The OD I work for wants me to match every BC, how important do you think it really is? Also arent lens clocks calibrated for crown glass? So how do you determine the true BC of a pair of glasses?
The OD I work for wants me to match every BC, how important do you think it really is? Also arent lens clocks calibrated for crown glass? So how do you determine the true BC of a pair of glasses?
It depends on the situation. If you're talking PALs, unless there's been no change in the script, no change in the frame measurements, and no change in heights, matching BCs isn't going to really do anything, because everything has changed. SV I'm a little bit more willing to say that it'll make a difference. The thing about lens clocks not being calibrated to plastic, it doesn't really matter because everyone is in the same situation, including labs. If you clock his current pair at a 6, you ask for a 6, and clock the new one at 6, you're fine. Even if the true BC isn't 6, as long as it's matching the pair he's wearing, you're good.
You can buy a clock calibrated at 1.49 if you wish; it isn't important though. Old clocks used to be calibrated to 1.53, an index not available at the time (now it is in trivex). Why not exactly like crown glass? I have never heard a convincing answer to that.
Because the glass catalogues of those companies originally catalogued the most popular crown glass as index 1.53.
Interesting. How did that 1.53 glass differ from today's glass aside from index? And where do you get all this arcane info? Your brain is full of interesting stuff.Originally Posted by Barry Santini
Tell your Doc to never change a script, and you won't ever have to change a base curve.
Now.......we all know that making the boss happy is important!
I would want the (G)OD I work for, to be the happiest of campers.....so match away, until the lab calls and informs you different!
I find it very unimportant to ask the lab to match base curves. The correction of the lens determines the best base curve. It is very rare that a different base curve is needed.
Geeze, How old is your OD and when did he take his last CE?
Forget it. You will likely never have a problem related with that. Fact is, if you use a lens clock to attempt this it will only drive you and your lab nuts. Once a non glass lens is mounted it flexes the lens and you will get all kinds of readings. Just compare the 90 and 180 readings and you will see. Show this to the OD if nessessary. Just forget it.
Late 19th-century glass indeed had an index of 1.53 and lens clocks matched that index. Then crown glass was developed, and its index was 1.523. But glass manufacturers didn't want to re-tool their equipment for this very small change in glass index, which only produced an error of only 1/16th of a diopter (0.06D). But the lens clocks stayed calibrated for the old 1.53 glass to this day.
It's important that we do not to match the base curves of the old glasses. Spacial disorientation will not be any more significant than the client's first pair of eyeglasses, the off-axis performance will be better, minification will be less with myopes, and PALs will benefit by coming closer to the lens designers intended performance.
Search for "best form lenses".
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Match base curves belongs with expiration dates on Rxs.
B
Robert is ENTIRELY correct... once again.
In "best form" or "true form" optics every RX requires its own individual base curve. A -2.00 even requires different BC from a -2.25. The Vikings knew this in the 1000's when they made lenses.
Any time we step away from True Form or Best Form Optics we induce distortion. As I said in another BC Forum, I cannot think of many instances where distortion is good for a patient.
I am trying really hard to be polite... but if a Dr. didn't know enough about Optics to not understand the detrimental optical effects of always matching base curves, I don't think I would be able to stay working there... even one minute.
From Opticampus' Ophthalmic Lens Design course by Darryl Meister--
Base Curve Selection
The form of a given lens is determined by "base curve selection." The base curve of a lens is the surface curve that serves as the basis or starting point from which the remaining curves will be calculated. For semi-finished lens blanks, the base curve will be the factory-finished curve, which is generally located on the front of the blank. The surfacing laboratory is ultimately responsible for choosing the appropriate base curve for a given prescription (or focal power) before surfacing the lens. For finished lens blanks, which have already been fabricated to the desired power, the curves are chosen beforehand by the manufacturer.
Manufacturers typically produce a series of semi-finished lens blanks, each with its own base curve. This "base curve series" is a system of lens blanks that increases incrementally in surface power (e.g., +0.50 D, +2.00 D, +4.00 D, and so on). Each base curve in the series is used for producing a small range of prescriptions, as specified by the manufacturer. Consequently, the more base curves available in the series, the broader the prescription range of the product. Manufacturers make base curve selection charts available that provide the recommended prescription ranges for each base curve in the series.
The base curve of a lens may affect certain aspects of vision, such as distortion and magnification, and wearers may notice perceptual differences between lenses with different base curves. Consequently, some practitioners may specify "match base curves" on a new prescription. Some feel that these perceptual differences should be minimized by employing the same base curves when the wearer obtains new eyewear. This would conceivably make it easier for particularly sensitive wearers to "adapt" to their new eyewear.
A Typical Base Curve Selection Chart Power Range Base Curve +8.00 D to +4.75 D 10.00 D +2.25 D to +4.50 D 8.00 D +2.00 D to -2.00 D 6.00 D -2.25 D to -4.00 D 4.00 D -4.25 D to -7.00 D 2.50 D -7.25 D to -12.00 D 0.50 D
However, changes in the spectacle prescription will also create unavoidable perceptual differences. Moreover, the wearer will generally adjust to these perceptual differences within a week or so. If the same base curve is continually used as the wearer's prescription changes, which might necessitate a change in the manufacturer's recommended base curve, the peripheral optical performance of the lens may suffer as a consequence. When duplicating lenses of the same lens material, design, and power, matching base curves should not pose a problem—and is a recommended practice. Otherwise, unless the wearer has shown a previous sensitivity to base curve changes, you should use the manufacturer's recommended base curve when changing the prescription, or when using different lens materials and/or designs.
There are some exceptions to this rule, though they are rare. Some wearers with particularly long eyelashes may have been given steeper base curves at some point in order to prevent their lashes from rubbing against the back lens surface when their vertex distance—or the distance between the lens and the eye—is small, though this practice is very uncommon. Additionally, some wearers with a significant difference in prescription between the right and left eyes may suffer from aniseikonia, or unequal retinal image sizes, and require unusual base curve combinations in order to minimize the magnification disparity produced by the difference in lens powers. In these situations, a discussion with the prescriber may be in order before changing base curves.
Maybe show this to the Doc to change their mind that it should be done regardless of rx change.
I think the concept of "match base curves" is a misnomer. What it should say is: match surface power relationships.
Using 1.530 tooling, which was standard in most optical surfacing laboratories, a 5.00 diopter curve has a radius of curvature of (1.530 - 1)/5 or .106 meters. A CR-39 lens with this same radius of curvature would have (1.49 – 1)/.106 = 4.62 diopters of surface power; a 3/8 diopter difference. To further explain surface power, a 1.74 index lens with the same curvature (.106 meters) would have (1.74 – 1)/.106 = 6.98 diopters of surface power, rounded up to 7.00 diopters; a 2 diopter increase in surface power due to index while retaining the same curvature.
"match base curve" has no meaning when mixing indices, and not much when discussing aspherics, atorics, and progressives.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Maybe the OD wants you to match the frame base curve? This is what the patient tried on and expects the frame to fit like; it is hard to put a 7 base lens in a 4 base frame and make it look/fit like the plano they purchased.
Craig
Nowadays, Trying to match the (SV) base curve to the *frame* eyewire is my mantra...not what they're wearing now. With FF, it's no problem: Better Vision, better lens fit, better frame fit (less splay).
B
Don't tell them the secret, they will not believe it anyway.
Craig
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