With all this talk of HOAs*/free form optimization and the like, I think its time to take a step back and decide what you boarderd think impacts acuity the most.
* Higher Order Abberations
Not specifying Rx in precision of 0.12D
HOAs not being optimized
Testing acuity at 20 feet
Congratulating a client when there's little change Rx
With all this talk of HOAs*/free form optimization and the like, I think its time to take a step back and decide what you boarderd think impacts acuity the most.
* Higher Order Abberations
Last edited by Barry Santini; 01-03-2008 at 04:36 PM.
I'm sure I'll feel stupid when you tell me; but what does HOA stand for? Thanks.
Higher Order Abberations
Sorry, didn't mean to come across that way....
Barry
I voted for the congratualteing the patient, as we have more issues that result from a change in Rx with the patient being told it's not much. Although I do question 20ft as the testing distance for optical infinity.
Some other issues I have seen that impact the accuracy of an Rx and eyeglasses:
As far as HOA's are concerned I don't know how relevent they are in spectacles since what's present on the cornea is not going to be easily translated to a surface 12mm or so away without comprimise, and I find that management of off axis powers should be part of a good progressive lens design anyway.
- Illumination and Contrast - most office I have seen used poorly illuminated projection of a chart for testing combine that with an off white wall, and the texture of the wall and there may be issues.
- Increments of a diopter - 0.12 used to be the standard and when we look at 0.12 being the average error allowed in glasses the Rx may come to the optician a 0.12 off if we are now using 0.25 as the standard.
- Manufactureing Tolerances for Progressives - The progressive lens tolerance is very loose and does not differentiate between a FF or traditionaly surfaced product so even if a FF product is ordered the lab now has more room to work with as far as error is concerned. Plus the tolerance for warpage is 1.00D which is pretty high IMO.
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Harry:
Great reply, as always...
I think what we're currently seeing in HOA correction is about what the near future will see: Just an attempt to correct the population's mean spherical abberation, SA, (for larger pupil sizes=dusk/night).
Otherwise, I think HOA correction is overplayed with eyewear and CLs.
FWIW
Barry
I recall my early days in the business that refracting to the eighth of a diopter was the accepted practice. We also stocked B&L Orthogons in 1/8D increments. As some practices began dropping the eights we lamented that the optical world was going to hell in a hand basket and that folks were going to be walking off cliffs and driving into one another as a result. Guess what – nothing of the sort happened.
So, now we are going to go back to the good old days and try to improve on something that is really immeasurable. If we can refract and fabricate to a degree smaller than 0.25 diopters and irregular astigmatism it is a moot point. The diurnal change in the refractive state of the human eye varies by these small amounts.
Of course the marketing of goods and services to meet these higher order aberrations will be a new avenue to line ones pockets with gold.
Last edited by rbaker; 03-20-2008 at 04:41 PM.
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I was thinking about the illumination in our office and got to thinking everything in our office is set to use a reference wavelength of sodium 589nm and flourescent light 549nm is used in most office enviornments.
n = 589/549 = 1.07
Now if the doc prescribes a 4 diopter lens that would have a focal distance of
f = n/D = 1.07/4 = 0.2675
But then the optician makes it assuming that the reference wavelegth and the wavelength used were one and the same:
n = 589/589 = 1
D = n/f = 1/0.2675 = 3.74
So the refraction could end up being a 0.25D off. It's funny but I find that there are many chop shop docs around me that perfor quicie refractions and I would be curious how they had their chart illuminated and if the illumination was flourescent or not. Also when prescribing a business or office lens is this ever taken into consideration?
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I want to distinguish the following:
Refractions are best determined, where possible, to the nearest 0.12 diopter, along with the reference distance used for the testing.
I am not, however, recommending lens fabrication precision of 0.12D, rather, that ANSI tolerances are just fine as now. It's the refraction that needs more precision/overhaul.
IMHO
Barry
Not being an OD, I don't have much real world experience with refraction, but I do believe that a patient can see in one day 5 ODs and have 5 different refractions. So of these differences are not the OD making a mistake or doing anything wrong but just the patient being stressed or over tired, etc.
Yes; I can make a better lens with a better refraction.
All components are important in making a lens that performs. A bad PD=poorer vision, a patient that lets their frame get out of adjustment and wears it so the seg height is wrong= poor vision. The correct refraction is an assumption the person making and fitting the lens assumes is correct since most do not have a way to easily check this.
Seemed to have more complaints (as an Ophthalmologist in Pvt. practice)when changing an Rx when there is little refractive change. Patients always think they "don't see as good as they used to" but often if told the change is minimal and likely to "not make much difference" they can adjust. Of course, I always will Rx new spectacles if they WANT them, but with the disclaimer of minimal change.
Deshmd
Welcome to OptiBoard, Deshmd! :bbg:
Huh?:finger:I|'m confused, I must be getting something wrong here!?
