Note: Descriptions are shown in the official language in which they were submitted.
W094/08506 2 1 4 6 1 3 ~ PCT/US92/08~
NONCONTACT INFRARED TYMPANIC T~ERMOMETER
R~ BOUND OF T~E l~.V~h~ lON
Field of the Invention
The present invention relates to a device for
measuring temperatures by quantifying infrared emissions, and
more particularly, to a device which measures patient body
temperature by quantifying the infrared emissions from the
tympanic membrane.
DescriDtion of the Prior Art
The tympanic membrane has long been known to be an
excellent spot to check body temperature because it shares the
blood supply that reaches the hypothalamus, the center of core
body temperature regulation. Also, the ear is generally
considered to be a more acceptable site than the mouth or
rectum for temperature measurement, for use of the external
auditory canal eliminates common problems such as breakage or
perforation of the rectal wall or biting or gagging on a probe
placed in the mouth. As early as the 1960's, thermistors in
contact with the tympanic membrane were routinely used in the
treatment of severely burned patients. However, because of
the risk of injury and inconvenience of application, such
contact type temperature sensors have not been widely used to
measure temperatures in the ears of awake, alert patients.
Noncontact infrared thermometry differs from the
above-referenced contact thermometry in that a sensor is
placed at the external opening of the auditory canal for
sensing the infrared energy emitted from the tympanic
membrane, without contacting the tympanic membrane.
w094/08506 2 1 4 6 1 3 ~ PCT/US92/08~
.
Noncontact infrared thermometry is routinely used in industry
to remotely measure process and machinery temperatures, and
techniques for this purpose are described in detail in an
article by C. Hamel entitled "Noncontact Temperature Sensing
With Thin Film Thermopile Detectors, n SENSORS, January 1989,
pages 28-32. However, although noncontact infrared
thermometry is common in industrial applications, this
technology has only recently been applied to medical
temperature measurements.
In clinical application, the end of the probe
portion of a noncontact infrared sensor must be small enough
to be placed in the outer portion of the auditory canal (just
past the cartilage of auricula), where the sensor can get a
clear "view" of the tympanic membrane. Placing such a sensor
requires only a little training and is very similar to the
maneuver used to visualize the eardrum with an otoscope.
Typically, there is no risk of eardrum injury because the
probe is not long enough or small enough to be inserted past
the mastoid process. Moreover, normal amounts of cerumen (ear
wax) in the auditory canal typically do not interfere with an
accurate temperature reading.
The most advantageous feature of infrared tympanic
thermometry is that it takes very little time. Temperature
readings typically can be taken in a few seconds. Speed is
inherent in infrared thermometry because the sensors measure
emitted infrared radiation instead of being brought into
contact with the body until thermal equilibrium is reached as
with typical oral thermometers. The speed means less
discomfort to patients, which is especially important for
children and in emergency situations. Another widely
appreciated advantage over conventional thermometers is that
tympanic thermometry uses the ear, which is less likely to
harbor pathogens than the mucous membrane-lined mouth or
rectum.
An early device for such a method of measuring body
temperature using infrared emissions from the tympanic
membrane is disclosed by Barnes in U.S. Patent No. 3,282,106.
W094/08506 2 1 4 6 1:3 ~ PCT/US92/08~
Barnes therein discloses the general configuration of a
tympanic membrane directed infrared thermometer which is
inserted into the auditory canal so as to sufficiently enclose
the detector apparatus such that multiple reflections of the
radiation from the tympanic membrane transform the auditory
canal into a "black body" cavity, a cavity with emissivity
theoretically equal to one. However, in his early example of
a noncontact infrared tympanic thermometer, Barnes does not
consider, or in any way describe, how the device of that
patent is calibrated, how accuracy is maintained for clinical
use, or how contamination of the instrument by cross-infection
can be prevented. Such problems have restricted the
usefulness of the tympanic thermometer of Barnes in clinical
settings.
Subsequent patents have addressed the problems with
the thermometer of Barnes. For example, O'Hara et al.
describe in U.S. Patent Nos. 4,602,642 and 4,790,324 a
noncontact infrared tympanic thermometer which addresses the
calibration issues left unexplored by Barnes. In fact, the
infrared sensor of O'Hara et al. is calibrated before each
use. For this purpose, the sensor is housed in a temperature
controlled cavity and special care is taken to maintain a
constant temperature in the waveguide which directs the
infrared emissions from the tympanic membrane onto the
detector. In particular, the hand held probe unit of the
tympanic thermometer of O'Hara et al. has an infrared
sensitive thermopile mounted in a metal housing which is kept
at a constant reference temperature by a regulator circuit.
