parallax

[artag]

I like the effect of an edge-sharpened visual camera superimposed on a thermal image to make it easier to recognise the view (though increasing resolution is making that less important) but actual implementations suffer from parallax and need a usually manual correction to line up the images accurately. Adding an infra-red macro lens usually makes the visible image unusable - either the supplementary lens obscures the other sensor or the changed FOV makes the images unmatched.

It seems as though there's a few ways to fix this. My knowledge of optics isn't great so perhaps someone could tell me why these wouldn't work :

1. Perform autofocus on one or both images and use that to measure the distance (like an old-fashioned rangefinder). Then use the measured distance to correct the image overlap in software.

2. Use a semi-clear lens such as zinc selenide to cover both sensors. This will result in a  yellow-tinted image and a different focal length. Image colour is not a huge problem (it's often just monochrome for edge sharpening and maybe some colour correction is possible) and an approximation to focal length correction could be made by placing the sensors in different planes or even moving one of them.

3. Place a half-silvered, front-silvered mirror in front of the visible light lens at an angle. Reflect the IR sideways into the thermal sensor. It won't pass through any glass so will be subject only to the thermal lens. Part of the light will pass through the mirror and into the visible-light lens system. The two images will then have separated optical paths.

4. In an inversion of 3, have a flat plate mirror of germanium or chalcogenide glass which would reflect the visible image sideways and pass the IR image through. Maybe even have a plano-convex lens where the plano side acts as a mirror and the convex side focusses the IR.

 

[Bill W]

1. Perform autofocus
2. Use a semi-clear lens such as zinc selenide to cover both sensors.
3. Place a half-silvered, front-silvered mirror in front of the visible light lens
4. In an inversion of 3, have a flat plate mirror of germanium

1 would work - but only at the [scene] focal plane
2 won't work because you still have two sensors on separate optical axes.
3 & 4 would work.

There is also 5 - where manual focus of the thermal lens is used to converge the optical sensor axis mechanically at the desired distance

With relatively wide angle lenses, and being at the 'cheap' end of things, these are not going to be worthwhile commercially.  The old rule that spending too much on fixing a 'cheap' system means any buyer could simply buy the more expensive system that works properly in the first place.

You would also still get the phantom images issue of looking through glass - where 'MSX' tries to show a tree through the window in thermal.

Bill

[artag]

Thanks for the feedback.

I guess you're right that they wouldn't be cheap any more, though might be fun DIY.  The cheapest would be the zinc selenide lens, as you say it wouldn't be ideal but maybe puts the sensors closer together than the common solutions.

Is there any one doing a 'proper' system, albeit expensive ?  Maybe using some improved version of one of the above.

[CatalinaWOW]

You can only correct parallax if the subject has no depth.  You can sorta-kinda correct but there will always be pieces of the scene invisible to each camera when the scene has depth. 

The only fix for this is to have a common optical path from the scene.  It can be achieved by a partially reflecting mirror.  If the mirror is an interference filter the split can be nearly lossless with each sensor receiving nearly all of their in band photons.  Chief drawbacks are bulk and fiddly alignment + cost of the filter.