Are you assuming that there is a linear relation between index/refraction and wavelength? The actual index change of a lens is just what is described by 1/nue, relative to the mean index-1, so the maximum change from red to blue is in the order .6/42 (1.6 index example) or .014. From 589 to 549 the change is much smaller, just checked with a database, your ratio n is more likely in the order of 1.002.
小卫
I voted congrats. Most people won't see the 12th difference* (I've never called it an 8th :D). Most people won't be bothered by the HOAs*, and frankly, I'm not sure what the 20feet option is referring to, so I'm not clicking it. To me, out of all of those options, the patient's mindset is going to play the
biggest part in how well they'll see out of the end product. What exactly does "small change" mean? quarter sphere? Little bit more cyl? 5 degree change? Everyone's tolerance to changes like these are different, and if you tell them that the change won't make them see any different, and it DOES, they'll think something is wrong.
*Obviously there are exceptions to both of these. There are some that will be bothered by both. From my experience though, most won't.
I believe Harry answered that question on how you can get 5 different rx's.
Most offices don't even consider what the lighting is and that is probable the single most important element.
Second Harry you have sodium lighting in your office? The only sodium light ( that was designated sodium were orange-yellow in color used at Kodak) I ever came across unless I was just unaware.
Third you make a valid point as to stress and tired. Your other points are valid too, I was just pointing out that lighting is rarely ever sited as to why a patient does not see well.
The quality of the tear film at the moment of imaging with an aberrometer, refracting with a phoropter or even corneal topography is a HUGE factor in what one measures.
Heck, when we cannot get a good image with our GDx, we add art tears and voila! we can now get a GDx.
John
Hi Barry,
Great question:
I voted 'refraction' as well...we get so caught up in making the best possible lenses, when the original Rx may not be the best. This is one of the reasons why I believe that an optician should be able to refract, especially when a recent Rx is in hand (validating that a pathology check was done by a doctor).
As far as aberrations go, I think the main ones to focus on are off-center errors, spherical aberration, marginal astigmatism and chromatic aberration.
If those are in place through design, base curve selection and material selection with a good abbe value, then it is fun to dabble in the HOA.
: )
Laurie
This is the correct answer. For patient-related variables.
Not test conditions such as lighting, contrast, clean phoroptors, etc., nor opticians or labs (who usually hold up their end of the bargain).
The main variable for non-optimal prescribing is a non-statistically significant approach to refraction by the refracter. That is, the time is not taken to check and double-check and triple-check for reliability. It's on the person doing the refraction, actually.
And Barry, your instincts are correct: HOA is HOGwash.
Adding artificial tears may give you a refraction for when artificial tears are in place only. Patient's tears may not be as thick or as smoothe.
Don't take much in corneal radius to change Rx.
Chip
Drk,
I don't think that's going to change in the doctors hands. Unfortunately the expanded scope of practice that an optometrists enjoys also works against them. With all the procedures that need to be performed to meet the minimum standard of care compared to the price charged for the exam to stay competitive I don't see optometrists taking more time to make sure the refraction is as accurate as can be.
I think that if the refraction were to be improved upon the equipment is where the attention is needed. The equipment used today is outdated in my opinion. With CCD Cameras and computer components becomeing cheaper and cheaper it would'nt be unreasonable to have a phoropter that would integrate various equipment in it that would allow the prescriber to more accurately guage the refraction. For instance of tear films how about an interferometer, place the data on an LCD where the doctor can see it right before refraction? I know their are systems now that do integrate various components in the phoropter, but we're only scratching the surface. I think a road block to improvement is the ugliness of refraction to new optometrists, it seems that mroe and more optomtrists seem to find the refraction part of the exam as trivial and boring, this IMO is the meat of the exam and should be treated that way but it's not. I don't think manufacturere's can improve upon the refraction unless optometrists value it more.
Just my thoughts.
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Yeah, Harry, but don't forget....
there's a person's brain (indecision, past experience, anxiety, etc.) behind that phoropter (and not a CCD chip), and its an imperfect conciousness at that.
So even though I jones for better refractions, I'm a realist enough to thank *** that...
we have lens warranties....
FWIW
Barry
Last edited by Barry Santini; 04-02-2008 at 05:29 PM.
When talking about the accuracy of a refraction, one must consider that the endpoint of a good refraction does not necessarily equal a final written Rx. A final Rx is always tempered by a patient's symptoms, expectations, habitual Rx, VA, age (especially as it relates to presbyopia), visual demands and occupation (driving, heavy computer use, etc.), magnitude of the change, etc.
I find that most Rx "problems", occur when the prescriber fails to consider one or more of the above points.
Most of the time, autorefractors can get accurate results,... but that does not a final prescription make. So even if you could get all refractionists to work like robots, and be precisely accurate to .125D, you would still have remakes. That's why the .125D standard was abandoned. The other factors are more important.
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