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This invention relates to an apparatus for effecting rapid and accurate
ultrasonic measurements of the axial length of the eye.
A compact and simple ultrasonic instrument (echo-oculometer) for measuring
the axial length and anterior chamber depth of the eye has been described
in the literature by Mortimer et al. in the Proceedings of the 11th Intl.
Conf. on Medical and Biol. Engng. 1976 pp. 508-509 and by Mortimer et al.
in the Canadian J. Ophthal. Volume 12, 1977 pp. 318-320.
Advantages of this instrument over conventional A-Scan devices employing
cathode ray tubes for display are that a display of the A-Scan is not
essential, the results are immediately available on a counter and are
expressed in convenient numerical units.
The above echo-oculometer utilizes techniques similar to those used in the
echo-encephalograph invented by Hudson et al. and described in U.S. Pat.
No. 3,872,858 issued Mar. 25, 1975 and its corresponding Canadian Pat. No.
973,632 issued Aug. 26, 1975. The echo-oculometer employs a transducer
which emits a short pulse of ultrasound aimed along the ocular axis. The
echoes returning from the various surfaces within the eye are received by
the same transducer and the time required for the sound pulse to return is
converted to a length measurement. Two range gates consisting of
electronic logic circuits allow echoes from particular interfaces to be
selected and the corresponding time interval to be measured. The retinal
echo is selected for the axial length and the anterior lens echo is
selected for the anterior chamber depth measurement. The statistical
accuracy of the determinations may be increased by averaging several
readings.
Three important features of the above echo-oculometer device are that it
employs a slow gain sweep, a crystal controlled time base of a particular
frequency, and a special delay circuit which determines the time at which
the counter starts to count.
In contrast to the echo-encephalograph which employs a second fast gain
sweep (funcioning analogously to the time varied gain or TGC of the
conventional A and B scan equipment), the oculometer has only a slow gain
sweep since compensation for tissue attenuation is not needed. For the
slow gain sweep, the gain does not vary significantly during the time a
given pulse is transmitted and its echoes are received. Rather the
receiver gain increases from a transmitted pulse to the next until the
range-gated signal exceeds a predetermined threshold, stoping the counter
(displaying the accumulated count), or until the maximum receiver gain
level is reached whereupon the measurement cycle is automatically
repeated.
The gain sweep has two functions. It is supposed to compensate for
differences in the amplitudes of the received echoes (in different eyes)
and, more important, it is supposed to insure that the strongest echo
detected in the range gate interval selected will be the first to exceed
the detection threshold thereby stopping the counter.
Key assumptions made in the operation of the above echo-oculometer device
are that (1) when the beam is directed along the axis of the eye, the
first echo in the selected range gate will be the largest echo, and (2) if
the beam is directed off-axis, the echoes received will be too weak (due
to the inclination of the reflecting interface and the transducer
directivity) to exceed the range gate detection threshold.
While these assumptions are generally true for the anterior lens echoes
(anterior chamber depth), research and clinical experience conclusively
show that this is not always true for the much more important retinal
echoes (axial length). In a significant number of cases, other interfaces
and structures behind the retina give rise to the largest echoes. This can
result in errors in the axial length determinations of as much as 3 mm
which corresponds to an error in the lens power determinations of about 8
or 9 diopters. An eror of this magnitude is altogether unacceptable.
Furthermore, it must be emphasized that although this problem is much more
common for the case of off-axis beam incidence, it will still sometimes
occur when the beam is properly aligned.
Finally, if readings are taken with the beam improperly aligned, the
accuracy of the axial length determinations will be decreased either due
to the problem just described or in the case where the retinal echo does
stop the counter, the fact that a chord shorter than the axial diameter is
being measured.
One method of counteracting these problems would be to increase the beam
directivity but this is subject to both theoretical and practical
limitations. Another approach might be to lower the upper limit of the
swept gain. However, there are obvious constraints since the instrument
must accommodate a considerable range of ultrasonic and geometrical
characteristics for different eyes.
