What can cause a horizontally flipped ghost image when using a monochrome line-scan camera?

Question

What can cause a horizontally flipped detailed ghost image in a monochrome line scan camera?

I am new to photography but I have been seeing horizontal reflections in pictures with bright objects.

I was wondering if this effect has a name or if anyone may have an idea of what could cause it?

Here is a real world example of picture taken looking down on a remote controlled boat on a sunny day, using a monochrome line scan camera, but I have seen this phenomena with almost all of my camera/lens combinations. This is the full image – note the wobbles from my shaky hand. Real image of boat is on the left.

Answer 1

Aside from the question of cause, the term for reflections directed toward the source are generally referred to as a retro-reflections or a mirror image from a planar surface.

Top-for-bottom images are said to be flipped while left-for-right images are said to be flopped.

So much for terminology. Insofar as a cause, there is insufficient information to go much further than presented by scottbb in his response.

I do favour an electronic over an optical cause, possibly file corruption, due to the symmetry and weak signal to noise ratio without attenuating the highlight (boat? you say) in the weak image "reflection."

Why do you mention "you use a lot of unusual cameras" as though it might have a bearing on this question? What unusual cameras have this trait?

(You can talk dirty to us.) We'll understand. Don't be afraid of being too technical.

Answer 2

Assuming standard optics (lens elements that are spherical or pseudo-spherical, rotationally symmetric about the optical axis), and "standard" cameras (no beam-splitting, or mirrors in the optical path), there is nothing optically that will cause single-axis lateral reflection ghost images whether they be left-right, top-bottom, or even diagonally oriented. This is because lenses perform transformations for any set of orthogonal input axes (i.e., x and y axes). Both dimensions are transformed: left is swapped for right, and top is swapped for bottom (in addition to scaling, and probably some degree of distortion as well). In linear algebra, swapping x for –x and y for –y is mathematically equivalent to a 180° rotation about the z-axis (that is, about the optical axis of the lens). Thus, in optically-generated ghost images (again, with "standard" optics), elements of the ghost are all reflected through the center of the image, not merely across a vertical or horizontal "fold line".

Stepping away from standard optics, cylindrical-sector lens elements, which curve in one dimension (usually laterally) but not in the orthogonal (vertical) dimension, could cause left-right symmetric ghost patterns. Anamorphic lenses, or at least current day anamorphic filters and adapters, come to mind. They compress the lateral field of view of a taking lens, which when printed or processed allows for much wider lateral fields of view than the camera is normally capable of. This is often used to film wide screen cinema.

Besides optics, I suppose it's possible that some sensor technologies could be susceptible to lateral "ghost images", perhaps due to how the sensor data is read or scanned. But that would be pure speculation on my part.

The last thing I can think of, at least within the optics or the camera itself, is some sort of reflection of the image from the sensor, back to some plane in the optical path (such as a filter plate or something behind the lens, fairly close to the film/sensor plane), and then back to the sensor. But in order for the reflected image to appear even somewhat in focus, the added reflection path must be fairly short compared to the back focus distance of the lens. That implies that the lens is focused extremely closely to the subject, and there would be quite a bit of distance from the exit pupil of the lens to the sensor. Furthermore, that reflection surface would have to be concave (looking from the face of the lens) in the lateral dimension only. Frankly, this last possibility is even more speculative and unlikely than the previous paragraph.

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Outside of the camera, the most obvious explanation is a reflection through a window, automotive glass, or other largely transparent but semi-reflective surface. That would explain the same degree of magnification and in-focus reflected object as the real object in the image.

Answer 3

You may be shooting at or through glass or some other transparent or translucent object. Here is an image I captured shooting through a piece of plastic using a zoom lens set at about 50mm with a polarizing filter. The strength of the reflection is adjustable with the polarizer.

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There could be a defect in the optical system that produces ghost images (such as astigmatism). However, in this case, the ghost image would not be mirrored.

Consider the following image in which ghost images were formed by the optical system. The camera does not normally produce ghost images with other lenses. No filters were present. The image was not shot through any glass.

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I doubt an electronic cause because one of the cameras you state has the problem is a line scan camera. The sensor has long moved on by the time the portion of the image containing the mirror-ghost is scanned.

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You mention having a polarizer in front of your "lenses assemblies". What other equipment do you have present that you haven't mentioned? How are the filters held in front of the lens? What specific cameras, lenses, and other equipment are you using? These, and other details, can affect the presence and appearance of reflections.

Please provide detailed information and sample images from each of your camera setups so they can be evaluated.