated electrons.

The present invention can be used to fabricate a lamp for lighting a large area. The present invention makes it easier to fabricate individual field emission cells in a smaller package. For example, a spotlight 30 with a tilt panel structure is shown in FIGS. 2 and 3 . The spotlight 30 has many individual field emission elements 32 in a specially oriented array. Each single field emission element 32 is a full function vacuum package. The spotlight 30 is configured with a particular number and arrangement of the emission elements 32 and is adjusted to fit the shape of a spotlight holder 34 . The focal point of the spotlight 30 can be adjusted by the shape of spotlight holder 34 . The panel size can be changed depending on the size of the spotlight holder 34 .

For a better understanding of the present invention, examples of specific embodiments of the invention are given below.