A waveguide tube surrounded by a thermally insulative probe
directs infrared emissions from the tympanic membrane to the
thermopile. The thermopile and regulator circuit of the probe
unit are then electrically connected to a processing circuit
in a chopper unit. Prior to taking a patient's temperature,
the probe unit is mated with the chopper unit so that the
thermopile detects infrared emissions from a reference target
which is also kept at a constant reference temperature by
another regulator circuit. The processing circuitry
W094/08506 ~ 1~ 6 13 6 PCT/US92/08644
repeatedly acquires the output level of the thermopile and
stores calibration data. The probe unit is then removed from
the chopper unit, the probe is covered with an infrared
transparent disposable speculum and is inserted into the
patient's external ear canal. The patient's core temperature
is then determined by comparing the stored calibration data
to the maximum output of the thermopile during a succession
of auditory canal samplings.
Another technique for calibrating a noncontact
infrared thermometer is disclosed by Berman et al. in U.S.
Patent No. 4,784,149. Berman et al. therein disclose an
automatic calibration technique for an infrared thermometer
whereby the housing of the device is provided with a chamber
shaped to receive the probe and a target for viewing by the
infrared sensor when the probe is in the chamber. An error
signal is thereby generated which is added to the output
signals of an ambient temperature sensor and an infrared
sensor within the probe when they view a body tissue for
temperature measurement.
Fraden discloses in U.S. Patent No. 4,797,840 a
further calibration technique for an infrared thermometer in
which a pyroelectric sensor in the thermometer housing is
shielded from infrared radiation from exterior to the
thermometer housing and is then selectively exposed to
infrared radiation from the object to be measured to generate
a first electrical signal related to the absolute temperature
of the object to be measured. The ambient temperature of the
pyroelectric sensor is then sensed and a second electrical
signal proportional thereto is generated. The first and
second electrical signals are then processed to calculate the
temperature of the object to be measured. Errors due to
temperature differences are minimized by thermally isolating
the barrel of the sensor (which is in thermal equilibrium with
the pyroelectric sensor) from ambient heat sources such as the
human body by a protective thermoisolator coating.
Calibration is accomplished by electrical calibration using
a calibration circuit (Figure 9) including a piezoelectric
W094/085~ 2 1 4 6 1 36 PCT/US92/08~
- 5 -
element which creates a mechAnical stress calibration signal
when a shutter is closed. The value of the electrical signal
at the time of calibration of the thermometer is stored in
memory. The shutter is then opened for temperature readings,
and the resulting reading is adjusted by the stored value.
Wood discloses in U.S. Patent No. 4,895,164 a
tP~hnique for maintaining accuracy in clinical settings when
using a noncontact infrared tympanic thermometer. During use,
the radiation sensor is held in isothermic condition with a
waveguide at ambient temperature. A thermistor or some other
temperature sensor is thermally coupled to the radiation
sensor for compensating for changes in ambient conditions.
Also, the infrared radiation sensor of the device of Wood is
constructed and configured so as to remain in an isothermic
state, even during changes of ambient temperature, by
positioning the infrared radiation sensor assembly within the
housing so as to form an insulative air space between the
housing wall and the isothermic assembly. Low emissivity
barriers such as polished or gold plated aluminum tubing are
also placed around the protruding portion of the waveguide in
order to insulate the waveguide to limit the effects of
temperature changes. Thus, accuracy is maintained in the
device of Wood by maintaining a thermally stable environment
around the detector.
On the other hand, Beckman et al. disclose in U.S.
Patent No. 4,900,162 a radiometer thermometry system for
measuring the temperature of a target such as a tympanic
membrane in which the temperature of the radiation detector
can be changed so as to minimize the difference between the
temperature of the target (tympanic membrane) and the
temperature of the radiation detector (ambient temperature).
The target temperature is thus detected by a sort of
successive approximation (null seeking) technique. A related
technique is taught by Egawa et al. in U.S. Patent No.
