How does blowing a puff of air into the eye indicate as to whether you have glaucoma or not?

Ok. This is how it works. An eye is "inflated" by fluid called the aqueous humor much like a balloon is inflated with air. This pressure from inside the eye exerts force against the cornea of the eye from the inside. The higher the pressure, the more force exerted on the cornea. A Non Contact Tonometer does not blow a single puff of air. It actually blows a number of closely spaced puffs of air that increase in strength from one puff to the next. The puffs are so closely spaced, though, it feels like just one puff.

At the same time the tonometer is blowing the series of puffs it is also reflecting a light off of the cornea. As the puffs increase in intensity they each progressively indent the surface of the cornea, thereby progressively changing the angle at which the light reflects off of the surface. The tonometer also has a sensor that detects light. When the strength of the puff finally gets strong enough to flatten the cornea adequately to deflect the light reflection into the sensor, the tonometer shuts off the puffs. The tonometer then remembers which exact puff at which strength was necessary to indent the cornea to exactly the point that caused the light to hit the sensor.

Since the tonometer knows how much pressure was needed to indent the cornea to the right point, it can tell us how much pressure was inside the eye resisting the external pressure exerted by the puff of air. The stronger the final puff was, the more pressure there was inside the eye resisting that puff.

So, the tonometer doesn't tell you whether you have glaucoma or not. It simply tells you what the pressure in the eye is. More tests are necessary to find out whether or not the pressure found by the tonometer is causing glaucoma. What that means is, some people can have relatively high pressures but have no glaucomatous damage. Likewise, there are some people who can have relatively low pressures but for some reason their nerves are not robust enough to withstand even these relatively low pressure. That is why the doctor performs a SERIES of tests to decide whether you have glaucoma or not. A pressure test by itself is not adequate to make a diagnosis of glaucoma.

There ya go.

By itself, doesn't. It measures your eye pressure, if pressure is high it is a risk factor for glaucoma. Risk factors for glaucoma include large cup/disc ratio (appearance of optic nerve), corneal thickness, nerve fiber layer thickness, field appearance and others.

Your eye doctor considers some or all of these factors when deciding if you have glaucoma.

This article will provide all the information you need. As you will find this type of eye test is not the best measure of glaucoma.

http://www.webmd.com/eye-health/tonometr...

The instrument measures how much the eyeball reacts to the puff. If it's a BIG reaction, it means the pressure is high and high pressure usually is a warning sign for glaucoma.
I really hate those "puff" tests. I ALWAYS jump half a foot out of my chair when the doctor does that thing.

The air puff is an indicator of your intraocular pressure. A high pressure is an indicator of glaucoma, but is not the definitive diagnosis for glaucoma. There are other ways to measure pressure - ask your eye doctor if he/she can perform one of these other tests.