MEASURING VERTEX DISTANCE (PUPILLOMETER)
USING A PUPILLOMETER
This method sometimes works if the pupillometer can get close enough to the eye so the image is not too blurred. To help, pull back the forehead bar.
Set the distance dial to infinity. Move the thumb slides to the temporal most direction, look at the value in mm (will be about 38.5mm).
Holding the Pupillometer at the patient's right side, view through the left window and place the hairline at the front of the cornea. Next, slide the hairline to the back of the lens. Read the new mm value, the difference is the vertex distance.
Repeat the procedure for the left eye using the right window.
MEASURING VERTEX DISTANCE (MM RULER)
When wearing glasses, the distance from the front of the cornea to the back surface of a lens is called the Vertex Distance. Why is this measurement important?
Prescription lenses in eyeglass frames have different effective powers at different distances, tilts and wrap angles from the eye. That means that the same prescription on two different people, one wearing lenses farther than the other would actually have different powers. Lenses become more plus (less minus) when moved away from the eye (increasing the vertex) and less plus (more minus) when closer. That explains why telling a progressive or bifocal wearer to slide their glasses to the tip of their nose provides a stronger add to read the phone book or a medicine bottle.
TO MEASURE VERTEX DISTANCE - Using a mm ruler, measure from the back of the lens to the front of the cornea
If you can’t see the back of the lens - measure from the front of the lens to the front of the cornea (then subtract lens thickness).
MEASURING VERTEX - DISTOMETER
An original device for measuring vertex distance is called a Distometer. While unavailable for awhile, it's been remanufactured and sold through Western Ophthalmics (click on the image for the link).
To Use a Distometer
Image, Andrew Bruce, ABOM
Ask the patient to close their eyes. Place the end of the device between the lid and the lenses. Slowly press the plunger and the two ball ends will separate. When they touch the lid and back surface of the lens, read the scale. Add a mm for the thickness of the lid and you have the vertex distance.