4,932,789, who teach preheating the probe to a reference
temperature close to normal body temperature despite the
ambient temperature.
W O 94/08506 2 1 4 6 1 3 ~ PC~r/US92/08644
Junkert et al. disclose in U.S. Patent No. 4,722,612
an attempt to stabilize the detector rather than the
detector's environment. In the device of Junkert et al., an
optical blocking baffle is placed over one-half of a two
s element pair so as to render that half insensitive to incoming
radiation. The thermopile detectors are connected in series
opposition and adjacent to each other. Both detector halves
are sensitive to ambient temperature, and since only one
detector is sensitive to incoming radiation, the Junkert et
al. detector is "stabilized" against some ambient temperature
fluctuations. However, by totally blocking the radiation
falling on a portion of the detector in this manner, Junkert
et al. cause a temperature gradient to be generated in the
detector substrate, thereby inducing errors. Moreover, by
blocking radiation at the substrate level, unwanted radiation
from the walls of the detector package is not eliminated and
thus causes further errors.
Devices for preventing contamination of a tympanic
thermometer because of the accumulation of ear wax and the
like are also known. For example, O'Hara et al. disclose in
U.S. Patent No. 4,662,360 a protective, disposable speculum
for use with an infrared tympanic thermometer. The device
includes an infrared transparent window and method for
manufacturing it in a manner that would insure repeatable
infrared transmission properties through the window, for if
the transmission properties vary from unit to unit the
calibration of the instrument would not be stable. Since the
device is disposed of after each use, cross-contamination by
ear wax and the like is avoided. Another example of a
disposable speculum is disclosed by Twentier in U.S. Patent
No. 3,878,836, while a speculum for use with an otoscope is
described by Kieffer, III et al. in U.S. Patent No. 4,380,998.
Although prior art infrared thermometers of the type
described above have addressed many of the shortcomings of the
Barnes thermometer, several problems remain. For example,
since the prior art noncontact infrared thermometers propose
to maintain accuracy primarily by maintaining a thermally
W094/08506 21 4 6 13 G PCT/US92/08~W
- 7 -
stable environment around the detector during operation, their
accuracy is limited by the ability to maintain isothermic
conditions in areas where there may be substantial temperature
differences. In other words, since the environment around the
detectors may not be made truly isothermic, more accurate
detectors which need not be maintained under such conditions
and which do not induce temperature gradients are desired for
clinical use.
Reliable infrared detector devices which are
extremely accurate in clinical use have been previously
developed by the present inventors for use in a capnograph.
As described in our U.S. Patent Application Serial Nos.
07/401,952 (abandoned) and 07/522,208 and 07/522,177 (now U.S.
Patent Nos. 5,095,913 and 5,081,998 respectively), we have
previously developed a detector device which eliminates
thermal drift of thermopile detectors used in detecting the
concentration(s) of at least one gaseous component of gases
expired by a patient. The apparatus described in those
applications comprises at least two series opposed infrared
detectors which generate electrical signals when illuminated
by optical energy provided by an infrared radiation source.
In other words, the detectors are connected so that their
outputs are subtracted. Outputs from the detectors are thus
possible only if the same energy does not fall equally on both
detectors. For this purpose, means for attenuating the
infrared energy illuminating at least one of the detectors is
provided. An optically stabilized signal is thereby produced
since the undesired infrared signals fall upon both detectors
equally and are effectively cancelled out, while the desired
signal is maintained because of the difference provided by
using the attenuating means over one of the optical detectors.
The resulting information may then be processed and displayed
as representative of the concentration of elements such as C02
expired by a patient. However, no such tech~ique has
previously been used to overcome the above-mentioned problems
with noncontact infrared thermometers so as to permit more
widespread clinical use of such devices.
W O 94/08506 2 1 4 6 1 36 PC~r/US92/08644
- 8 -
Accordingly, it is desired to overcome the
aforementioned problems with prior art noncontact infrared
tympanic thermometers by utilizing optically stabilized
infrared detectors. The present invention has been designed
to adapt our above-mentioned optical stabilization techniques
into noncontact infrared tympanic thermometers so as to
improve their accuracy without adversely affecting their ease
of use.
