|This optician has received a prescription from her patient’s ophthalmologist and is discussing how the balance lens she will provide will make the eyewear comfortable and natural looking.|
EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH WE LOOK AT USING A BALANCE LENS.
HOW TO BALANCE THE WEIGHT OF EYEGLASSES.
If you’re a dispensing optician, you’ll occasionally come across a prescription that has the notation, “Balance Lens” on it. This statement will be written in the location where either the right or left lens prescription should be. Here’s what a balance lens is all about and how to handle this kind of Rx.
Some patients have an eye condition that makes them blind in one eye or they have vision that is unusable in one eye. An example is amblyopia, which results in a best corrected visual acuity of 20/100. Patients see light and blurred images with this eye but no crisp, usable vision with it. When this occurs, the optometrist or ophthalmologist might prescribe a plano lens power for that eye, but if the patient needs a corrective powered lens in the other eye, the resulting eyeglasses will look a bit odd. Instead, the eye doctor may prescribe a balance lens in the amblyopic eye.
A balance lens is one that is not made to its correcting power. Instead, it’s made as a cosmetic feature of the eyewear, not a refracting feature. The concept of a balance lens is to balance the weight, thickness, and appearance of the eyewear by supplying an Rx lens in each eye. For the “good” eye, you’ll be supplying what the doctor prescribed, but for the “balance” eye, you’ll supply an Rx lens that is chosen to make the eyewear as cosmetically appealing as possible.
One objective of a balance lens is to balance the weight of the eyeglasses. Consider how much heavier a prescription lens is compared to a plano lens and you’ll get the idea. If a doctor prescribed a plano lens for balance in the right eye and a +3.00D lens in the left eye, the +3.00D lens might be two, three, or more times thicker than the plano. This would create eyewear that always wanted to droop on the left side due to the excessive weight. The way to overcome this is to supply an Rx lens that has a weight that’s close to the Rx lens in the “good” eye.
If you supply a plano lens in the eye that is blind or has unusable vision, the right and left lenses will not have the same thickness. This will make the lenses look odd since most patients wear lenses that are identical or nearly identical. To avoid this cosmetic problem, supply a balance lens that has the same or similar center thickness to the Rx lens in the “good” eye.
There are two factors in appearance: eye magnification and cosmetic appearance. If you don’t supply lenses that are identical or at least similar in power, the wearer’s eyes will be magnified or minified by different amounts. This makes their eyes appear unnatural through the lenses. This can be avoided by using a balance lens with a power similar or identical to the “good” eye’s power. You’ll also want to use the identical lens design. Even though the patient might not get the refractive benefit from a flat-top trifocal, if his “good” eye requires a trifocal, the balance lens eye should have the same lens style and it should be positioned the exact same way as the “good” eye. You should also use the same lens material and tint. This provides a very acceptable cosmetic appearance.
WHAT TO CHOOSE
The textbook method of selecting a balance lens is to use a spherical equivalent of the power of the “good” eye’s lens. In practice, the eyecare professional (ECP) usually talks to a lab which offers a lens that was rejected from a prior job that is close enough in power for cosmetic reasons to be serviceable. This lens is billed at a substantially reduced cost to the ECP, who passes on those savings to the patient. Providing an identical Rx and design is the best outcome, but it’ll cost the patient the full expense of both lenses, which seems unfair since they don’t have usable vision in one eye. Having something close enough for the balance lens works just fine and saves money.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.