How to measure subjective visual axis separation.

[Kujiradesu]

I have made the long trip over internet deserts and jungles from Ask the Lab Guy on Facebook with a task from Barry Santini to ask Robert Martellaro how to measure subjective visual axis separation without a specialized measuring device. Presumably you know how to do this because of your extensive knowledge of low vision aids and how to fit them (which is another area that I would love to know more about). Robert Marellaro help youre my only hope.

[Kujiradesu]

Help Robert Martellaro. I need your expertise with this one.

[Robert Martellaro]

I have made the long trip over internet deserts and jungles from Ask the Lab Guy on Facebook with a task from Barry Santini to ask Robert Martellaro how to measure subjective visual axis separation without a specialized measuring device. Presumably you know how to do this because of your extensive knowledge of low vision aids and how to fit them (which is another area that I would love to know more about). Robert Marellaro help youre my only hope.

Sorry for the later response- it's been busy (thank goodness!).

I'm not low a vision expert, and I'm not absolutely certain what you're looking for, but if it's a manual method to determine the position of wear, I can help.

At the top left (see below) is is a plumb line device to measure pantoscopic or retroscopic tilt. Hold it about 12" from the side of the eyeglasses. Use the Panorameter to measure the wrap/dihedral angle. It's hard to find though- look for protractors online like this one...

http://www.opticalprescriptionlab.co..._Wrap_Tool.pdf

I use the bottom left device with light/corneal reflex to get a starting point for the IPD, but one could use most anything, as long as you transfer the value to the lenses of the preadjusted frame. I use a fine point pen and short vertical lines, remarking until the reflex is occluded by the ink line. Use a layout chart to assist with marking the lens and confirming the final values.

https://www.essilorusa.com/content/d...ur%20chart.pdf

Pupil/fitting heights are determined in a similar fashion except with horizontal lines.

1. For every 1mm diference between the PD of the fitter and the subject, the fitter will over-estimate
the subject's PD by 1/16 mm if the fitters PD is larger than that of the subject.

Not an issue if your PD and the client's PD is in the average range.

2. Angle Kappa

I typically ignore it, but keep in mind that shorter eyes (hyperopes) may have larger values, decreasing the PD by 1mm or so. Ask Barry about a tricked-out pupilometer that the client can use to determine there own mono PD.

3. Head turns.

Not uncommon, but tricky to determine. Observe in both primary and near gaze (subject may turn head strongly in the primary gaze but straighten out at near. The mirror method may help with monocular vision.

https://www.optiboard.com/forums/showthread.php/32267-Mirror-Method-for-fitting-progressives?p=268122&viewfull=1#post268122

Digital centration devices are recommended for the busy offices, but continue to measure pupil heights manually.

Hope this helps,

Robert Martellaro

[Kujiradesu]

Thanks for youre thorough response. A bit of background: Barry turned me on to the articles and research of Tom Clark, OD and his modified pupillometers used to have a Cx take their own subjective pd/ visual axis separation. Im working on maybe getting my hands on one of these devices, but Barry said I should ask you about the best way to take angle kappa into account when taking an IPD measurement. Reason being knowing now how important angle kappa is to accurately fitting a PAL I want to do it right or learn how.

I have a distometer, pantometer, and a panorameter. What is the jig in the bottom left and how can I get my hands on one? What kind of light do you use to create the corneal reflex?

Any other insight you may be able to share would be much appreciated. Thanks again.

[Robert Martellaro]

Thanks for youre thorough response. A bit of background: Barry turned me on to the articles and research of Tom Clark, OD and his modified pupillometers used to have a Cx take their own subjective pd/ visual axis separation. Im working on maybe getting my hands on one of these devices, but Barry said I should ask you about the best way to take angle kappa into account when taking an IPD measurement. Reason being knowing now how important angle kappa is to accurately fitting a PAL I want to do it right or learn how.

Your welcome. My approach would be a conservative one (yes, I'm old), primarily due to so many unknowns. As previously discussed, "rounding down" for moderate to high hyperopes is as far as I'm willing to deviate from my objective measurements. FWIW, it's probably more accurate to call it self-testing than subjective.

This may be of interest...

https://www.researchgate.net/publica...mal_population

I have a distometer, pantometer, and a panorameter. What is the jig in the bottom left and how can I get my hands on one? What kind of light do you use to create the corneal reflex?

Any other insight you may be able to share would be much appreciated. Thanks again.

I describe it here...

https://www.optiboard.com/forums/sho...ll=1#post56606

There are rulers like this, but this is hands free so I can occlude with one hand and hold the light under my other eye. The idea is to measure the near PD, and by using a chart included with the device, based on simple trigonometry, determine the monocular distance PD. One would sit at the work distance, typically 40cm/16", occlude one eye, and line up the two marks on the bridge to minimize parallax error. As mentioned previously, I use it to get a distance PD starting point, transferring this value to the lens for observation.

The light that came with the "meter" is pictured above looks exactly like the device used in "Men in Black" to wipe the citizen's memory. The kids liked that, but I needed more light to see through dark tinted lenses so I switched to Lumistar from the now defunct Radio Shack Corp., shown here...

http://ledmuseum.candlepower.us/tenth/lumistar.htm

At least Milwaukee still has a science and surplus store!

https://www.sciplus.com


Best regards,

Robert Martellaro