~MMARY OF T~F INVENTION
The present invention relates to a noncontact
infrared tympanic thermometer which utilizes the optical
stabilization techniques of our capnograph inventions
referenced above. However, the optical stabilization
technique of the present invention differs from that employed
in our capnograph applications in that the filters of the
present invention are not mounted on the detectors but rather
in the "window" of the detector package which is inserted into
the auditory canal. The attenuating filter is also placed so
as to eliminate the thermal effects of the detector package
itself. In other words, the attenuating filter of the
invention is disposed such that the undesirable thermal
effects caused by the detector device walls are effectively
cancelled out. As will become more apparent from the
following detailed description, this feature of the invention
enables all undesirable infrared energy to be cancelled out
and thus removed from the infrared energy detection.
In accordance with a preferred embodiment of the
invention, an apparatus is provided for measuring the
temperature of an infrared radiation emissive target, such as
a tympanic membrane, comprising:
means for sensing the amount of incident infrared
radiation emitted by the infrared radiation emissive target,
comprising:
first and second thermopiles, connected in
opposed relation to each other and positioned on
the same substrate so as to receive incident
W094/08506 2 1 4 6 1 3 6 PCT/US92/08644
g
infrared radiation emitted by the infrared
radiation emissive target and by an ambient
environment of the first and cecond thermopiles,
for producing ouL~L electrical signals
representative of the intensity of the incident
infrared radiation,
a bandpass filter, disposed in an energy path
between the infrared radiation emissive target and
the first and cecond thermopiles, for passing a
predetermined range of infrared wavelengths of
incident infrared radiation from the radiation
emissive target to the first and second
thermopiles, and
an attenuating neutral density filter disposed
in the energy path so as to attenuate the incident
infrared radiation impinging upon only one of the
thermopiles from the radiation emissive target
without attenuating incident infrared radiation in
the energy path due to infrared emissions by the
ambient environment of the first and second
thermopiles; and
means for processing the output electrical signals
to determine the temperature of the radiation emissive target
substantially independent of ambient temperature variations
of the ambient environment.
In a preferred embodiment, a reference detector is
also provided for detecting the ambient temperature of the
ambient environment and generating a reference temperature
signal which is proceC-ce~ with the output electrical signals
by the processing means to determine an absolute temperature
of the infrared radiation emissive target. Such processing
means preferably comprises an amplifier for increasing the
gain of the output electrical signals, a filter for
eliminating noise in the output electrical signals, and a
microprocessor for determining the temperature from the output
electrical signals and the reference temperature signal. As
would be apparent to one of ordinary skill in the art, the
w094/08506 2 1 4 ~ 1 3 6 PCT/US92/08644
-- 10 --
infrared radiation emissive target may be other internal
tissues of a patient as well.
The arrangement of the invention may be incorporated
into a noncontact infrared tympanic thermometer or a multi-
function otoscope. Such~-devices also preferably include a
display mounted on its housing for displaying the measured
temperature. To facilitate such display, the device of the
invention may include a tilt sensor for inverting the
temperature display to read upright and left to right
independent of the orientation of the device's housing. In
addition, a speculum may be used for covering the end of the
device's probe to prevent contamination of the directing means
when the device is inserted into the patient's ear for the
measurement of the amount of infrared radiation emitted by the
patient's tympanic membrane. Also, the attenuating neutral
density filter preferably has a transmission coefficient of
approximately 0.50 in a preferred embodiment.
The invention also comprises a method of measuring
the temperature of an infrared radiation emissive target, such
as a tympanic membrane, comprising the steps of:
sensing the amount of incident infrared radiation
emitted by the infrared radiation emissive target, comprising
the steps of:
positioning first and second thermopiles in
opposed relation to each other and on the same
substrate so as to receive incident infrared
radiation emitted by the infrared radiation
emissive target and by an ambient environment of
the first and second thermopiles,
passing a predetermined range of infrared
wavelengths of incident infrared radiation from the
infrared radiation emissive target to the first and
second thermopiles,
attenuating the incident infrared radiation
impinging upon only one of the first and second
thermopiles from the radiation emissive target
without attenuating incident infrared radiation
W O 94/08506 ~ 1 4 6 1 3 6 PC~r/US92/08644
received from the ambient environment of the first
and s~con~ thermopiles, and
producing output electrical signals
representative of the intensity of the incident
infrared radiation received by the first and second
thermopiles; and
processing the ouL~u~ electrical signals to
determine the temperature of the radiation emissive target
substantially independent of ambient temperature variations
of the ambient environment.
