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Consider this photo from the Ingenuity navcam on the first (?) flight: Shadows with different opacity

You can see that the shadows of the blades seem brighter than the shadows of the legs. It's especially noticeable when they overlap.

It looks like the blades are kinda transparent. But that's not the case: Even if the camera works in near IR, carbon blades shouldn't be transparent.

So what is the reason?

Does Ingenuity make HDR images by combining "short-exposure" (blades look still) and "long-exposure" (blades do full revolution)?

And why does the shadow of the upper-left (on photo) blade show a gradient? (It's brighter closer to the body of the copter)

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    $\begingroup$ @BMF: No, the Martian atmosphere is much too thin to produce such a scattering effect (and if it was due to scattering, the outlines of the blades would be much more blurred). The blades must be translucent. $\endgroup$
    – TonyK
    Jul 15 at 20:55
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    $\begingroup$ @TonyK translucent? Huh, that's a possibility. businessinsider.com/… I can find some images where the blades' shadows look just as dark as the body. Maybe it's somehow the motion of the blades. $\endgroup$
    – BMF
    Jul 15 at 21:04
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    $\begingroup$ @BMF: Interesting! I don't have an explanation for that. But note that the body of the helicopter has a square cross-section, whereas the solar panel is rectangular. And the OP's picture shows an opaque rectangle, which can only be shadow of the solar panel. But the solar panel is higher than the rotor blades, so it really can't be due to scattering. $\endgroup$
    – TonyK
    Jul 15 at 21:11
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    $\begingroup$ @BMF: See the last three or four frames of this video. It strikes me that 2400 rpm is very fast, so it's surprising that they don't show movement blur. Perhaps the satnav camera is taking a wide-aperture, short-exposure shot and blending it with a longer-exposure shot? I don't know why it would want to do that, but it would explain the partial shadow effect. $\endgroup$
    – TonyK
    Jul 15 at 21:27
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    $\begingroup$ @antlersoft I don't think I'm right. The solar panel is actually above the blades. Legs, body, blades, then solar panel. $\endgroup$
    – BMF
    Jul 16 at 14:18
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I believe I got the answer.

Let's check this explanation:

it's unintentional and an artifact of how the sensor works. Basically it still works as a much lower performance image sensor after the shutter is closed but before the stored pixel data is scanned out. Essentially photoelectrons can still get into the storage node. https://mobile.twitter.com/sdamico/status/1384221915422724096

So the light leaks to the pixel during readout. In extremely low rate compared with exposure time, but exposure time is about 100us, which is 100 times smaller than 10ms of readout.

(You can estimate exposure time by the angle of smearing in blade shadow. Readout time can be estimated by maximum FPS - 100fps gives 10ms for frame)

Can we reproduce that effect?

Luckily, I found a the video where exactly same camera filming small propeller under the bright light:

propeller

You can pause video and use < > buttons to navigate frame-by-frame and observe exactly the same translucency.

Also this post https://www.raspberrypi.org/forums/viewtopic.php?t=267563 by the same author, shows still image of the blades with prominent translucent circle:

white propeller global shutter

Note that effect is much more prominent here. I believe thats because:

a) White propeller and dark background gives higher contrast

b) Exposure time was 4 times shorter (0x04 in the video, 0x01 in the forum post) which makes readout/exposure relation 4 times larger.

UPD 21 Jul 2021: Infrared disclaimer

It seems that I was wrong assuming that Ingenuity navigation camera works in IR. OV7251 can work in IR and it's funny little pdf says about "integral 850 nm bandpass filter" (from which I was thought that it has only IR-vesrion). But as it turns out that there are different versions for IR and visible-light-only. And now I can't find any proofs that Ingenuity is indeed using IR version. But IR part still seems plausible for other cases, so I won't delete it.

I also think that effect is more prominent with infrared light, because silicon (from which sensor is made) kinda translucent in IR.

silicon and IR LED (https://www.flickr.com/photos/imager/3380554807)

Thus make it easier to scatter around and reach storage nodes:

storage node on CMOS sensor

It can explain why amount of effect for Ingenuity and for video looks the same, despite that readout/exposure is much higher for video (I've estimate exposure in order of 10us): in video some LED was used for lighting, and LED has orders of magnitude less IR than the sun.

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  • $\begingroup$ Beautiful! This is a really interesting answer. The cross-section drawing shows that they've included metalization over the storage area so it won't detect photons directly, but for some reason it's not sufficiently isolated to prevent a low rate of photoelectrons from elsewhere within the pixel from drifting into it during those "long" times between shuttered exposure and readout. Cool! $\endgroup$
    – uhoh
    Jul 20 at 0:12
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    $\begingroup$ I've held double-side-polished silicon wafers in front of thermal IR cameras and they are as transparent as glass at those wavelengths as well as in any substnatially-longer-than-band-gap near IR. Electronics-grade, high resistivity single crystal silicon is incredibly transparent in the IR, but with an index of refraction of 2.5 to 3.5 out there there is still wicked fresnel reflection at each interface. $\endgroup$
    – uhoh
    Jul 20 at 0:15
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    $\begingroup$ @uhoh I now think I was wrong about IR sensitivity of Ingenuity's camera (see updated answer). But I still love that scattering-idea. Gonna get global-shutter camera and test. $\endgroup$
    – dcoder_mm
    Jul 21 at 14:28

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