Page 8 13900-103 Rev. A
Theory of Operation (Continued)
Air Puff System
The air puff system produces a small puff of ambient air
out of the nozzle of the air tube when the patient is fully
aligned in front of the instrument. The puff of air is
directed at the center of the cornea which causes the
cornea to applanate. The air puff system consists of a
piston within a stainless steel cylinder, a solenoid drive for
the piston and a "nosepiece" assembly that contains the
air tube.
The piston and solenoid provide the force to push the air
out of the air tube. The nosepiece assembly combines the
air puff, target illuminator, pressure monitoring,
applanation detection and the proximity detection
functions together to enable a closed loop system.
When the patient is positioned correctly in front of the air
tube and observes the green target, the piston is
activated. After the air is delivered to the cornea and the
cornea has applinated, light from the emitter diode is
sensed by the detector. When the detector has reached
its highest output signal (applanation of the eye), the
solenoid drive to the piston immediately stops and air
pressure quickly ramps down. When the retaining spring
attached to the piston overcomes the ramp down
pressure, the piston is reset to its original position. The
internal manifold inside the nosepiece assembly monitors
the pressure just behind the air delivery tube. The forced
air from the tube is proportional to the pressure inside the
manifold. This pressure is constantly monitored and the
pressure at the time of applanation is detected and
recorded. This pressure is a measure of the force
required to applanate the cornea and is displayed as the
IOP reading.
Applanation System.
The applanation system consists of an infra red emitter
(875 nm) and detector placed so that the energy from the
emitter is directed obliquely toward the cornea. The
detector is placed tangentally from the emitter so that light
reflected from the flattened cornea will be directed to the
detector. An undisturbed cornea (non-applanated) will
direct very little amount of infrared light toward the
detector. As the curvature of the cornea starts to
applanate by the air pulse, the infrared light increases to
the detector until a maximum level is detected at the exact
instant of flattening (e.g., applanation). This is followed by
a rapid decrease in the amount of infrared light detected
as the cornea becomes slightly concave. At the exact
instant of applanation, the pressure is recorded. This
pressure is then converted directly to an IOP reading and
is displayed on the LCD screen.
Proximity Detector
The proximity detector is a sensor which is attached to the
end of the air tube. This circuit prohibits the air tube from
contacting the eye.
This circuit contains an oscillator and its associated
circuitry to detect external capacitance (the human body
or an eye which is within approximately 1/4" of the cornea)
near the airtube. This circuit contains a copper ring and
two semi-circular rings surrounding the end of the air
tube. The capacitance between these rings will change
which will then disable the motor drive circuits when it
detects thre external capacitance .
Target Illuminator
The target illuminator is a visible target which the patient
utilizes to align their eye to the instrument. This target is
located at the back of the nosepiece assembly at the
opposing endof the air tube. During alignment of the
patient to the instrument, the patient initially finds the
green target, then moves forward to place their head
against the headrest. Once against the headrest the
instrument quickly finds the eye and takes a measurement.
Touch Sensor
This instrument contains a touch sensor which will stop
the motion of the alignment system if activated. If
someone (e.g., patient) touches the inside edge of the
touch plate (grey plate which surrounds the nosepiece
cover) the message "Touch Sensor Activated" will appear
and the alignment system will stop its movement. After the
alignment system stops, it will then retract to the home
position.
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