Such a method in accordance with the invention
preferably also comprises the further steps of detecting an
ambient temperature of the ambient environment, generating a
reference temperature signal corresponding to the detected
ambient temperature, and processing the reference temperature
signal with the output electrical signals to determine an
absolute temperature of the infrared radiation emissive
target. In addition, the processing step preferably comprises
the steps of amplifying the output electrical signals so as
to increase their gain, filtering the output electrical
signals so as to eliminate noise, and determining the
temperature from the output electrical signals and the
reference temperature signal.
Preferably, the infrared radiation emissive target
whose temperature is measured in accordance with the invention
comprises an internal tissue of a patient and the positioning
step includes the step of placing first and second thermopiles
in a body cavity cont~ining the internal tissue. In addition,
when used in conjunction with a tympanic thermometer, the
positioning step of the method of the invention may further
include the step of placing a device housing the first and
second thermopiles into the patient's ear canal. Also, the
resulting temperature is preferably displayed on a temperature
display. Such a displaying step may also comprise the step
of inverting the temperature display to read upright and left
to right independent of the orientation of a device housing
the first and second thermopiles. Moreover, to prevent cross-
W094/085~ 2 1 4 6 1 36 PCT/US92/08~
- 12 -
contamination, the method of the invention may include the
further step of co~vering the device housing the first and
second thermopiles with a speculum before the device is
inserted into the patient's ear for the measurement of the
amount of infrared radiation emitted by the patient's tympanic
membrane.
BRIEF DE8CRIPTION OF T~E DRAWING8
The above and other objects and advantages of the
present invention will become more apparent and more readily
appreciated in the following description of presently
preferred embodiments, taken in conjunction with the
accompanying drawings, of which:
FIGURE 1 illustrates a cutaway view of a noncontact
infrared tympanic thermometer in accordance with the
invention.
FIGURE 2 illustrates a perspective side view of an
optically stabilized detector for use in the tympanic
thermometer of the invention.
FIGURE 3 illustrates a preferred embodiment of the
processing circuit for processing the outputs of the optically
stabilized detector and reference detector so as to produce
a temperature signal.
DET~TT~D DESCRIPTION OF T~E PRE8ENTLY PREFERRED EMBODIMENT8
A device having the above-mentioned beneficial
features in accordance with presently preferred exemplary
embodiments of the invention will be described below with
reference to FIGURES 1-3. It will be appreciated by those of
ordinary skill in the art that the description given herein
with respect to those figures is for exemplary purposes only
and is not intended in any way to limit the scope of the
invention. All questions regarding the scope of the invention
may be resolved by referring to the appended claims.
FIGURE 1 illustrates a cutaway view of a noncontact
infrared tympanic thermometer 10 in accordance with the
invention. As shown, the tympanic thermometer 10 of the
W094/08506 21~6136 PCT/US92/08~4
invention is similar in ~hape to an otoscope and is comprised
primarily of a housing or body portion 12 (which is generally
made of plastic or some other rigid material), a protruding
probe portion 14 (preferably made of aluminum or plastic) for
insertion into the patient's ear, an optically stabilized
thermopile detector 16, and a disposable speculum 18 which is
placed over the probe portion 14 before insertion of the probe
portion 14 into the auditory canal for a temperature
measurement. Within the housing 12 is preferably disposed a
circuit board having a processing circuit 20 powered by a
battery 22 upon depression of a button 24. The resulting
temperature reading is preferably displayed on a digital
display 26.
The tympanic thermometer 10 of the invention is
preferably engineered to fit comfortably in the human hand.
It is a single piece device with a temperature display 26 on
one end and a curved design and tapered ear probe which make
it easier to hold and aim at the tympanic membrane. The body
of the device can be made small by incorporating "surface
mount" electronic technology, and in a preferred embodiment,
the ear piece of the invention can be made as small as 0.16
inches in diameter, the same size as a conventionally used 4mm
otoscope speculum.
