Calibration method and apparatus for tonometers

• US 4,834,105 A
• Filed: 11/09/1987
• Issued: 05/30/1989
• Est. Priority Date: 11/07/1986
• Status: Expired due to Fees

• Current Assignee: Keeler Company Incorporated
• Original Assignee: Keeler Company Incorporated
• Inventors: Fisher, John, Little, Mervyn A., Matthews, James R. A.
• Primary Examiner(s): NOT, DEFINED
• Application Number:
  US07/118,159
• Time in Patent Office: 568 Days
• Field of Search: 128/648, 128/652, 128/645
• US Class Current: 600/401
• CPC Class Codes: A61B 3/165 Non-contacting tonometers

Abstract

A tonometer is described for measuring the intra-ocular pressure of an eye in which a pulse of fluid is projected towards the cornea of the eye to distort the corneal surface by the pressure of the pulse, the distortion in the corneal surface being detected by a change in the reflection of light directed onto the cornea. A fluid delivery system (36, 44, 48) leads to an outlet through which the fluid pulse is applied to the cornea, and a branch (54) leads to a pressure measuring chamber (52), containing a transducer (170) which produces an electrical signal whose value depends on the pressure in the branch. A flow restrictor is removably fitted in the branch (54) and signal processing means (176, 178, 180) (the gain and rise time of which are adjustable by (182) or (186)), is supplied with the output from the transducer. Adjustment of 182, and 186 enable the electrical signal output of the processing means to be adjusted, for calibration. Calibration of the instrument is achieved by selecting an appropriately dimensioned restrictor and by adjusting the gain, and where appropriate the time constant, of the signal processing means, so that the plot of a pressure-time signal curve of the processing means output corresponds to the pressure-time curve for such an instrument when set to operate in line with a clinical standard.

Claims

1. A tonometer for measuring the intra-ocular pressure of an eye in which a pulse of fluid is projected towards the cornea of the eye to distort the corneal surface by the pressure of the pulse, the distortion in the corneal surface being detected by a change in the reflection of light directed onto the cornea comprising:

(1) a fluid delivery system leading to an outlet through which the fluid pulse is applied to the cornea,

(2) a branch in the fluid delivery system which leads to a pressure measuring chamber,

(3) a transducer in the chamber which produces an electrical output signal whose value depends on the pressure in said branch,

(4) a fluid flow restrictor removably located in the branch leading to the transducer, said restriction being selectable to provide selectable flow restrictions in said branch so as to correlate said electrical output signal from said transducer to the pressure of the fluid pulse delivered by said fluid delivery system, and

(5) signal processing coupled to said transducer so as to receive said electrical output signal means the gain and rise time of which are adjustable, to enable the electrical signal output of the processing means to be adjusted so as to effect calibration of the tonometer when a selected restrictor is in place.

2. A tonometer as set forth in claim 1 wherein the signal processing means comprises an analogue amplifier the gain and time constant of which are adjustable, wherein adjustment of the gain enables the amplitude of the output signal to be adjusted and variation of the time constant enables the rise time of the output signal to be adjusted.

3. A tonometer as set forth in claim 1 wherein the fluid delivery system includes a housing having, at one end an apertured lens comprising the said outlet for the fluid pulses, inlet means to which fluid under pressure can be supplied when a pulse is required, and a connection for the said branch leading to the pressure measuring transducer.

4. A tonometer as set forth in claim 3 wherein the housing includes a first section which extends between the lens at the one end and a viewing eyepiece at the other and includes an optical focussing and reflecting system.

5. A tonometer as set forth in claim 4 wherein the housing includes a second section within which a source of illumination is located.

6. A tonometer as set forth in claim 4 wherein the housing includes a third section containing light detection means, the fluid inlet means, and the connection to the said branch leading to the pressure measuring transducer.

7. A tonometer as set forth in claim 6 a housing comprising said third section and a portion of the first section leading from the third section to the apertured lens, is separated from the remainder of the housing by means of a semi-reflecting mirror fitted so as to create a fluid-tight seal between the said first housing part and the remainder of the housing.

8. A tonometer as set forth in claim 3 wherein a part of the wall of the housing is comprised of a printed circuit board forming a light tight seal with the housing, and pressure measuring transducer is mounted on a region of the printed circuit board which communicates directly with the interior of the housing.

9. A tonometer as set forth in claim 3 wherein the apertured lens comprises a pair of aligned centrally apertured plano-convex lenses, a tube forming part of the fluid delivery system extends through the aligned apertures in the lenses, and a mask which defines two opposed V-shaped windows on opposite sides of said tube, to determine the shape of the final image on the light detection means is formed on one of the plane surfaces of the lenses.

10. A tonometer as set forth in claim 9 wherein the mask comprises two slit like windows symmetrically arranged relative to the central opening and bounded by an opaque surround.

11. A method for calibration of a non-contact tonometer which has a pressure measuring transducer subject to fluid pressure delivered through a fluid path branched from an air delivery system, which in use of the instrument delivers fluid pressure pulses to be incident on the eye, and wherein the branched fluid path has a removably locatable fluid flow restrictors selectable to correlate the pressure of the fluid pressure pulse delivered by their delivery system with an electrical output signal produced by the measuring transducer, said method comprising the steps of:

locating the selection restriction and the branched fluid path;

positioning the instrument incorporating the selected restrictor and to be calibrated according to the output of the air delivery system a fixed distance from a test pressure transducer, which has previously been calibrated to produce output signals which are proportional to the pressure readings which would be obtained using a standard tonometer, as a reference,

establishing electrical connections whereby the output signals from the test transducer and from the signal processing means of the tonometer under test would be supplied to a double beam oscilloscope having signal storage facilities;

during the release of a single pulse of air from the instrument under test, simultaneously plotting the instantaneous electrical signal values produced by analog circuits coupled with the pressure transducer within the instrument under test and analog signal values from said test transducer;

comparing the two curves so obtained on said oscilloscope and adjusting at least one of the gain and time constant of the signal processing means of the instrument under test, so as to correct the curve produced by the signals from the instrument under test, so as to approximate the curve from said test transducer; and

repeating the release of a single pulse of air towards the test transducer and the comparison adjustment steps until a desired level of identity between the pressure/time curves is obtained.