| 摘要 |
<p>26 APPARATUS FOR MEASURING THE AXIAL LENGTH OF AN EYE An apparatus for measuring the axial length of the eye is disclosed. The apparatus comprises a transducer adapted to transmit repetitive ultrasonic pulses along the ocular axis of the eye of a patient and to receive echo pulses reflected from the retina of the eye. A fixed gain amplifier is connected to a transducer for amplifying the reflected echo pulses. An automatic gain controlled amplifier is also connected to the transducer for amplifying the reflected echo pulses. Control means are coupled to the automatic gain controlled amplifier for gradually increasing the gain of the amplifier to a predetermined maximum gain. First and second gate circuits are coupled to 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 is connected to the second gate circuit and adapted to display the axial length of the eye 27 as a function of the distance travelled by the retinal echo pulses. A gate width generator is connected to the second gate circuit for generating a time slot during which echo pulses originating from the posterior wall of the eye can be received. A retinal echo triggered gate width generator is connected between the first and second gate circuits and is responsive to the first gate circuit for enabling the second gate circuit to pass logic signals 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 is also connected to a latching circuit which is connected to the first gate circuit 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 circuit by echoes originating from structures behind the retina.</p> |