FIGURE 2 illustrates an optically stabilized
infrared detector 16 in accordance with the invention. As
shown, the optically stabilized detector 16 of the invention
generally comprises a sensor housing 161 having disposed
therein a hAn~pAss filter 162, a two-channel thermopile
detector 163, a neutral density filter 164 and a reference
thermistor 165. The sensor housing 161 or "can" is designed
to hold the detector circuitry in a predetermined relationship
as will be described below. Bandpass filter 162 is designed
to pass infrared wavelengths in a range of approximately 8-14
microns, which corresponds to emissions in the range of the
internal temperature of a human being, for example. The two-
channel thermopile detector 163 is preferably a pair of series
opposed thermopile detectors of the type described in our U.S.
W094/08506 2 1 4 6 1 3 6 PCT/US92/08~
application Serial Nos. 07/401,952 (abandoned), 07/522,208
(USP 5,095,913) and 07/522,177 (USP 5,081,998), referenced
above, the contents o~'which are hereby incorporated by
reference as if set forth herein in their entirety.
Generally, the two-chAn~el thermopile detector 163
in accordance with the invention comprises first and second
series opposed thermopile detectors mounted on a common
ceramic substrate as described in our aforementioned
applications. As described therein, the individual thermopile
detectors are formed by depositing a bi-metallic circuit upon
a polyester film, such as Mylar, or another suitable
substrate. Preferably, the thermocouples have a substrate
thickness of approximately 1 mil. The thermopile detectors
of the two-channel thermopile detector 163 are connected in
series opposition so that the substraction of their outputs
is inherent in their interconnection. The importance of this
interconnection will be described more fully below.
The neutral density (attenuation) filter 164 of the
detector 16 uniformly attenuates all wavelengths of energy
which are incident upon it. In a preferred embodiment, the
neutral density filter 164 may have a transmission coefficient
of 0.50, but a transmission coefficient of 0 is also possible
in accordance with the invention. Of course, one skilled in
the art may use other neutral density filters with different
transmission coefficients as desired. However, as will be
apparent to one of ordinary skill in the art, the transmission
coefficients preferably are not chosen to be close to 1.00
since that would inhibit operation of the detector.
Generally, transmission coefficients of less than 0.75 are
preferred. The neutral density filter 164 is placed within
the housing 161 so as to overlap or "shadow" only one of the
thermopile detectors of the two-channel thermopile detector
163 so that the received infrared radiation is only attenuated
before impinging upon the "shadowed" detector. This enables
a differential signal to be developed as will be described
below.
W094/08506 2 1 4 6 1 3 6 PCT/US92/08~
- lS -
Finally, reference thermistor 165 determines the
ambient temperature of the detector 163 so that an absolute
temperature measurement may be determined in accordance with
known principles. This feature of the invention also will be
described in more detail below.
The optically stabilized infrared detector 16 of the
invention is unique in that the detector 16 is sensitive only
to radiation from the target (the tympanic membrane) and not
the side walls of the detector package. As a result, the
device does not need to be maintained under isothermic
conditions or repeatedly calibrated as in prior art devices.
This selectivity of the invention is accomplished by locating
the neutral density filter 164 over half of the optical
aperture of the detector package. As shown in FIGURE 2, the
detector's active element consists of two thermopile channels
connected in series opposition such that any optical signal
equally present in both channels will yield a zero net output.
Signals that reach each detector substantially equally include
the infrared energy emitted by the walls of the detector
package 161. In a tympanic thermometer, the target
temperature is typically 37 C, while the detector wall
temperature varies from 20-37 C. Because the two surfaces
(the tympanic membrane and the detector wall) are at
substantially the same temperature, emissions from the
detector wall 161 that are not in some way eliminated from the
measurement can cause large errors in the measured
temperature, as recognized in the prior art.
The above problem is overcome in accordance with the
invention because the infrared energy from the detector walls
161 falls substantially equally on the two thermopile channels
and is thus cancelled by the series opposition connection.
If it were not for the neutral density filter 164, infrared
energy from the tympanic membrane would also fall equally on
the two thermopile detectors and similarly cancel. However,
the neutral density filter is aligned so that it "shadows" one
of the detector channels as described above so that one
detector receives less target energy than the other, thereby
w094/08506 2 ~ 461~ 6 PCT/US92/08~
--16 -
allowing a differential signal to be developed. This
differential signal is then ~o~-e~ in order to determine
the temperature as will be described below with reference to
FIGURE 3.
