As some of you may or may not know I passed 3 parts of the State Board Exam, but failed the neutralization. This got me thinking as to what I may be doing wrong. After reading anything I could get my hands on about neutralization, I think I may have found out what I have been doing wrong. My problem thus far were determining the adds. I would always seem to get exactly 1/8 of a diopter off. This would invariably leave me with the problem of which way do I estimate. It turns out that I wasn’t doing this one very basic thing. So I decided to write this to help out others, and myself, as to the textbook way of neutralizing a pair of spectacles. Hopefully it will lead to you (provided you have to retake the neutralization portion) and I passing this coming March 2006. This will be edited and updated periodically and I would hope that anyone who sees any errors will email me.

I’ve looked at 3 textbooks, which as I understand, are industry standard. I’ve also re-read some of my notes System for Ophthalmic Dispensing 2nd Ed. by Brooks and Borish, Clinical Optics by Fanin and Grosvenor, and Ophthalmic Dispensing 3 Ed. by Russell Stimson. The Stimson book is a bit older but agrees pretty much with the other books and attests to the historical use of the method.

Keywords: Lensmeter, Lensometer, Focimeter, Vertometer

This is a schematic of a lensometer:

The standard lens is usually a +20 or +25 achromat doublet lens that moves along with the power drum.

Focusing The Eyepiece

Make sure you rotate the eyepiece all the way out and turn the eyepiece until the crosshairs and concentric circles (the reticles) first come into focus. This is your zero point and you should compensate for whatever your illuminted target is at. If the lensometer reads an eighth strong without a lens, make sure you subtract an eighth for your readings. Remember we are only allowed an eighth (0.125 D) in the sphere and cylinder powers. This allows your eye to remain relaxed.

Determining The Prescription (distance Rx in multifocal, or single vision)

The spectacles should be placed into the lensometer with the ocular side away from you. This is known as back vertex power. As you may already know, the thin lines are your sphere lines and the thick lines your cylinder lines. If everything is clear you have a spherical lens. The illuminated target should be in the center of your crosshairs when you dot it up. If you have a lens with cylinder, start with high plus and work your way down using the power drum until one set of lines come into focus. If the lines are not straight/look broken turn your axis wheel until the thin lines look unbroken. This would be your axis. You might have to repeat the focusing of the sphere lines. Make note of the power and dot up the lens. Next you will continue turning the power drum in the minus direction until the thick lines come into focus. Note the power drum reading of the cylinder and dot up the lens. The difference in both power drum readings will be the cylinder. The horizontal measurement will be the PD and any horizontal prism that has been prescribed.

First, center the lens with the higher power in the 90th meridian. Neutralize that lens in the normal fashion. Without adjusting the lens table, center the other lens horizontally. Any vertical displacement of the target lines is your vertical prism. Measure with the reticles. If it is in the center of the reticles still, you have no vertical prism. Anything else will have to be calculated using the reticle scale of the eyepiece. You can only figure out the amount of horizontal prism if you know the patients PD. The only exception to this is when you have a plano lens with horizontal prism. It will seem as if you aren’t able to center the illuminated target. Whatever the deviation is, is the amount of prism in that lens. It may be negligible though.

For progressives you have to check for prism at the “prism reference point” which may be anywhere from 0 to 4 mm below the fitting cross. Since prism thinning is used, it may seem as if the target has prism, but this must be compared with the other lens. If the other lens has the same amount of prism and in the same direction, then there isn’t any total prismatic effect.

Determining The Add Power

For adds on the front surface, you need to take the front surface power. This means the ocular side is towards you. Hopefully you didn’t erase the dots on the O.C. since you will need to know where it is. Once you have that, you will take the measurement in the add by paying attention to just the sphere lines only and dot this. You only care about the readings, and not about centering the illuminated target. Now you have to retake the distance Rx by placing the lens above the OC exactly the same amount you were below the OC when you took the add reading. Subrtacting the add drum reading from the distance (the one you retook) drum reading will give you the add. The reason you do this is because you want to compensate for any aberrations. If you go the same distance away from the OC, there should be the same amount of aberrations in the lens. This is what I have been doing wrong. I have tried this at work and I have gotten rid of the pesky eighth.

If the segment is on the back or ocular side, then you would take back surface power. An Ultex would be an example of a segment with the segment on the ocular side.

So why do we take the front surface power if the seg is on the front and the back surface power if the seg is on the back? The answer has to do with the lensometer and the purpose of the segment. The distance portion of the lens is primarily meant to bring parallel light to a focus. In the lensometer, the light is coming from the back of the lens towards your eye as if you were wearing them. The light coming out of the front of the lens into the eyepiece is parallel. Since the add is meant to make divergent light parallel, we need to turn the lens around in order to find out what the focal length of the segment is.
Q & A

Q. How do I know if my lensometer is reading accurately.?
A. The only way to check a lensometer is with a Younger gauge-master.

Q. Why does my lensometer not read properly?
A. There can be several reasons for this. In higher Rx powers the vertex distance may come into play. The gauge-master should let you know if this is the case. If your lensometer reads fine at low powers, but is slightly off at higher values, you may need to adjust or recalibrate the lensometer.