# Is there a lens that would invert my vision?

Would I be able to invert my vision by mounting a particular lens in front of each eye?

I am currently able to achieve this by mounting a right-angled triangular prism across both eyes. The specific rotation can be thought of as follows:

The effect is that things in the "bottom" of my visual field appear at the "top" and "upsidedown". For example, if I stand and move my head to look down, I see my legs apparently "in front of me" and my feet pointing "towards me".

The prism arrangement is heavy, uncomfortable (there is an edge with an acute angle against my forehead), and severely restricts my view (which can be fully occluded by an A3 sheet of cardboard held at arms length). This is a problem as the scientific value of this arrangement comes from being able to wear them for extended periods (e.g. a whole day, or 4 hours a day for a month).

Is there a lens that would have essentially the same inversion effect, without all of the problems in the previous sentence? Suggested solutions don't have to be inexpensive.

Two lenses placed such that the focal points of each overlap.

I'll give you an image to help...

The blue dot is the focal point of both lenses. The red and green paths are simple enough, and the brown shows how the angle of incoming light is flipped, so it looks like the light is coming in from the top, not the bottom.

BTW: I used what looks like to convex lenses, this could also be two flats. So even something as simple as a glass ball given the right index could do this.

EDIT:

Reading this a second time over, makes me think you might want something different than two glass spheres. This would make things on the left appear to be on the right and things on your right, left.

I think a glass cylinder lens would be more like it. That way the ray's horizontal angle remains unchanged.

Basically take two of those prisms you are using and sandwich them with aluminum or some other reflective material.

• So this is more like an engineering question? If you read my last comment, I think a glass cylinder lens is the best method. We just need a mounting strategy, like glasses. I can look into it. – Luke Burgess Nov 13 '13 at 18:10
• @LukeBurgess I am trying to build a non-electrical solution to the link you provided, which I will then wear for extended periods (hence me trying to maximise comfort and field of view). – Andrew Martin Nov 14 '13 at 12:47
• What if an electronic solution offered the most comfort and most field of view, that solution would already be wrong from your point of view? Besides that if optical waves are involved than electronic waves are involved. This is clearly an engineering problem, no physical concepts are seeking to be better understood. It is the design that we are seeking to improve. – Luke Burgess Nov 14 '13 at 15:41

Binoculars include a prism that invert the image as the lenses already do an inversion. The lens apparatus would be a bit heavy to use as an inversion mechanism, but the prisms (Porro prims for instance) could do the job. They can provide a decent field of view. Even if they take some space and weight, I doubt any other apparatus, such as a lens arrangement, could do better.

This outfit sells goggles that do that. It's just a matter of prisms and some engineering. Alternatively, you could just have them look down/up into a mirror, or even make a periscope like this, where the blue parts are mirrors:

or this:

• Looking down is hard on the neck :) TBH, I like the periscope solution the best. If you're going to go around bumping into things, you might as well look awesome doing it! flickr.com/photos/83287853@N00/4848187328 – lionelbrits Nov 13 '13 at 23:45
• @lionelbrits: Google should pick up on that! – Mike Dunlavey Nov 14 '13 at 3:46

I think you should consider a catoptric or catadioptric system. The heart of the system is two parabolic mirrors facing each-other (can be light weight if you aren't overly concerned about a flawless image) , and they produce an inverted image.

Here is a diagram showing the light path of a Cassegrain telescope. Obviously you would use different curvatures for both primary and secondary because your objective is not to magnify but to invert.

• One mirror has a hole in it. – lionelbrits Nov 13 '13 at 22:33
• You don't see the hole, because a single point in the image is formed by many rays travelling along different paths. Put differently, the "hole" is so far out of focus that it is invisible. As for the particulars, I did mention that you would have to tune the positions and curvatures of the mirrors to the application. – lionelbrits Nov 14 '13 at 0:24

This a very old question and it's practical application is related to neuroscience. If you read this Wikipedia article: Perceptual adaptation - Experimental support you will learn of 1890s experiments with inverted vision glasses aimed to verify that brain will adapt and flip the images at some point after long glasses usage. Here is description of glasses that were used by the experimentator George M. Stratton found here on 1st page:

This was done by binding on the eyes a simple optical contrivance constructed on the following principle : If two convex lenses of equal refractive power be placed in a tube at a distance from each other equal to the sum of their focal distances, the eye in looking through the tube sees all things inverted, but in other respects the image remains unchanged. The image cast on the retina is as if the whole field of view had been revolved on the line of sight through an angle of 180o. All light other than that which comes through the lenses must, of course, be carefully excluded by making the instrument fit exactly the inequalities of the face by means of black linings and pads. For if light were permitted to enter the eyes otherwise than through the lenses, the observer would be subjected to- both upright and inverted images, and the purity of the experiment would be lost. The size of the visual field was a matter requiring some care. The size and refractive power of the lenses are the determining factors here, and in the desire to obtain a reasonably large visual field one is tempted to use large thick lenses. But they are soon found to be too heavy to wear on the head for a considerable length of time. I found it best, therefore, to modify the instrument above described, by substituting two double convex lenses (placed close together on the same axis line) for each of the lenses in that description. I had thus for each eye a short adjustable tube, and at either end of the tube a pair of good lenses of equal focal length. The instrument by this means gave a clear field of vision with a compass of 45o, and at the same time was light enough to be worn without discomfort.

• This is a very useful description, before asking this question I didn't realise Stratton used a lensed solution. I assume the "too heavy" apparatus is the same as the one drawn in Luke Burgess's answer, but I'm not clear on how Stratton adapted it to make the lighter apparatus. Can someone "substituting two double convex lenses (placed close together on the same axis line) for each of the lenses in that description" for me? – Andrew Martin Nov 18 '13 at 13:44

Andrew, do you have to have an optical solution ? An electronic solution, combining a Google glass with an electronic image inversion would do the trick. I know that HP made some small LED displays, that you wore like glasses, which projected a heads up display. could easily be programmed to invert the image.

• This is a great suggestion, but I think I do need an optical solution to help reduce lag between motor actions and change in sensory stimulus. If I can find a solution within my budget I'll give it a go anyhow. Cheers – Andrew Martin Nov 14 '13 at 11:59
• @Andrew oculus rift is pretty cheap, and I think the delay can be made pretty small. I know someone who's using it for rubber hand illusion type experiments, and I would guess you need a pretty low latency for that. (I could ask him about it if that's a path you think you might take.) – Nathaniel Nov 14 '13 at 14:40

Not quite the type of solution you're looking for (and I know George E. Smith already suggested something similar), but lots of people have modified the Oculus Rift consumer virtual reality headset by mounting dual wide-angle webcams on the front, for augmented reality applications. It seems there are kits available for this.

This solution would have the advantages of being ergonomically designed and having a good field of vision and excellent isolation of light from outside the helmet. It would also be trivial to program it to flip horizontally rather than vertically, or to make other transformations to the visual field, if you need that.

On the other hand it does need to be plugged in all the time, and at 379 grams (according to Wikipedia) it might be a bit heavy and bulky for long-term wear. Still it seemed worth mentioning.

You can invert you sight using a dove prism.

• A dove prism is just a truncated version of the triangular prism in the question. – Brandon Enright Nov 13 '13 at 22:08