While there are other techni ques for eliminating
detector side wall emissions, such as controlling the side
wall temperature and subtracting a constant from the signal
as in the prior art, the present invention is more advanced
because it allows "dynamic" elimination of the effect of the
side wall emissions. Dynamic elimination is critical in
situations where the detector side wall temperature changes,
as is the case when the speculum of the tympanic thermometer
is inserted into the auditory canal. At that time the
detector begins a rapid transition from substantially room
temperature to substantially body temperature. The present
invention is designed such that this temperature transition
does not adversely affect the temperature reading. This is
so because the signals from any infrared emissions originating
from behind the neutral density filter 164 in the sensor
assembly are effectively canceled at the thermopile detectors
as described above.
Thus, with the neutral density filter 164 in place
and properly aligned and the two detector channels connected
in series opposition, a signal will be developed as a result
of infrared energy from the target, but none will result from
detector package wall emissions. In general, no emissions
occurring on the detector side (h~h i n~) the neutral density
filter 164 will result in an output signal because there is
no element to produce a differential signal. Only infrared
emissions from in front of the filter 164 result in an
electrical output signal because only those emissions pass
through the neutral density element 164 which "shadows" one
of the detector channels causing the differential signal at
the detector. Accordingly, the neutral density filter 164 is
preferably placed as close to the infrared energy source
(tympanic membrane) as possible in order to cause cancellation
W094/08506 2 1 ~ 6 1 3 6 PCT/US92/08~
- 17 -
of all undesired infrared emissions h~hi~ the neutral density
filter 164.
In an alternate embodiment, the neutral density
filter 164 can be placed in the speculum 18 so as to be as
close as possible to the tympanic membrane. In such an
embodiment, infrared emissions from the ~peculum itself as
well as the detector walls, both of which are located behind
the filter, can be cancelled so as to eliminate both sources
of error. However, as shown in FIGURE 2, in the presently
preferred embodiment the neutral density filter 164 is placed
within the housing 161 of the optically stabilized detector
16 since it is difficult to place the neutral density filter
164 on the speculum 18 without casting a "shadow" on both
thermopile detectors 163. Of course, it is believed to be
well within the skill of one of ordinary skill in the art to
properly align a speculum having a neutral density filter 164
such that the "shadow" of the neutral density filter 164 only
covers one of the thermopile detectors.
FIGURE 3 illustrates the interconnection of the
thermopile detector 16 and the processing circuit 20 of the
invention. As described above, the infrared detector 16
comprises thermopiles 163 which are connected series opposed.
The received infrared radiation is received by these
thermopile detectors 163 to create output signals. Also, the
reference temperature of the detector 16 is determined by
reference thermistor 165, and the output of the reference
thermistor 165 is connected across resistor 201 to a voltage
reference Vref and also applied to a positive input of an
operational amplifier 202. A negative input of operational
amplifier 202 receives a fed back output of operational
amplifier 202 across resistors 203 and 204 as shown. The
result is that the output of operational amplifier 202 is
amplified so as to boost the signal gain of the output of
reference thermistor 135 to a level which is acceptable by
signal processing and display circuit 205. The detected
reference temperature signal is therein processed as will be
described below.
w094/08506 2 1 4 6 1 ~-~ PCT/US92/08~
- 18 -
The series opposed outputs of thermopiles 163 are
output to respective inputs of an operational amplifier 206.
The output of a feedback network is also applied to the
positive input of the operational amplifier 206, this network
including a capacitor 207 and a resistor 208 which together
provide a roll-off capacitance for rejecting high frequency
noise. The ouL~uL of operational amplifier 206 thus
represents the difference signal between the respective
detector outputs. This difference signal is applied across
resistor 209 to a positive input terminal of operational
amplifier 210. A feedback resistor 211 is also provided to
connect the output of operational amplifier 210 to the
positive input terminal of operational amplifier 210. The
negative input terminal of operational amplifier 210 receives
a feedback signal from a network comprising resistor 212,
resistor 213 and capacitor 214 as shown. The output of
resistor 213 is also applied to a positive input terminal of
operational amplifier 215, and the output of operational
amplifier 215 is fed back to the negative input terminal
thereof as shown. Finally, a capacitance 216 connects the
common node between resistors 212 and 213 to the output of
operational amplifier 215 as shown. As would be apparent to
one skilled in the art, the circuit comprising elements 209-
216 functions as a low pass filter with a predetermined time
constant. Preferably, the time constant is chosen to be on
the order of the time response of the thermopile detector 163
so as to roll off high frequency noise such as the 60 Hz line
frequency noise common in the United States. In a preferred
embodiment of the invention, the resulting low pass filter has
a 100 msec time constant. The resulting low pass filtered
target signal is then applied to the signal processing and
display circuit 205.
