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Thread: How does the nct work?

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    How does the nct work?

    I have been in the field for awhile and use the nct everyday. I really should know the answer to this question. How does the machine know what the eye pressure is with just a puff of air.

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    Master OptiBoarder OptiBoard Silver Supporter optical24/7's Avatar
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    The non-contact tonometer is an applanation tonometer and works on the principle of a time interval. Measuring the time it takes from the initial generation of the puff of air to where the cornea is exactly flattened (in milliseconds) to the point where the timing device stops. It takes less time for the puff of air to flatten a soft eye than it does a hard eye.

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    OptiBoard Professional Ory's Avatar
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    Quote Originally Posted by Steph26 View Post
    I have been in the field for awhile and use the nct everyday. I really should know the answer to this question. How does the machine know what the eye pressure is with just a puff of air.
    There is a beam of light directed at the cornea at about 45 degrees. There is a corresponding photosensor. When enough air hits the cornea it flattens the surface and the light beam is reflected onto the photosensor. Too much or not enough air and the light beam doesn't hit.

    Based on how much pressure is used the computer works out how firm the cornea is, which can be translated into an intraocular pressure. This assumes an average corneal thickness.

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