Coupling a ccd to a surgical loupe

[Rodrigo Bordin]

Hi!
I´m a dentist and I own a couple of surgical loupes... I´m a DIYer for the fun of it, but I´ve got a limited knowledge in physics...

I´d like to adapt a CCD (I´ve got one from a webcam) to the eyepiece of my loupe´s ocular... So I can take pictures or even film what I´m seeing so I can show my client what I´m seeing in realtime. There´s some DIY WIFI camera kit on ebay, so this is where probably I´ll follow after solving the optics problem...

Here´s what I´ve done so far, and what I don´t understand... and maybe some of you could light me a solution...

My loupe´s ocular is a small telescope with prisms inside to correct the position of the image, like a binocular. And I can adjust the focus and distance from the subject by making the eyepiece farther or shorter away (rotating it). So the farther I focus, the smaller the magnification is, the closer I focus, the higher the magnification is. My loupe has 4.5x magnification.

The weird thing (to me, of course, an ignorant in optics...) is that... If I place the disassembled webcam CCD in front of my eyepiece...the way I would in front of my eye... the magnification is not the same as it´s in my eyes... In fact, the distance from the subject gets super-short and the magnification increases a lot!

What I did, for testing so far, is to remove the eyepiece from the ocular´s barrel, and I put then the CCD right in front of the prisms, where the eyepiece would be attached to. When I did this, the magnification is similar from what I see in my eye, and it seems to focus a little shorter than it would be with the eyepiece and looking through it with my eyes.

But by doing so, I loose the ability to adjust the focus distance and magnification. In fact I don´t. If I move the ccd farther from the prisms (just a mm or so), the focus distance decreases more and the magnification increases...

I´d like to do the opposite... be able to increase or decrease a little bit the focus distance, to match the exact distance my eyes sees through the lens... so the ccd would see exactly what I see and at the same distance. But I can´t increase the focusing distance.. the CCD is already almost touching the prisms... I can´t reduce the size anymore of the loupe´s barrel...

So.. is there a solution to this? Is there an... alternative eyepiece I could use to couple with my loupe (by removing the loupe´s original eyepiece) that would focus and work normally with the CCD?

I need also a IR cut filter (removed the webcams lens, lost the IR filter) , but I think it´s and easier task to adapt...

English is not my first language, so...sorry if all this got bad explained...

Thanks a lot!

 

[Danger]

HI. Fascinating problem. Unfortunately, I know pretty much nothing about optics. In fact, having seen some of the equations, the subject scares the hell out of me.
I'm wondering about one thing, though. The human eye constantly adapts to the owner's intent, including autofocus. Could that be what's making the CCD "see" things differently?

 

[Drakkith]

You say you disassembled your webcam. Are you still using the lens of the webcam? Is it still in front of the sensor?

 

[Andy Resnick]

<snip>

I´d like to adapt a CCD (I´ve got one from a webcam) to the eyepiece of my loupe´s ocular... <snip>

Sounds similar to what I tried once- adapting a camera to some old endoscope/cystoscopes I have. I couldn't do it easily and gave up, but here's some information that may help you:

1) The loupe, if it works on the same general principle as an endoscope, puts the 'intermediate image' at infinity. That is, when you are looking into the loupe, your eye is relaxed and focused at a distant object. The optics of your eye places the final image properly at your retina. Your CCD (if you've removed the lens) cannot 'focus at infinity" (or focus at all), and so you have to alter the loupe optics to compensate, leading to all the problems you describe.

2) The CCD is much smaller than your retina- this effectively increases the magnification and focal length of your optical system. There's not much you can do to alter this (the situation is identical to digital camera lenses specifed as "35mm equivalent focal length", due to having a smaller sized CCD).

What you could try is to re-assemble the webcam play around with the webcam/loupe spacing to try and get a functional system; alternatively try the bare CCD, remove the eyepiece, and again play with the CCD/loupe space to try and get a functional system. My guess is that if you wanted to use this as a routine instrument you will be better off purchasing something:

http://www.surgitel.com/surgicam
http://www.la-lens.com/pages/minicam-hd-for-surgeons

etc.. These systems appear rather expensive, and I'm not entirely sure why since the camera shouldn't cost more than $100.

 

[Rodrigo Bordin]

<snip>

I´d like to adapt a CCD (I´ve got one from a webcam) to the eyepiece of my loupe´s ocular... <snip>

Sounds similar to what I tried once- adapting a camera to some old endoscope/cystoscopes I have. I couldn't do it easily and gave up, but here's some information that may help you:

1) The loupe, if it works on the same general principle as an endoscope, puts the 'intermediate image' at infinity. That is, when you are looking into the loupe, your eye is relaxed and focused at a distant object. The optics of your eye places the final image properly at your retina. Your CCD (if you've removed the lens) cannot 'focus at infinity" (or focus at all), and so you have to alter the loupe optics to compensate, leading to all the problems you describe.

