which is the principle and technology that enables us to measure objectively myopia?

mather

hello!

which is the principle and technology that enables us to measure objectively myopia, astigmatism and presbyopia?

thanks!

Simon Bridge

Well, it's the principles and methodologies of opthalmology of course - the technology is that of optics.

I don't think is is rigorously objective - it's more about what counts as an optimum performance of your visual apparatus based on it's "construction" and comparison with other examples.

mather

Any more in-depth info?

Evo

Please fully explain what infomation you want.

mather

I want to know how is myopia measured

What the machine that measures it, does exactly

Evo

Have you done a search on this? It took me 30 seconds to type this into google. We encourage people to do an initial search first, then if they have specific questions about the information, to ask those here.

http://en.wikipedia.org/wiki/Phoropter

mather

umm, I think there is another way...

Simon Bridge

...don't keep it a secret!

mather

isnt there a way to actually detect that at a current eye focus, the idol does not fall exactly to the retina?

Simon Bridge

Yep - it's called "blurry vision". The sensitive optical processor used to determine this is called a "human brain" and the methodology is called "asking the patient".

A non-subjective way, that does not rely on the patient's report, would involve shining light into the eye while examining the retina... to be sure you'd have to dissect the eye and patients tend to object.

But if the retina were at the focal length (the focus falls on the retina) then doesn't that mean the image will form behind the retina? (I am unfamiliar with the terminology of "the idol does not fall exactly on the retina".)

mather

thankfuly Mr. Bridge, technology has already moved forward from what you describe

Simon Bridge

The autorefractor involves shining light into the eye and examining the retina. That goes double for retinoscopy. :P I'm guessing it's the dissection part you were talking about.

Shining light into and onto stuff and watching what bounces off is a pretty common technique for investigating lots of things. Dangerous to drive at night without it for example.

Ambient light, and light spontaneously emitted from stuff is also commonly used for other things but not so useful when examining the interior structures of a living eyeball.

Presumably the fine detail of the operation of retinoscopy would be most relevant to the OP but the optics is basically what you'd do with a ground-glass rig in a lab. Actually, this may make a good optics teaching experiment ... finding the focal length of a lens when you cannot get behind it.

mather

this is what I need! and to measure it realtime and in nanolevel! :D

Evo

That link was in the article I gave you. You didn't bother to read it?

All you had to do was click the links provided.

Simon Bridge

You shine light through it an observe what happens.

Imagine a laboratory rig - you have a standard lab converging lens (biconvex and made of solid glass say). The lens is fixed over a hole in one side of a box that is closed on all the other sides.

The back of the inside of the box acts as a screen.

If you shine parallel light through the lens, you will get a spot on the back of the box where the light scatters. The size of the spot depends on the focal length of the lens and the internal dimensions of the box.

This spot will act as an "object" - since it is a diffuse light source.
Looking obliquely through the lens you'll be able to see it and take measurements.

If the back of the box is also patterned in some way - like the back of the eyeball usually is - and you have a good idea of the internal dimensions of the box, you can determine quite a lot about the size of the spot and deduce the focal length. You can also tell if the image is virtual or real. Get the idea?

You don't have to rely just on the lens in the box - you can add your own (taking their known properties into account) to adjust the image to something that is, perhaps, easier to handle.

Of course - you can always just use a autorefractor ;)