Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-01-06
2001-02-06
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S551000, C250S216000
Reexamination Certificate
active
06184521
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of photodiode detectors, the field of electromagnetic interference and the field of band-limiting optics. In particular, this invention relates to electromagnetic and optical shielding to reduce background noise from photodiode detectors.
2. Description of the Related Art
A photodiode is a semiconductor device which converts the photon energy of light into an electrical signal by releasing and accelerating current-conducting carriers within the semiconductor. A photodiode behaves like an ordinary signal diode, but is specialized with respect to spectral response and efficiency to optimize internally generated current derived from illumination. In applications, a photodiode is often used as a detector which is optically coupled to a light-emitting-diode (LED) emitter. Examples of such applications include solid-state relays, remote control devices, optical communications and noninvasive biomedical sensors.
A limitation in many photodiode applications is a background noise floor which masks the signal detected by the photodiode. A contributing factor to background noise in a photodiode detector circuit, as in most electronic circuits, is the parasitic coupling of electromagnetic interference (EMI) into the circuit. External sources of EMI vary from power lines and cellular telephones to medical devices such as diathermy, MRI and lasers.
Conventionally, an electromagnetic shield is utilized as an effective method of reducing the effect of EMI-induced noise. Typical shielding techniques involve surrounding potentially affected parts with a “Faraday cage” of conducting material. However, conducting materials are typically opaque to optical signals. Hence, for photodiode applications, prior art electromagnetic shields have typically consisted of optically-transparent conductive materials, such as thin film silver or silver alloy or conductive “screens” having optically transmissive openings. This is illustrated in
FIG. 1
, which is a cut-away view of a prior art cage
100
containing an optical detector
110
. The portions of the cage
100
within the optical path
140
between an emitter
150
and the detector
110
are constructed of a transparent or transmissive conductive material
120
. The remainder of the cage
100
is conductive material
130
which may be opaque.
Besides electromagnetic interference, a contributing factor to background noise in photodiode detectors is ambient light. For photodiode applications, prior art ambient light reduction techniques typically consist of placing opaque, polarized or similar light-blocking material externally around the signal optical path and external wavelength filters within the signal optical path. This is illustrated in
FIG. 2
, which is a cut-away view of a prior art optical enclosure
200
containing an optical detector
110
. The portion
220
of the enclosure
200
within the optical path
140
between an emitter
150
and the detector
110
is constructed of a wavelength filtering material. The remainder of the enclosure
200
is light blocking material
230
.
SUMMARY OF THE INVENTION
A particularly advantageous application of a photodiode with integrated noise shielding according to the present invention is in pulse oximetry, and in particular, as a detector 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 which measures oxygen saturation by computing the differential absorption of these two light wavelengths after transmission through the finger. A probe may also be reflective, with the emitter and detector on the same side of vascularized tissue. This is sometimes referred to as “backscatter” oximetry. The LED emitters are alternately activated by the pulse oximetry instrument which then reads voltages indicating the resulting intensities (I
rd
and I
ir
) detected by the photodiode, where I
rd
is the detected intensity of the red light and I
ir
is the detected intensity of the infrared light. 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
Unfortunately, pulse oximetry probes are adversely affected by background noise generated in the photodiode detector by both EMI and ambient light. EMI-generated noise enters an unshielded detector through parasitic capacitive coupling, i.e., through the mutual capacitances that exist between any two objects. Noise from ambient light is generated by the detector when light not generated by the emitters illuminates the photodiode. A significant portion of ambient light induced noise may result from light having wavelengths outside the emitter bandwidth but within the detector bandwidth.
The detector output from both signal and noise sources can be represented as:
I
rd
/I
ir
=(
S
rd
+N
rd
)/(
S
ir
+N
ir
)
where S
rd
is the signal component of the red light, N
rd
is the noise component of the red light, S
ir
is the signal component of the infrared light, and N
ir
is the noise component of the infrared light. If the noise level becomes large in relation to the signal, the ratio I
rd
/I
ir
approaches 1, which corresponds to a false saturation reading of 85%. This noise problem is compounded by the critical human life mission of pulse oximetry devices. Thus, in pulse oximetry applications, there is a particular need for both EMI shielding and ambient-light shielding in order to increase the detector signal-to-noise ratio.
The use of conventional external noise shielding for photodiode detectors, including detectors used in pulse oximetry, has a number of drawbacks. Any practical external shielding enclosure includes openings which reduce shield effectiveness. For electromagnetic shields, shielding effectiveness (SE) can be expressed as
SE=
20 log(&lgr;/2
L
)
where &lgr; is the interference wavelength and L the longest dimension of any opening. Thus, a mere ½ inch opening in a shield reduces shielding effectiveness beyond a minimally acceptable 20 db at frequencies as low as 1 GHz. Likewise for optical shields, small openings in opaque or wavelength filtering materials can allow noise-producing ambient light to reach the photodiode. This is particularly problematic for pulse oximetry probes, where the optical path from emitter to detector includes, for example, fingers and feet having a variety of sizes and shapes which frustrate achieving a light-tight seal.
In large-scale manufacturing applications, external shielding devices, both electromagnetic and optical, can add significantly to the cost of photodiode detectors, both in terms of additional parts and additional assembly steps. Conductive and optically transmissive shielding deposited directly on a photodiode substrate might overcome some limitations of external shielding but, generally, would require extra processing steps in photodiode fabrication, which would also increase final detector cost. A photodiode with integrated noise shielding according to the present invention is intended to eliminate or reduce these drawbacks encountered with conventional noise shielding techniques.
Another aspect of the present invention is a shielded detector which comprises a photodetector having an active area exposed to receive light. The photodetector is responsive to light of a first band of wavelengths. The shielded detector further comprises a shield deposited on at least portions of the exposed active area. In preferred embo
Coffin, IV James P.
Gerhardt Thomas J.
Kiani Massi E.
Mills Michael A.
Knobbe Martens Olson & Bear LLP
Le Que T.
Luu Thanh X.
Masimo Corporation
LandOfFree
Photodiode detector with integrated noise shielding does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photodiode detector with integrated noise shielding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photodiode detector with integrated noise shielding will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2570222