Aother method would be to require the presence of the anterior and
posterior lens echoes (either one but preferably both) of a magnitude
equal or greater than some specified fraction of the retinal echo
threshold (typically 1/2 or more) as a necessary condition for a valid
reading. In fact, the standard A-scan technique (used in determining the
axial length) consists of insuring that both the anterior and posterior
lens echoes are simultaneously present together with the retinal echo, and
then maximizing the two lens echoes while maintaining a good clean and
large retinal echo. The technique is illustrated in detail by Leary in
Ultrasonics April 1967, pp. 84-87. Under normal conditions, the symmetry
of the eye is such that following the above procedure will insure good
axial alignment.
Implementing the above lens echo conditions in the form of electronic
circuits to assure that readings are obtained only under conditions of
good alignment is straightforward and is obvious to those skilled in the
art. However, while this would greatly reduce the likelihood of incorrect
triggering by structures behind the retina, it does not entirely eliminate
the problem. Also the difficulty of achieving exact alignment without
reference to an A-Scan can make the actual obtaining of readings very
problematic. Certainly adding the lens echo condition reduces the speed
with which valid readings can be obtained. This is an important
consideration when dealing with older or uncooperative patients.
It is therefore the object of the present invention to provide an apparatus
for effecting rapid and accurate ultrasonic measurements of the axial
length of the eye.
The apparatus, in accordance with the invention, comprises a transducer
adapted to transmit repetitive ultrasonic pulses along the ocular axis of
the eye of a patient and receive echo pulses reflected from the retina of
the eye, a fixed gain amplifier connected to the transducer for amplifying
such reflected echo pulses, an automatic gain controlled amplifier also
connected to the transducer for amplifying the reflected echo pulses,
control means coupled to the automatic gain controlled amplifier for
gradually increasing the gain of the amplifier during a measurement cycle,
first and second gate circuits controlled by the output of the fixed and
automatic gain controlled amplifier, respectively, and adapted to pass
logic signals triggered by retinal echo pulses exceeding first and second
predetermined thresholds, a digital counter connected to the second gate
circuit and adapted to display the axial length of the eye as a function
of the distance travelled by the retinal echo pulses, a gate delay
initiated by a slow clock, a gate width generator connected to the gate
delay for generating a time slot during which echo pulses originating from
the posterior wall of the eye can be received, a latching circuit
responsive to the gate delay for enabling the first gate circuit to pass
logic signals triggered by echo pulses exceeding the first threshold, a
retinal echo triggered gate width generator interconnecting the first and
second gate circuits and responsive to the first gate circuit for enabling
the second gate circuit to pass logic singals triggered by retinal echo
pulses exceeding the second threshold in the time slot generated by the
gate width generator, the output of the retinal echo triggered gate width
generator being also connected to the latchng circit for blocking the
first gate circuit immediately after receipt of the first logic signal
triggered by a retinal echo pulse, thereby preventing mistriggering of the
retinal echo triggered gate width generator by echoes originating from
structures behind the retina.
The above control means includes a slow ramp generator adapted to generate
a ramp voltage which is applied to the automatic gain controlled amplifier
in such a manner that the gain of the amplifier varies from a minimum at
the start of the measurement cycle to a preset maximum after a number of
cycles.
A pulser is connected to the transducer for applying sharp high voltage
spikes to such transducer to shock excite it so as to direct an ultrasonic
pulse into the eye being examined. The pulser is triggered by the slow
clock. A fast clock, operating at a frequency in MHz which is 1/2 times
the average velocity of ultrasound in the eye expressed in units of 0.1 mm
per microsecond, provides the counter times base and is used to
synchronize the slow clock.
A pulser delay is located between the slow clock and the pulser to insure
that the counter starts counting at the correct time.
The invention will now be disclosed, by way of example, with reference to
the accompanying drawings in which:
FIG. 1 is a block diagram of an exemplary embodiment of the invention;
FIG. 2 is a series of waveforms produced at various points in the block
diagram of FIG. 1; and
FIG. 3 is a circuit diagram of the echo-oculometer constructed according to
the prsent invention.
