Red-deficiency-compensating phosphor LED

Details Red-deficiency-compensating phosphor LED Red-deficiency-compensating phosphor LED

1999-02-18

2002-02-26

Day, Michael H. (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S503000, C313S504000, C313S501000

Reexamination Certificate

active

06351069

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to light emitting diodes and more particularly to a phosphor light emitting diode.
DESCRIPTION OF THE RELATED ART
Light emitting diodes (LEDs) are well-known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. Traditionally, the most efficient LEDs emit light having a peak wavelength in the red region of the light spectrum, i.e., red light. However, a type of LED based on Gallium Nitride (GaN) has recently been developed that can efficiently emit light having a peak wavelength in the blue region of the spectrum, i.e., blue light. This new type of LED can provide significantly brighter output light than traditional LEDs.
In addition, since blue light has a shorter wavelength than red light, the blue light generated by the GaN-based LEDs can be readily converted to produce light having a longer wavelength. It is well known in the art that light having a first peak wavelength (the “primary light”) can be converted into light having a longer peak wavelength (the “secondary light”) using a process known as fluorescence. The fluorescent process involves absorbing the primary light by a photoluminescent phosphor material, which excites the atoms of the phosphor material, and emits the secondary light. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength. An LED that utilizes the fluorescent process will be defined herein as a “phosphor LED.”
With reference to
FIG. 1
, a prior art phosphor LED
10
is shown. The LED
10
includes a GaN die
12
that generates blue primary light when activated. The GaN die
12
is positioned on a reflector cup lead frame
14
and is electrically coupled to leads
16
and
18
. The leads
16
and
18
conduct electrical power to the GaN die
12
. The GaN die
12
is covered by a layer
20
that includes fluorescent material
22
. The type of fluorescent material utilized to form the layer
20
can vary, depending upon the desired spectral distribution of the secondary light that will be generated by the fluorescent material
22
. The GaN die
12
and the fluorescent layer
20
are encapsulated by a lens
24
. The lens
24
is typically made of a transparent epoxy.
In operation, electrical power is supplied to the GaN die
12
to activate the GaN die. When activated, the GaN die
12
emits the primary light, i.e., blue light, away from the top surface of the GaN die
12
. A portion of the emitted primary light is absorbed by the fluorescent material
22
in the layer
20
. The fluorescent material
22
then emits secondary light, i.e., the converted light having a longer peak wavelength, in response to absorption of the primary light. The remaining unabsorbed portion of the emitted primary light is transmitted through the fluorescent layer
38
, along with the secondary light. The lens
24
directs the unabsorbed primary light and the secondary light in a general direction indicated by arrow
26
as output light. Thus, the output light is a composite light that is composed of the primary light emitted from the GaN die
12
and the secondary light emitted from the fluorescent layer
20
.
The output light may have a spectral distribution such that it appears to be “white” light. The color composite of the output light will vary depending upon the spectral distributions and intensities of the secondary light and the primary light.
PCT Application No. PCT/JP97/02610 by Shimizu et al. describes various phosphor LEDs that generate white output light having a color temperature somewhere between 5,000 to 6,000 degrees Kelvin. The LEDs of Shimizu et al. are schematically identical to the LED
10
of FIG.
1
. In one embodiment, the LED of Shimizu et al. utilizes Yttrium Aluminum Garnet (YAG) phosphor to convert some of the primary light into secondary light having a peak wavelength of about 580 nm. The spectral distribution
28
of the output light from the Shimizu et al. LED is shown in FIG.
2
. The spectral distribution
28
has two peaks
30
and
32
. The peak
30
is predominately caused by the primary light emitted from the GaN die of the Shimizu et al. LED. The peak
32
is predominately caused by the secondary light emitted from the YAG phosphor.
A concern with the Shimizu et al. LED is that the “white” output light has an undesirable color balance for a true color rendition. The output light of the Shimizu et al. LED is adequate for applications in which simple illumination is required. However, for applications in which a high color rendition is desired, the output light is deficient in the red region of the visible light spectrum (647-700 nm range). When used for such applications, the red deficiency in the output light causes illuminated red objects to appear less intense in color than they would under a white light having a well-balanced color characteristic. In particular, when used as a backlight for color liquid crystal displays (LCD), the output light of the Shimizu et al. LED causes red colors to be weakly displayed on the LCD. A separate red light source may have to be used in conjunction with the Shimizu et al. LED to compensate for the red deficiency of the output light generated by the Shimizu et al. LED, adding complexity to the system embodying the Shimizu et al. LED.
What is needed is a phosphor LED that can generate white output light having a well-balanced color characteristic for a true color rendition.
SUMMARY OF THE INVENTION
A light emitting device and a method of fabricating the device utilize a supplementary fluorescent material that radiates secondary light in the red spectral region of the visible light spectrum to increase the red color component of the composite output light. The secondary light from the supplementary fluorescent material allows the device to produce “white” output light that is well-balanced for true color rendering applications. As an example, the device can be used as backlight for a color LCD or a light source for a color scanner.
The light emitting device is an LED that includes a die that emits primary light in response to an electrical signal. Preferably, the die is a Gallium Nitride (GaN) based die that emits blue light having a peak wavelength of 470 nm. The die is encapsulated by an optional transparent layer. The optional transparent layer provides a generally uniform surface for the next layer. Preferably, the optional transparent layer is made of clear resin. The next layer is a fluorescent layer that contains the supplementary fluorescent material. The fluorescent layer also includes the main fluorescent material that radiates broadband secondary light having a first peak wavelength in the yellow region of the visible light spectrum. Coupled to the fluorescent layer is a lens that operates to direct the lights from the die and the fluorescent layer in a direction generally normal to the upper surface of the die.
In operation, the GaN die is activated by electrical power that is supplied to the die via leads. When activated, the GaN die emits the primary light, i.e., blue light, away from the upper surface of the die. The emitted primary light propagates through the optional transparent layer to the fluorescent layer. A portion of the primary light impinges upon the main fluorescent material in the fluorescent layer. The main fluorescent material absorbs the impinging primary light and emits the secondary light having the first peak wavelength. Another portion of the primary light impinges upon the supplementary fluorescent material in the fluorescent layer. The supplementary fluorescent material absorbs the impinging primary light and emits the second light having the second peak wavelength in the red spectral region of the visible light spectrum. However, some of the primary light will not be absorbed by either the main fluorescent material or the supplementary fluorescent material. The amo

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