Learn how to use a distometer to help personalize your patients’ lenses.
Back in the days when eye doctors prescribed eyeglasses to correct a patient’s vision after cataract surgery, the distometer was an important tool in the drawer of many dispensing tables. Since post-cataract eyeglass lenses usually had a very high plus power, they were subject to positional effect, also known as vertex distance shift. In other words, if the position of the lenses in front of the eyes at the time of the refraction was different than the location of the eyeglass lenses at the time of delivery, patients experienced a different power than the one they received during the refraction. This positional effect occurs in any lens that has power but in most cases, the power error induced by the shift is very small and patients usually don’t notice it.
With the advent of premium progressive lenses that use free-form technology in their design and processing, some lenses give the eyecare professional (ECP) the option of specifying the vertex distance. If they do, the lens’ power can be compensated for the error that will occur. This is a high-tech way of personalizing lenses for a patient.
Below is an example of how to properly use a distometer when ordering premium progressive lenses that incorporate vertex distance measurements or for any strong powered prescriptions that require power compensation.
Nallibe Mehfoud is a licensed optician in Richmond, VA. She works in an ophthalmology setting where she practices fashion eyewear dispensing.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.
|1. This photo shows the distometer in the right compartment of its storage box and a circular “slide rule” type scale that enables the user to calculate what power is needed due to vertex distance shift. Without the circular scale, the ECP would have to use an algebraic formula to calculate the adjusted power, which can be time consuming, especially in a busy office.||2. The distometer’s construction is pretty straightforward. The parts that are black are moving parts while the gray body stays stationary. On the right is a plunger button and on the left is what looks like a dog’s leg. This is a foot that moves as the plunger is pushed. The scale in the middle of the instrument has a pointer that moves as the plunger and the foot move and reports in millimeters of vertex distance.||3. Here you can see how the moveable foot works. As the plunger is pushed, the foot moves away from the device and the scale indicates a vertex distance reading.|
|4. Once the reading is taken, you need to determine what power change has occurred and make the appropriate compensation. The scale can be used a couple of ways. In this photo, zero millimeters is next to +10.00D. If this lens was moved 5mm away from the eye, the scale indicates that slightly more than +9.50D would be needed to compensate. You could also have the refracting vertex distance value next to the prescribed power on the chart and read the compensated power next to the fitting vertex distance of the final glasses. Either method works.||5. Here the ECP is taking a vertex distance measurement of a patient’s eyeglasses. Notice that the patient must close his eyelid during the measure so that the moveable foot can rest upon it. You’ll need to add 1mm to your reading because of eyelid thickness.||6. Since the reading the ECP obtained is 10mm, the final result should be recorded as 11mm.|