Referring to FIGS. 1 and 2, there is shown a fast clock 10 which generates
a continuous signal at a frequency in MHz which is numerically equal to
1/2 times the average velocity of ultrasound in the eye (1553 m/sec) in
units of 0.1 mm per microsecond, that is 7.765 MHz, and has a sinusoidal
waveform as illustrated at A in FIG. 2. The clock 10 feeds a signal to a
counter-display 12 and to a slow clock 14. The slow clock 14 generates a
square wave at a frequency of about 60 Hz as shown at B in FIG. 2 of the
drawings. The operation of the slow clock 14 is synchronized to the fast
clock 10. The output signal of the slow clock is applied to a pulser delay
16 which generates a repetitive pulse signal such as shown at C in FIG. 2.
The pulser delay 16 triggers a pulser 18 on the positive going edge of the
waveform C and at the same time resets the counter 12. The counter begins
counting on the negative going edge of the waveform C to compensate for
the propagation time of the ultrasonic pulse from the transducer to the
eye and from the eye to the transducer. Pulser 18 generates a sharp high
voltage spike, such as shown at D in FIG. 2, whish is used to shock excite
a transducer 20 to direct an ultrasonic pulse through the eye being
examined. This pulse travels through the eye and is reflected by the
various surfaces and returned to the transducer. These echo pulses are
picked up by the transducer and converted back to electrical signals which
are directed to a preamplifier 22 providing an output such as shown at E
in FIG. 2. The output of the preamplifier 22 is applied to a fixed gain
amplifier 24 providing an amplified output such as shown at F in FIG. 2
and to an automatic gain control amplifier 26 providing an amplified
output such as shown at G, H or I in FIG. 2 depending on the gain of the
amplifier as controlled by the voltage V applied to its automatic gain
control terminal. The output of the fixed gain amplifier 24 is fed to a
first gate circuit 28 through a comparator 30 which sets a signal
threshold level for the output of amplifier 24 as illustrated by a dashed
line through waveform F in FIG. 2. Similarly, the output of the automatic
gain control amplifier 26 is fed to a second gate circuit 32 through a
comparator 34 which sets a signal threshold level for the output of
amplifier 26 as illustrated by a dashed line through waveform I in FIG. 2.
The present apparatus is capable of measuring not only the axial length
(AL) of an eye by detecting the retinal echo pulses but also and anterior
chamber (AC) depth by detecting the anterior lens echo pulses. As
mentioned previously, the measurement of the anterior lens echoes is not
the object of the present invention, therefore the portion of the
circuitry which is concerned with the measurement of the retinal lens echo
will be primarily disclosed. In order to permit gate circuits 28 and 32 to
pass logic signals triggered by the echo pulses which are reflected from
the desired surfaces of the eye, there is provided a gate delay 36 which
is triggered by the positive leading edge of the signal B appearing at the
output of the slow clock 14. Gate delay 36 provides an output waveform
such as illustrated at L in FIG. 2, and triggers the gate width generator
38 on the positive edge of the waveform L. If the anterior chamber depth
was to be measured, the delay would of course be much shorter so as to
allow gating of the echo pulses originating from the anterior lens surface
of the eye. A function switch 37 is provided for selecting which one of
the measurements is to be performed by the apparatus. The gate width
generator 38 generates a signal M as shown in FIG. 2. Signal M is fed to
gate 32 and controls the time slot during which gate 32 is opened. Gate
width generator 38 is also responsive to function switch 37 for selection
of the desired measurement to be performed. The output L of gate delay 36
is also applied to a spike generator 40 which generates a signal shown at
N in FIG. 2. Signal N is fed to a latching circuit 42 which generates a
signal P, as shown in FIG. 2, for controlling the opening of gate circuit
28. Latching circuit 42 is reset by the output C of the pulser delay at
the beginning of each cycle.
