Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1998-03-10
2003-02-25
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S432000, C257S079000, C257S080000, C257S081000, C257S082000, C257S083000, C257S084000, C257S099000, C257S100000, C257S428000, C257S229000, C257S430000, C257S431000, C257S436000, C257S787000, C257S435000, C257S434000
Reexamination Certificate
active
06525386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of optoelectronics, which includes light emitting components, such as light emitting diodes (LED) and laser diodes, and which also includes light detecting components, such as photodiodes, phototransistors, photodarlingtons and photovoltaic cells. Optoelectronics also includes various devices which incorporate optoelectronic components, such as displays, photosensors, optocouplers, and fiberoptic transmitters and receivers. In particular, this invention relates to lenses to increase the efficiency of optoelectronic emitters and the sensitivity of optoelectronic detectors.
2. Description of the Related Art
A prior art LED
100
is shown in FIG.
1
and consists of a semiconductor diode element
110
electrically connected to a leadframe
120
and surrounded by an encapsulating material
130
. The diode element
110
is typically mounted to one lead
122
of the leadframe
120
and connected to a second lead
124
of the leadframe
120
by a wire bond
140
. These two leads provide an electrical connection between an external current source and the anode and cathode of the diode element
110
. The external current source supplies power to the diode device
100
that is converted to emitted light by the photoelectric effect, which occurs at the semiconductor junction within the diode element
110
.
Internal inefficiencies within a semiconductor diode result in very low net efficiencies, which is the ratio of emitted light power to input power. Internal inefficiencies arise from a low ratio of minority carriers injected into the diode semiconductor junction to photons generated at the junction; photon loss due to internal reflection at the semiconductor/encapsulant interface; and absorption of photons within the semiconductor material. Because of these low net efficiencies, many LED applications require high input current, resulting in heat dissipation and device degradation problems in order to obtain sufficient light.
As illustrated in
FIG. 1
, the encapsulant
130
forms a flat light-transmitting surface
150
. A flat surface is convenient in many applications where the LED is mounted to another surface that is also generally flat or in applications that otherwise cannot accommodate a protruding surface. The inefficiencies described above, however, are compounded by the configuration of the LED encapsulant/air interface. An encapsulant having a flat surface, such as in
FIG. 1
, allows photons transmitted by the diode element
110
to have considerable dispersion. A flat encapsulant surface also results in internal reflection at the encapsulant/air interface, further reducing photon transmission and increasing photon absorption within the encapsulant material.
FIG. 2
illustrates a prior art LED
200
having an encapsulant
230
that forms a spherical surface
250
. A spherical or other curved surface gives a larger angle of incidence for photons emitted from the semiconductor diode element
210
, reducing losses due to internal reflection. Further, this surface
250
acts as a lens to reduce the dispersion of generated photons. Unfortunately, a protrusion, such as this curved surface, is difficult to accommodate in many applications.
SUMMARY OF THE INVENTION
An optoelectronic device according to the present invention incorporates a lens that increases component performance. For example, the output of an LED utilizing the lens is increased by, in part, reducing internal reflection. Internal reflection results from the differing indices of refraction at the interface between the LED encapsulant and the surrounding air.
As shown in
FIG. 3
, when a light ray
310
passes from a media having a higher index of refraction
320
to a media having a lower index of refraction
330
, the ray
310
is refracted away from the normal
340
to the surface
350
. The angle, &thgr;
1
, is customarily referred to as the angle of incidence
370
and the angle &thgr;
2
is customarily referred to as the angle of refraction
380
. As the angle of incidence
370
is increased, the angle of refraction
380
increases at a greater rate, in accordance with Snell's Law:
sin &thgr;
2
=(
N
1
/N
2
) sin &thgr;
1
,
where (N
1
>N
2
). When the angle of incidence
370
reaches a value such that sin &thgr;
1
=N
2
/N
1
, then sin &thgr;
2
=1.0 and &thgr;
2
=90°. At this point none of the light is transmitted through the surface
350
, the ray
310
is totally reflected back into the denser medium
320
, as is any ray which makes a greater angle to the normal
340
. The angle at which total reflection occurs:
&thgr;
c
=arcsin
N
2
/N
1
is referred to as the critical angle. For an ordinary air-glass surface, where the index of refraction is 1.5, the critical angle is about 42°. For an index of 1.7, the critical angle is near 36°. For an index of 2.0, the critical angle is about 30°. For an index of 4.0, the critical angle is about 14.5°.
An optoelectronic device according to the present invention has an encapsulant that functions as a lens. For emitter applications, the lens reduces internal reflection and dispersion without having a protruding curved surface. Thus, LEDs utilizing the present invention have an improved efficiency compared with prior art flat-surfaced LEDs and similar devices, without the physical interface difficulties of the prior art curved-surface LEDs and similar devices. For detector applications, the lens focuses photons on the active area of the detector, increasing detector sensitivity. This increased detector sensitivity allows a detector having a reduced size, hence a reduced cost, to be used for a given application.
A particularly advantageous application of an optoelectronic device with a non-protruding lens is in pulse oximetry, and in particular, as an emitter in pulse oximetry probes. Pulse oximetry is the noninvasive measurement of the oxygen saturation level of arterial blood. Early detection of low blood oxygen saturation is critical because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. The use of pulse oximetry in operating rooms and critical care settings is widely accepted.
A pulse oximetry probe is a sensor having a photodiode which detects light projected through a capillary bed by, typically, red and infrared LED emitters. The probe is attached to a finger, for example, and connected to an instrument that measures oxygen saturation by computing the differential absorption of these two light wavelengths after transmission through the finger. The pulse oximetry instrument alternately activates the LED emitters then reads voltages indicating the resulting intensities detected at the photodiode. A ratio of detected intensities is calculated, and an arterial oxygen saturation value is empirically determined based on the ratio obtained:
I
rd
/I
ir
=Ratio→% O
2
Saturation
Typically, a look up table or the like correlates the Ratio to saturation. The use of conventional LEDs within pulse oximetry probes has a number of drawbacks. Pulse oximetry performance is limited by signal-to-noise ratio which, in turn, is improved by high light output emitters. LEDs without lenses, such as illustrated in
FIG. 1
, are not optimized to transmit the maximum amount of light into the skin. LEDs with protruding lenses, such as illustrated in
FIG. 2
, create increased pressure on the skin, resulting in perfusion necrosis, i.e. a reduction of arterial blood flow, which is the medium to be measured. A solution to this problem in accordance with the present invention is an LED incorporating a non-protruding lens.
One aspect of the present invention is an optoelectronic device that comprises an encapsulant having a surface, a lens portion of the surface, and a filler portion having a generally planar surface. The filler portion is disposed around the lens, and the lens does not extend substantially beyond the plane of the generally planar surface. The optoelectronic device also comprises an optoelectronic e
Coffin, IV James P.
Mills Michael A.
Knobbe Martens Olson & Bear LLP
Masimo Corporation
Williams Alexander O.
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