A simple question about spot retinoscope

[t1a2n3]

I have a question which I think should be very simple for you optometry students, but I cannot get a clue. I'm a physics PhD student, currently concentrating into the simulation of retinoscopy. I have calculated the simulation for a streak retinoscope and now have some trouble on spot retinocope. I chose the parameters of the product from Welch Allyn.
From my reading of manual, I know the Welch Allyn streak retinoscope can be easily converted to spot retinoscope by replacing the linear light source with a spot light source. However, I could not find any description about the slit aperture in front of the light source. Should this aperture be removed for a spot retinoscope? Also, what is the dimension of the spot light source? I've never seen a real one. Thank you for any comments.

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[rpames]

WOW...those are some serious physics I don't think any of us actually every thought about. We just shine the thing at the eye and look at the reflex. But, the spot scope is just a regular ret with out the slit aperture in it. I will check the dimension and get back to you. You need to talk to Neil Hodur, OD. He helped develop the prototype for the current WA ret and is the professor of geometric and theoretical optics at the Illinois college of optometry. You can reach him 312-949-7000 and then when asked, just say his name. It is pronounced Ho-der. If you get his voice mail just leave a message and then call back. He may not get back to you right away...he is a busy guy, he has to steal his coffee from security and wonder the halls. He is a great guy so just call him, he would love to talk optics with you b/c none of us will.

 

[t1a2n3]

Thank you so much, rpames! The info helps me alot, I will try to ask Dr. Hodur.

 

[fjpod]

I know the WA streak retinascopes can be converted to spot by simply replacing the bulb and keeping the collar in the lowered position ( at least on mine). I didn't have to change any aperature. It's the bulb that creates the streak or the spot.

I learned retinoscopy on a streak, but in my first year in practice I tried the spot retinascopes that my associates were using. I found it to be so much faster and no less accurate. I haven't used a streak in 29 years.

 

[t1a2n3]

I know the WA streak retinascopes can be converted to spot by simply replacing the bulb and keeping the collar in the lowered position ( at least on mine). I didn't have to change any aperature. It's the bulb that creates the streak or the spot.

I learned retinoscopy on a streak, but in my first year in practice I tried the spot retinascopes that my associates were using. I found it to be so much faster and no less accurate. I haven't used a streak in 29 years.

Thank you for inputing. So the beam on the cornea looks like an oval shape instead of a real circle because the aperture is rectangular shape? I've never really used ret. Can you explain what you mean "faster and no less accurate"? And which one is better for a KC case? Thanks again.

 

[fjpod]

Thank you for inputing. So the beam on the cornea looks like an oval shape instead of a real circle because the aperture is rectangular shape? I've never really used ret. Can you explain what you mean "faster and no less accurate"? And which one is better for a KC case? Thanks again.

Well...the beam is not really seen on the cornea. It is seen through the pupil. It is the light reflected off the retina after it passes through all the optical media of the eye. The circular beam of a spot scope is curved into a streak if and only if the eye has astigmatism. The astigmatism of the eye bends the circular light beam into a cylinder (streak), thus establishing the axis. It's simple geometrical optics, really.

By KC, you mean keratoconus, correct? I personally think a spot retinascope would be better as it wouldn't bias your perception of the axis of the reflex with the position you happen to start with if you had a streak, but this is debatable. The trouble with a keratoconic cornea is it does not follow the rules of simple geometrical optics. "Regular" astigmatism, as we call it, depends on a an optical system having only two major meridians of differing power. The trouble with a keratoconic cornea is that it has no regular shape to it and no spectacle lens can correct it. They can only approximate a correction. Rarely do you see a streak in a keratoconic eye. You usually see gobbley-gook, or as we call it, scissors motion. Only a rigid contact lens can get the eye close to normal vision...and that depends on no underlying corneal changes such as scarring or striae.

I think spot is faster because you can determine the axis of the astigmatism without bothering to rotate the collar on a streak scope. With a spot scope, you let the orientation of the streak formed automatically by the astigmatic error of the eye guide you to the cylinder axis. Many would argue with me. I am in the minority on this...but I would match my accuracy against anybodys.

 

[PBEA]

Thank you for inputing. So the beam on the cornea looks like an oval shape instead of a real circle because the aperture is rectangular shape? I've never really used ret. Can you explain what you mean "faster and no less accurate"? And which one is better for a KC case? Thanks again.

Sounds like you are fishing to me. Why do you want to know whether spot or streak is better for KC?

 

[fjpod]

On second thought, I don't think either retinascope will give you a better correction for KC. It's only a matter of preference of the examiner. Getting a wearable spectacle Rx for a keratacone has a lot more to do with the experience of the examiner than it does with the type of retinascope used. At this point, I have probably given too much extraneous information without knowing more about what you are trying to do/solve/explore. There is no cookbook...one way is better...method for KC correction.