A retinal echo triggered gate width generator 44 is connected between gates
28 and 32 and is triggered by output signal Q of gate circuit 28 when a
retinal echo signal exceeding the threshold of comparator 30 is present.
The retinal echo triggered gate width generator 44 provides an output R,
as shown in FIG. 2, which is applied to the gate circuit 32 to permit the
gate to pass logic signals triggered by echo signals originating from
automatic gain control amplifier 26 exceeding the threshold of comparator
34. The output R is also fed to the latching circuit 42 to cause the
latching circuit to immediately disable gate circuit 28 after receipt of
the first logic signal triggered by an echo signal originating from the
fixed gain amplifier 24. Thus, gate circuit 28 is latched out immediately
after the retinal echo pulses are detected to prevent retriggering of 44
thereby preventing gate 32 from passing logic signals triggered by echo
pulses originating from structures behind the retina. The retinal echo
triggered gate width generator 44 is disabled by the function switch 37
during anterior chamber measurement because it is not required.
The output T of gate 32 is fed to a display duration circuit 46 which
provides an output U to stop the counter and display, for a few seconds,
the distance travelled by the retinal echo pulse as an indication of the
axial length of the eye.
The gain of the fixed gain amplifier 24 is set by an amount approximately
10 to 14 dB greater (3 to 5 times greater) than the maximum gain of the
automatic gain control amplifier 26. This insures that the retinal echo
signal will be of sufficient amplitude to exceed the threshold of the
comparator 30 and that the retinal echo triggered gate width generator 44
will not be mistriggered by an echo pulse of greater amplitude originating
from structures behind the retina.
The gain of the automatic gain control amplifier 26 is varied by a slow
ramp generator 48 which generates a voltage of increasing negative
amplitude V starting from a minimum value at the beginning of the
measurement up to a maximum value set by a comparator 50. The output U of
the display duration circuit 46 is applied to the slow clock to enable the
same, and to the slow ramp generator 48 to reset the slow ramp generator
voltage V to its minimum value, when the automatic gain control amplifier
26 has sufficient gain to pass the signals exceeding the threshold set by
comparator 34.
The invention will now be disclosed with reference to the more detailed
circuit diagram of FIG. 3 which is intended to give a better understanding
of the invention but not to limit the scope thereof. The non-detailed
blocks as well as the circuit diagrams outlined in FIG. 3 by broken lines
carry the same references as the corresponding blocks of FIG. 1.
The fast clock 10 is a conventional crystal controlled oscillator operating
at a frequency of 7.765 MHz as mentioned previously. The output A of the
fast clock is fed to the counter-display 12 which is a conventional
digital counter capable of displaying a count when energized to do so. A
suitable example of such a counter is RCA No. ICAN-6733. The
counter-display is therefore operated by the fast clock to indicate
directly the axial length of the eye. For synchronizing of the clocks, the
output A of the fast clock is also applied to the clock input C of a
conventional type D flip-flop 60 which acts as a slow clock. The timing
period of the slow clock is about 60 Hz as mentioned previously and is
determined by a resistor R1 connected between terminals D and Q of the
flip-flop and capacitor C1 connected between terminal D and ground.
Flip-flop 60 may be enabled by clamping terminal D through diode D1 as
will be disclosed later.
The output of the slow clock is applied to pulser delay 16 through coupling
capacitor C2 and resistor R2. The pulser delay is comprised of a CMOS NOR
gate 62 and an inverter 64. Gate 62 has a first input connected to the
slow clock and a second input connected to the output of inverter 64. The
output of gate 62 is connected to the input of the inverter 64 through
capacitor C3. A positive potential V'+ is also applied to the input of the
inverter through resistors R3 and R4. Gate 62 and inverter 64 form a well
known monostable circuit. The output of the pulser delay is as shown at C
in FIG. 2 of the drawings.
