The present invention relates to a white light-emitting diode device which has an excellent color rendering property and which can be used in a back light and a front light as an externally irradiating light source of a color liquid crystal display and for room illumination. Particularly, the present invention relates to a covering member that enables a conventional blue or green light-emitting diode device to emit white light, and a white light-emitting diode device using the covering member.
A light-emitting diode (hereinafter, referred to as xe2x80x9cLEDxe2x80x9d) emits a light when electrons from an N-region and holes from a P-region are moved to a PN junction section and are recoupled in a PN junction semiconductor when a forward voltage is applied. Since the energy released when the free electrons are coupled is radiated as light, the light emitted from the LED is light in a narrow wavelength range, i.e. light of a single color such as red or blue.
On the other hand, a light source for illumination of a color liquid crystal display and for room illumination needs to emit fluorescent light or white light approximate to natural light.
Thus, a full-color LED device has been used as a light source for illumination of the color liquid crystal display and for room illumination. The full-color LED device generates white light by using a combination of a red light-emitting element for emitting red light, a blue light-emitting element for emitting blue light and a green light-emitting element for emitting green light.
However, with a full-color LED device as a light source results in a complicated design, since each of the light-emitting elements in the full-color LED device requires must have an anode terminal and a cathode terminal. Further, a larter space and higher costs are required since a full-color LED device uses three kinds of light-emitting elements. In addition, if one light-emitting element of a full-color LED device is broken, the desired color balance is lost, with the result that the intended color tone may not be obtained. In view of the above situation, many studies are being vigorously undertaken to develop an LED device capable of emitting white color using one kind of light-emitting element. Such an LED device is called a white LED device and is distinguishable from the full-color LED device. Using a white LED device enables a back light or the like to be driven by a simple electric circuit without requiring an inverter circuit and the like, and reduces power consumption. This presents advantages such as reducing the outer configuration of the driving circuit and eliminating electromagnetic noise. Because of such advantages, the white LED device is being developed.
Since a white LED device must generate white light by using only one kind of light-emitting element (single-color), it has been the general practice to use a single-color light-emitting element in combination with a phosphor that can convert the wavelength of light emitted from the light-emitting element to emit light of another color. For example, white LED devices which have a layer containing YAG (yttrium aluminate) phosphor provided on a blue light-emitting element are commercially available.
The YAG phosphor generally emits light in the green to yellow region, depending on the kind of activating element in the YAG. Thus, the red light obtained in a white LED by combining a blue light-emitting element for emitting light having an excitation wavelength of YAG and the YAG phosphor is insufficient. This leads to the problem that matters to be displayed in red look subdued.
To remedy this problem, it has been considered to use a YAG phosphor capable of emitting fluorescent light approximate red light, such as by activating YAG, for example, by substituting Gd at the Y-site of Y3Al5O12, to activate the YAG phosphor and shift the emission peak toward a longer wavelength, i.e. toward the red side of the spectrum. However, such a method cannot be used to provide a white LED device having an average color rendering evaluation number (Ra) of 85 or larger, the value of Ra being an index as to whether an illuminated object conforms to color perception under a reference light. Although more red light may be obtained by increasing the content of red YAG in the fluorescent layer, the emission intensity of yellowish light emitted from YAG also becomes higher, with the result that an average color rendering evaluation number Ra of 85 or larger cannot be obtained.
As a method of compensating for not only the blue, green and yellow lights, but also the red light in the white LED, the use of a red phosphor has also been considered. However, at present the manganese activated magnesium arsenate phosphor known as a dark red phosphor is not being produced for fluorescent lamps due to its toxicity. Further, since manganese activated magnesium fluoride germanium oxide phosphor is expensive, it is not suitable for the use of a white LED as a back light having the merits of being light-weight and thin and of requiring low power consumption.
The effect of adding red pigment to the fluorescent layer of an LED is also being studied (see, for example, Japanese Unexamined Patent Publication No. (HEI)5-152609). However, since the pigment acts like a light filter in making a specified color stand out by cutting other colors, addition of the pigment results in a reduced luminance of the LED device and, thus, is not suitable for a white LED device aimed at conserving space, costs and power.
An object of the present invention is to provide, without increasing manufacturing costs more than necessary, an LED device having an excellent color rendering property and a covering member used for such an LED device.
An inventive light-emitting diode comprises a light-emitting element for emitting light in the blue to green region and a fluorescent substance containing a CaS phosphor activated by Eu for converting the wavelength of the light emitted from the light-emitting element to a different wavelength.
Another inventive light-emitting diode device comprises a light-emitting element for emitting light in the blue to green region and a fluorescent substance containing a phosphor expressed by a general formula AEu(1xe2x88x92x)LnxB2O8 for converting the wavelength of the light emitted from the light-emitting element to a different wavelength, wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than 1.
An inventive covering member for a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light in the blue to green region comprises a base polymer and a CaS phosphor activated by Eu.
Another inventive covering member for covering a light-emitting diode device which includes a light-emitting element sealed by a transparent resin for emitting light in the blue to green region comprises a base polymer, and a phosphor expressed by a general formula AEu(1xe2x88x92x)LnxB2O8, wherein A is an element selected from the group consisting of Li, K, Na and Ag; Ln is an element selected from the group consisting of Y, La and Gd; and B is W or Mo; and x is number equal to or larger than 0, but smaller than 1.
The inventive light-emitting diode devices are better in emitting red light than conventional light-emitting diode devices using a light-emitting element that emits light in the blue to green region. Thus, according to the present invention, a white diode device having a color rendering property approximate to that of a fluorescent lamp or of natural light can be provided by containing YAG phosphor in the fluorescent substance.
Further, if the inventive covering member is mounted on a conventional light-emitting diode device, it can be used as the inventive white LED device having an excellent color rendering property.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.