Deciphering the Symbolic Shapes in Fundus Images: A Question on their Meaning

[myquestion]

Hi,

I currently have to do with fundus images ( https://en.wikipedia.org/wiki/Fundus_photography ). I am wondering, if someone has an idea, what the meaning of the rectangle on the sphere means? (marked in the following in green) At this images it looks like a rectangle, I also saw some fundus images, which seem to have triangles or semi-circles attached.
I searched around, but couldn't find an explanation so far, why fundus cameras attach these symbols to the images.

 

[Ibix]

You can tell which way up it is at a glance. Otherwise it's difficult to tell a left eye retina from an upside down right eye retina.

 

[Ibix]

It's not a symbol, by the way. Somewhere in the camera optics there's a mask with that circle-and-notch shape. You'll see it's always the same colour as the retina and sometimes you can see features like blood vessels going into it.

 

[myquestion]

Thanks @Ibix for your answers.

> You can tell which way up it is at a glance. Otherwise it's difficult to tell a left eye retina from an upside down right eye retina.

But are ophthalmological devices, ever used in an upside down way, such that such an indication would be necessary? I also thought that it could have to do something to differentiate left- and right-eye images. But at the example images at wikipedia (https://en.wikipedia.org/wiki/Fundus_photography) they show a left and right eye image and the notch does not seem to give an indication on the left or right eye.

> It's not a symbol, by the way. Somewhere in the camera optics there's a mask with that circle-and-notch shape. You'll see it's always the same colour as the retina and sometimes you can see features like blood vessels going into it.

Yes, that was my impression also, that it's not just a symbol (sorry for naming it like that!). Therefore my question would have also been in the direction, whether there could be some optical reasons that the mask has this shape?

 

[myquestion]

I am happy for further references, that give explanations on the background, why the camera optics in this case is as it is. (Unfortunately my search in the web was not that successful on it, but I might have searched for the wrong terms/words as I don't have a deeper background in optics.)

 

[DaveC426913]

It's not a symbol, by the way. Somewhere in the camera optics there's a mask with that circle-and-notch shape. You'll see it's always the same colour as the retina and sometimes you can see features like blood vessels going into it.

+1

 

[Ibix]

But are ophthalmological devices, ever used in an upside down way, such that such an indication would be necessary?

I think you are assuming that the images are always digital, which is not always the case. If the photo is hardcopy how do you know which way up it is if the image is purely circular? It's less important with digital images because the image will never be handed to you upside down (or back to front if it's on transparent film).

 

[sophiecentaur]

they show a left and right eye image and the notch does not seem to give an indication on the left or right eye.

I assume that the square is a registration mark - always top right of every image from every fundus camera, by convention. So whatever is done with the image (rotation / reflection ) you always know how to orientate what you are looking at. Then, the side where the optic disc is will tell you whether it's a right eye or left eye.
Registration marks are everywhere to resolve confusion. I use them all the time when I'm making things. That avoids (but, in my case, not completely) confusion.