The pulser 18, which is energized from a conventional high voltage source
66, is triggered on the rising edge of output C of the pulser delay and
produces a sharp high voltage spike D which is used to shock excite the
transducer 20 to direct an ultrasonic pulse into the eye of the person
being examined. This pulse travels through the eye and is reflected by
various surfaces of the eye, as mentioned previously, and returns to the
transducer. The echo pulses are detected by the transducer 20 and
converted back to electric signals which are fed to preamplifier 22. The
output of preamplifier 22 is applied to a fixed hi-gain amplifier 24 and
to an automatic gain control amplifier 26. The above mentioned circuit
elements 18, 22, 24 and 26 are conventional and need not be disclosed in
detail.
The output F of fixed hi-gain amplifier 24 is full-wave rectified by diodes
D2 and D3 and clamped to the voltage level determined by the voltage
divider resistors R5, R6 connected across a source V.sup.+. The clamped
signal (Gate trig) is fed to one of the inputs of a two input NOR gate 68.
The echo signal detection threshold voltage is equal to the difference
between the NOR gate logic threshold and the above clamping voltage. In
this particular embodiment, therefore, the resistor network R5 and R6 and
the logic threshold (approximately 1/2V.sup.+ for CMOS logic) constitute
essentially the equivalent of the comparator 30 while the NOR gate 68
constitutes the gating circuit 28.
Resistors R7 and R8 and capacitor C4 provide low-pass filtering for the
echo signals which improves the detection performance. While not
essential, full-wave rectification of the signal simplifies low-pass
filtering of the signal (in order to improve the signal to noise ratio).
In a manner similar to the preceeding, the output (G, H, I) of the
automatic gain control amplifier 26 is clamped to the voltage level
determined by the voltage divider, resistors R9 and R10, connected across
the V.sup.+ source. The clamped signal (stop) is fed to one input of the
NOR gate circuit 70. R9 and R10 and the NOR gate logic threshold thus form
the equivalent of the comparator 34, and the three input NOR gate 70
corresponds to the gating circuit 32.
The gain of the amplifier 26 is variable and depends on the "sweep" voltage
applied to its AGC terminal as will be disclosed later.
A diode D4 is connected across the resistor R9 to protect the CMOS against
overvoltage.
The output B of the slow clock 14 is also applied to the gate delay 36.
Gate delay 36 comprises a CMOS NOR gate 72 and an inverter 74 which are
interconnected in the same manner as in pulser delay 16 to form a
monostable. Gate 72 has a first input connected to the slow clock 14 and a
second input connected to the output of inverter 74. The output of gate 72
is connected to the input of the inverter 74 through a capacitor C5. A
positive potential V+ is also applied to the input of the inverter through
resistors R11-R14. The time constant of the monostable is controlled by
resistors R11-R14 and capacitor C5. The output L of the monostable is as
shown in FIG. 2 of the drawings. Gate delay 36 may also be used for
anterior chamber measurement and, in such a case, the time constant of the
R-C circuit may be changed by clamping the connecting point of resistors
R12 and R13 to the voltage source V'+ through a diode D5. The clamping
action is performed by operating a switch in function switch 37 (FIG. 1).
As mentioned previously, the purpose of the gate delay 36 is to delay the
operation of the gate width generator 38 which sets the time slot during
which the desired echo is to be detected.
The output L of the gate delay 36 is applied, through a capacitor C6, to
the gate width generator 38 which comprises a CMOS NOR gate 76 and an
inverter 78. The gate width generator is a monostable of the same type as
the one of the pulser delay 16 and the gate delay 36. The first input of
the gate 76 is connected to output L of the gate delay and its second
input is connected to the output of inverter 78. The output of gate 76 is
connected to the input of inverter 78 through a capacitor C7. A positive
potential source V+ is also connected to the input of inverter 78 through
resistors R16-R19. The time constant of the monostable is set by resistors
R16-R19 and capacitor C7. The output M of the monostable is as shown in
FIG. 2 of the drawings.