Signal processing and display circuit 205 processes
the detected reference temperature and target signals in order
to determine the absolute temperature of the tympanic membrane
and hence the patient's internal temperature. The functions
of signal processing and display circuit 205 may be programmed
WO 94/08506 2 1 ~ 6 1 3 6 PCI/US92/08644
-- 19 --
into a computer through software, such as a Mackintosh SE
computer, but in a preferred emho~liment of the invention, the
signal processing and display circuit 205 may comprise a
single chip microproc~s~or such as the Intel 8051 and an A/D
5 converter such as the Analog Devices AD7824. Of course, other
microprocessors and A/D converters may also be used in
accordance with the invention.
As noted in the article by C. Hamel referenced
above, the voltage ou~uL of a typical thin film thermopile
detector directed at a target of temperature, Tt, may be
simply determined in accordance with a known equation. In
particular, signal processing and display circuit 205
processes the detected reference temperature signal (Ta) and
the target temperature signal (Tt) in accordance with the
following equation:
Eo~t = M(Tt - T- )
where:
Eo~t = the detector output voltage;
Tt = target temperature in degrees K;
T~, = ambient temperature in degrees K; and
M = z (rAtl~) R,
II D
where:
z = 5.688 x 10 2 W/cm2/K (the Stefan-Boltzman
constant);
I~ = a constant representative of the optical
properties of the device (may be determined
empirically);
At = area of target in cm;
Ad = area of detector element in cm;
D = distance from detector element to target; and
R = responsivity of detector in V/W, where typically
R = 10 V/W.
From the foregoing equation, the target temperature
Tt in absolute terms is determined. The result is then
displayed on display 26 (FIGURE 1) in accordance with known
techniques.
Thus, the present invention as configured in the
preferred embodiments does not require environmental
stabilization or waveguide temperature control as in the prior
art because it utilizes an optically stabilized infrared
W094/08506 2 1~ 6 1 3 6 PCT/US92/08~
- 20 -
detector 16. The optical stabilization technique in
accordance with the invention renders the tympanic thermometer
incencitive to ambient temperature effects and allows it to
read true tympanic membrane temperatures. Moreover, by
placing the optically stabilized detector 16 as close as
possible to the end of the probe 18, all unwanted infrared
emissions may be cancelled out.
Accordingly, the present invention is a significant
improvement over prior art devices of its type in that the
device is small enough to be placed inside the ear so as to
eliminate the need for a waveguide and in that the device is
constructed of two detector elements and a neutral density
optical filter built into the window of the detector package
so as to eliminate the sensing of emissions from the detector
itself. The construction of the detector of the present
invention thus allows it to be located inside the auditory
canal, thereby removing the requirement for a waveguide to
bring energy from the tympanic membrane to the detector. This
eliminates the sensing of emissions from the waveguide and
correction for this unwanted signal.
As would be apparent to one of ordinary skill in the
art, the present invention need not use a closed end speculum
cover 18. Rather, the speculum cover 18 of the invention may
be similar to that used with conventional otoscopes whereby
an opening is provided through which to "view" the tympanic
membrane. It is accepted and understood that an open ended
otoscope probe cover provides acceptable safety from cross
contamination by exten~ing sufficiently beyond the reusable
parts to prevent patient contact. Also, the open ended probe
cover design provides for minimal manufacturing cost and is
presently preferred. On the other hand, as noted above, the
speculum cover 18 may be modified such that the neutral
density filter 164 is incorporated therein. Of course, such
a speculum cover would not be open ended.
Although a number of exemplary embodiments of the
invention have been described in detail above, those skilled
in the art will readily appreciate that many additional
W094/08506 2 1 4 6 1~6 PCT/US92/08644
- 21 -
modifications are possible in the exemplary embodiments
without materially departing from the novel teachings and
advantages of this invention. For example, the infrared
detector of the invention may be incorporated into a
conventional otoscope so that a multi-functional otoscope may
be formed. Accordingly, all such modifications are intended
to be included within the scope of this invention as defined
in the following claims.
