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At the London Science Museum some years back, I saw a device which projected a laser speckle pattern and had a sign explaining that it would appear stationary for people whose eyes were correctly focused, move in one direction for people who were near-sighted, and move in the opposite direction for people who were far-sighted. Looking at it with my glasses on and off suggested that the sign was accurate.

Merely saying that someone is nearsighted or farsighted doesn't seem like a very useful diagnostic tool in and of itself (since most near-sighted or far-sighted people could pretty easily tell whether near or distant objects appear clearer without need for any apparatus). On the other hand, it would seem like a device which could project a speckle pattern whose motion would depend upon how it was focused could provide more detailed visual diagnosis than conventional optometrist tools. Normal eye exams involve having the optometrist change lenses between the eye and what it's observing, and ask which changes make things better or worse. Since what's better or worse may depend upon how the eye refocuses, such questions can be hard to answer. If instead the patient stared at a test pattern through a consistent set of optics, and a speckle pattern was superimposed after going through adjustable optics, I would think the patient could judge the motion of the speckle pattern without having to focus on it, thus avoiding subjective issues caused by refocusing.

I don't know how old the concepts employed by that apparatus were when I saw it in the 1990s, but I've not seen anything similar since. Could the concept be used in diagnosing either human vision or artificial lens systems?

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It could, as it is possible to neutralize the movement with lenses hence determine the dioptric value. The value is limited due to lack of control over accommodation and the need to stand perfectly still during testing. Guess that's why they went off market quickly. I still have two devices in use for training purposes in my office though.

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  • $\begingroup$ Since I wrote the above, I've thought of another idea and would be curious what you'd think of it: have the left and right half of the lens receive slightly-different images (use a forehead rest to keep the person's eyes steady) which contain dots and dashes of a vertical dash-dotted line, such that if the image is in focus the dots and dashes will appear colinear, if it's near-focused the dots will appear to the left left of the dashes, and if it's far-focused the dots will appear to the right of the dashes. Then repeat with upper-left/lower-right, top/bottom, etc. $\endgroup$ – supercat Sep 26 '15 at 20:20
  • $\begingroup$ To avoid accommodation issues, start with both images showing the same thing, but periodically flash the separate images briefly--too fast for accommodation but long enough to give a quick impression of whether the line was straight, or which side had the dots sticking out. $\endgroup$ – supercat Sep 26 '15 at 20:22

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