The output M of the gate width monostable 38 determines the length of time
during which the echo returned from the posterior part of the eye will be
detected. Such output M is applied to the gate circuit 32 as will be
disclosed later. The gate width generator 38 may also be used for anterior
chamber measurement and, in such a case, the time constant of the R-C
circuit will be changed by clamping the connecting point of resistors R17
and R18 to V'+ through a diode D6. This is performed by function switch
37.
The output L of the gate delay 36 is also applied to a latching circuit 42
through a spike generator 40 formed by resistor R15 and capacitor C6. In
the embodiment disclosed, latching circuit 42 is a conventional type D
flip-flop 80. The output N of the spike generator is applied to the reset
terminal of flip-flop 80 to reset the output of the flip-flop to low at
the beginning of the time slot during which the echoes returned from the
posterior part of the eye are to be detected, as indicated by waveform P
in FIG. 2 of the drawings.
The output P of the latching circuit is applied to the CMOS NOR gate 68 to
enable the gate. When no echo signal (amplified by fixed gain amplifier
24) exceeds the threshold set by comparator 30, the output Q of gate 68
remains low.
CMOS NOR gates 68 and 70 are interconnected by a retinal echo triggered
(R.E.T.) gate width generator 44 comprising a CMOS NOR gate 82 and an
inverter 84 which operate as a monostable. Gate 82 has a first input
connected to ground through resistor R20, a second input connected to the
output Q of gate 68 and a third input connected to the output of inverter
84. The output of gate 82 is connected to the input of inverter 88 through
capacitor C8. The input of the inverter 84 is also connected to a positive
potential source V+ through resistor R21. The inverter is protected
against overvoltage by diode D7. The input of the inverter is normally
high as it is connected to V+ and its output low, so that when the output
Q of gate 68 is low (no echo signal exceeding the threshold of comparator
30), the output R of gate 82 is high. However, when an echo signal
exceeding the threshold level is received, the output Q of gate 68 turns
high and the output R of gate 82 turns low. The output R of gate 82 is
applied to terminal C (clear) of flip-flop 80 to turn output Q of the
flip-flop high to immediately block gate 68 and so prevent the R.E.T. gate
width generator from being retriggered after it has been triggered by a
retinal echo signal. Thus, output Q is only a narrow spike such as shown
in FIG. 2 of the drawings. After approximately one microsecond as set by
resistor R21 and capacitor C8, the output of inverter 84 returns to low
and the output of gate 82 to high thereby blocking gate 70. As a result,
any subsequent echos which exceed the threshold of comparator 34 are
prevented from triggering gate 32. If no retinal echo signal exceeding the
threshold level of the comparator 30 is detected, flip-flop 80 is set
(Q=1) by the output C of pulser delay 16 at the beginning of the next
cycle to block gate 68 and to prevent the latching circuit 42 from being
activated by echo signals detected in the gate delay interval which would
disable the R.E.T. gate width generator 44 prematurely. The R.E.T. gate
width generator 44 may also be disabled by applying a positive voltage V'+
to the first input of gate 82. This may be done by a switch of function
switch 37 when making an anterior chamber measurement as the R.E.T. gate
width generator is not needed for such an operation.
When a retinal echo pulse is detected, the output of the R.E.T. gate width
generator is applied to the first input of the three input NOR gate 70. As
long as the retinal echo signal amplified by the automatic gain control
(AGC) amplifier does not exceed the threshold set by comparator 34, the
clamped output signal stop applied to the third input of gate 70 is
logically high and the output T of gates 70 is low.
As mentioned previously, the gain of the AGC amplifier 26 is controlled by
a slow ramp generator 48 and a comparator 50. The slow ramp generator 42
is a conventional Miller integrator comprising a resistor-capacitor
charging network consisting of resistor R23 connected to the inverting
terminal of an operational amplifier 86 and a capacitor C10 connected
between the inverting terminal and the output terminal of the operational
amplifier. The output "Sweep" of the operational amplifier provides a
linear time-base voltage varying from a minimum voltage of say 5V to a
maximum voltage of say 12V under the control of comparator 50. The
comparator 50 comprises an operational amplifier 88 having its inverting
terminal connected to source V+ through resistors R24 and R25 and its
inverting terminal connected to the junction of resistors R26 and R27
which are connected in series with a diode D10 between the "sweep" output
of the operational amplifier 86 and ground. A resistor R28 is connected
between the non-inverting terminal and the output terminal of the
operational amplifier 88 for controlling the gain thereof in known manner.
