|Target vs. Reticle|
|Both the target and the reticle are viewed through the lensometer’s eyepiece.|
Each issue, Tech Tips will explore some interesting aspect of optical technology. This month, we look at neutralizing and verifying lenses.
Don’t hunt for lens powers-validate them!
There are two fundamental functions an optician performs with a lensometer: neutralizing lenses and verifying them. In the neutralization operation, the optician attempts to discover the power of an unknown lens. In the verification process, the optician tries to determine if the lens power that was ordered for a pair of eyeglasses is correct. While these two operations are very similar, they differ in important ways.
Let’s say you want to neutralize the lenses of the eyeglasses a patient is wearing. This is something opticians do repeatedly every day. The task begins with you ensuring that the eyepiece of the lensometer (assuming it’s a manual model) is properly focused…which you should always do. Even if you were not the last person to use the lensometer, you should focus it in order to be certain that it’s right for you. Next, place the eyeglasses on the instrument’s platform and position the right lens so that you observe the center of the target (the green lines you see in the eyepiece) located at the center of the reticle circles (also seen inside the eyepiece). Now you go hunting! Neutralization is a hunting exercise. In other words, when you perform neutralization, you don’t know the lens powers and you must search for them.
Verification is different although it starts the same way. After you’ve ensured that the lensometer eyepiece is properly focused, place the eyeglasses on the instrument’s platform and position the right lens so that you observe the center of the target located at the center of the reticle circles.
What many opticians will do at this point is hunt for the power, just like they do in neutralization. But since you already know what the powers are (they’re on the prescription), there’s no need. Verification is not a hunting exercise, it’s a validating exercise. In other words, I don’t want to hunt for the lens powers because I already know what they are-I want to validate that they are correct.
When you neutralize a lens, you’re asking the lensometer to tell you what the lens powers are. When you verify, you are telling the instrument what the powers are in order to determine if they are correct.
Here’s an example. You want to verify that the lenses in a pair of eyeglasses with the power of -1.00 -1.50 x 180 OU are correct. Before you place the lenses into the instrument as described above, set the power drum for the sphere power of the right lens (-1.00D) and set the prescription’s axis on the axis wheel (180 in this example). You know this is the right power and axis so tell the machine 1.00D at axis 180. Now look into the instrument. If the sphere lines are clear, the power is correct; if sphere lines are straight, the axis is correct. If the sphere lines are blurred, the power is inaccurate; if the sphere lines are broken, the axis is wrong. If there are errors, you’ll need to “hunt” for them to determine how much they’re off and if they are within tolerance.
Next, set the power drum for the total power in the cylinder meridian, which is -2.50D in this case (add the sphere and cylinder powers together) and look into the instrument again. If the cylinder lines are clear, the power is correct; if cylinder lines are straight, the axis is correct. If the cylinder lines are blurred, the power is inaccurate; if the cylinder lines are broken, the axis is wrong. If there are errors, you’ll need to “hunt” for them.
Following this procedure will make your verifications a lot more accurate and a lot more visually comfortable to perform. Remember that when verifying lenses, don’t hunt, validate.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.