2) The CCD is much smaller than your retina- this effectively increases the magnification and focal length of your optical system. There's not much you can do to alter this (the situation is identical to digital camera lenses specifed as "35mm equivalent focal length", due to having a smaller sized CCD).

What you could try is to re-assemble the webcam play around with the webcam/loupe spacing to try and get a functional system; alternatively try the bare CCD, remove the eyepiece, and again play with the CCD/loupe space to try and get a functional system. My guess is that if you wanted to use this as a routine instrument you will be better off purchasing something:

http://www.surgitel.com/surgicam
http://www.la-lens.com/pages/minicam-hd-for-surgeons

etc.. These systems appear rather expensive, and I'm not entirely sure why since the camera shouldn't cost more than $100.

Yes.. This surgitel system is lowres... I want it fullHD... And of course... cheaper!

I removed the Webcam's lens. I made an adaptor and playing around I could focus, but as you said, and it makes sense, the retina is bigger than the ccd... so, the magnification is higher in the ccd.. I can ever make it work through the loupe's eyepiece... but it works better with the eyepiece removed...

Would it be hard for me to... find out how to compensate the optics... or... use an alternative eyepiece??

Where could I find more information about this "focus at infinity you said"?

Thanks for the support!

 

[Drakkith]

Where could I find more information about this "focus at infinity you said"?

When we say that your loupe is focused to infinity we mean that the eyepiece is adjusted so that light leaving the eyepiece is parallel to itself.

Your loupe is focused at infinity because your eye doesn't have to bend its lens to bring the light into focus, so you expend no effort focusing on something viewed through the loupe. Try holding something very close to your eye. You'll notice that it takes effort and your eye gets tired after a while. This is because light from very close objects is diverging a great deal when it enters your eye, requiring the eye to flex and bend its lens, which causes eye fatigue. The loupe's eyepiece, if it is focusing at infinity, takes this diverging light and makes it parallel to keep this from happening.

I removed the Webcam's lens. I made an adaptor and playing around I could focus, but as you said, and it makes sense, the retina is bigger than the ccd... so, the magnification is higher in the ccd.. I can ever make it work through the loupe's eyepiece... but it works better with the eyepiece removed...

The magnification is different because you have a different optical system setup than you did before:

Setup 1: Loupe + Eyepiece + Eye
Setup 2: Loupe + Camera w/o lens

Your eye is like the webcam in that it has a lens to focus the light and a sensor to detect it (the retina). The magnification of the loupe is designed around a person's eye, and replacing the eye with a camera, which has a different focal length than they eye, will change the magnification. Removing the camera's lens further changes the magnification.

Would it be hard for me to... find out how to compensate the optics... or... use an alternative eyepiece??

I don't think there's much you can do. Lenses are not easy to make and can't simply be adjusted to fit any purpose, and optical systems have to be carefully designed around the specifications of the lenses or mirrors involved in order to work properly. If you want to setup a variable zoom system then you're probably going to have to buy something designed just for that, like a DSLR camera lens, which is not cheap.

Also, could you provide the exact model/type of your loupe? It would help greatly if we knew what kind of optical setup the loupe uses.

 

[Danger]

This is definitely going to demonstrate my ignorance, but I feel that I have to mention something off-the-wall. Is there any chance that a fluid coupling could help? I just very vaguely remember from grade 11 Biology (41 years ago) that sometimes a microscope snout was immersed into a water drop that was placed upon the sample. My understanding was that it somehow matched the refractive index better than air, or something else of that nature.

 

[Drakkith]

I don't see how, Danger.

 

[Danger]

As expected, but I felt obliged to mention it.

 

[Tishrei]

Hi I,m also a dentist and would love to know how to put together the ccd to a loupe.
If you get it operational I'd appreciate you dropping me a line with the info.
Tony

 

[Tom.G]

Essentially what you are trying to do is put a closeup lens on the camera. You may be able to use the existing camera lens though.

First try using the camera without the loupe but with it adjusted for closeup focus. The focusing is done by varying the distance between the CCD and the lens. The further the lens is from the CCD the closer the focus.

If that doesn't get acceptable results, focus the camera at infinity (or something across the street) and try it with the loupe.

The loupe should have its eyepiece in place and adjusted for normal usage. Place the camera lens so it is the same distance from the loupe as the lens of your eye would be. (Theoretically, with everything focused at infinity, the spacing "shouldn't" matter.)
This should give you a baseline configuration that works. The magnification and field of view will be wrong though due to the size of the CCD. You MAY be able to get an acceptable compromise by varying the loupe focus, camera focus, and the distance between the two. If not, you will need to either try a lower power loupe or add a lens to the existing setup.

@Danger referred to an "Immersion Objective". The expensive ones use oil as the fluid, water would be a more available and less messy substitute.