The output of operational amplifier 88 is connected to the non-inverting
terminal of the operational amplifier 86 through a coupling resistor R29.
The "sweep" output of the ramp generator is shown at V in FIG. 2 of the
drawings but not on the same time scale as the other waveforms. The time
lapse between t.sub.1 and t.sub.2 is about 0.7 sec and between t.sub.5 and
t.sub.6 about 2 sec. The amplitudes of the echo signals at times t.sub.3,
t.sub.4 and t.sub.5 are shown at G, H and I in FIG. 2 of the drawings. At
the beginning of the measurement cycle, the gain of the AGC amplifier 26
is low but it gradually increases until at time t.sub.5 the retinal echo
exceeds the threshold set by comparator 34. At such time, the "Stop" input
of gate 70 turns low and, if the other two inputs of gate 70 are also low
(in the time slots generated by the gate width generator 38 and the R.E.T.
gate width generator 44), the output T of gate 70 turns high.
The output T of gate 70 is applied to a display duration monostable circuit
comprising a CMOS NOR gate 90 and inverter 92. The first input of gate 90
is connected to the output of gate 70 and its second input is connected to
the output of the inverter 92. The output of the gate 90 is connected to
the input of inverter 92 through a capacitor C11. A positive potential
source V+ is also connected to the input of inverter 92 through resistor
R30. A protective diode D11 is connected across resistor R30 to protect
the inverter against overvoltage. The output of the monostable is shown at
U in FIG. 2 of the drawings. The output signal U is applied to the
"Display enable" terminal of the counter and display 12 to stop the
counter and show the axial length of the eye for a period of time
determined by the time constant of resistor R30 and capacitor C11. The
output U of the inverter 92 is applied to an inverter 94 which produces
the output "Display enable" which is applied to the slow clock 14 to
enable the slow clock. The output "Display enable" is also applied to the
inverting terminal of operational amplifier 86 through diode D12 to reset
the gain sweep ramp voltage generator 48.
It will be seen from the above description that the counter 12 will be
stopped and the display turned on only if (1) the output of the automatic
gain amplifier 26 exceeds the threshold set by the comparator 34 and if
(2) this occurs during the short time interval that the gate circuit 32
(or CMOS NOR gate 70) is enabled by the R.E.T. gate width generator 44 as
indicated by waveform R in FIG. 2 of the drawings. If the beam is
off-axis, the retinal echo will not have enough amplitude to stop the
counter (which is then automatically reset on the next pulse transmitted).
As a result, the echo-oculometer in accordance with the present invention
behaves as if the beam directivity was much greater than it actually is.
More importantly, mistriggering by echoes from structures behind the
retina is eliminated by the latching circuit 42 which prevents the R.E.T.
gate width generator 44 from being retriggered by echoes from structures
behind the retina after it has been triggered by the retinal echo pulses.
It is also important to note that the signal E appearing at the output of
fixed gain amplifier 24 is amplified by an amount approximately 10 to 14
dB greater than the maximum gain of the AGC amplifier 26 to make sure that
the first echo signal exceeding the threshold of the comparator 30 is
positively a retinal echo signal and not an echo signal originating from a
structure behind the retina as would happen if the retinal echo was of
lower amplitude than the threshold set by the comparator 30.
Although the invention has been disclosed with reference to a workable
embodiment shown in FIG. 3 of the drawings, it is to be understood that
other detailed circuit diagrams could be used for the blocks of FIG. 1 and
that the invention is not limited to such detailed circuit diagrams.
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