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For my current research project I need to take photos in a wind tunnel (A big enclosed rectangular 400cm x 100cm x 70cm). The objects are super tiny seeds that disperse on a white reflective sticky carpet inside the tunnel.

In this tunnel I installed 2 led light strips on upper both sides along its length. The lights are facing the ceiling because it will be super reflective if it’s facing down, towards the sticky carpet. However, with the lights facing towards the ceiling there are still reflections on the carpet.

Because the ceiling is white (actually all part of the wind tunnel is white) I took the initiative to paint black the ceiling, which help reduce the reflection, but now the light is less diffused and darker.

On the camera setting, I used the lowest ISO and aperture, and play only with the shutter (The camera I use is Canon 700D with 16mm lens. With this lens I took 6 photos along the 4m length of the carpet, I can not take more than 6 photos because I will have so many repetitions and will produce a lot of photos if it’s more than 6)

Until now I still get photos with very minimum quality to be analyzed on image processing software. Here I attached the wind tunnel photo (so you can have the idea) and the photo result I have.

Any suggestion to get better quality images (aka we can see more clearly the seeds) will be a huge huge help. Thank you!!

enter image description here enter image description here enter image description here

enter image description here

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  • \$\begingroup\$ Several questions. 1. Is the sticky carpet, really reflective by nature, like some special panel you need to use, Or you constructed a glossy panel and add a sticky aerosol or something? 2. In the photo, the black curtain is a rail where you slide the camera. So the led strip is the rail on the left? 3. Do you need a shadowless photo of the seed or some shadow is allowed? 4. Are the photos taken after you turn off the wind, so they are static and already stuck to the carpet? 5. What do you mean by 200x magnification? Is it 200x or 200%? \$\endgroup\$
    – Rafael
    May 23, 2023 at 18:51
  • \$\begingroup\$ To reduce reflections you could also try a polarizer filter. I can't guarantee it will work, but it's worth a try. \$\endgroup\$ May 23, 2023 at 22:18
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    \$\begingroup\$ 1) Yes the sticky carpet is really reflective by nature, it is a protective sticky carpet that you find before entering a sterile room normally, so all the dust on your shoes got off. 2) The led strip is inside the tunnel we can't see it from outside. 3) I need a total shadowless photo! 4) Yes that is right the photos will be taken after I turn off the wind and the seeds will be all stuck to the carpet 5) My bad I mean 200% \$\endgroup\$
    – Catharina
    May 24, 2023 at 6:12

3 Answers 3

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What you want is for the ceiling and walls to be white, and the lights to be installed on the floor with a baffle to keep them from shining directly onto the reflective sticky surface.

When photographing reflective surfaces I find it easier to think of it as creating the environment that is going to be reflected, rather than as lighting the subject/object. And the goal of this is to create an evenly illuminated white box (walls/ceiling) for the surface to reflect (to illuminate the surface with diffused/reflected light). Preferably the entire ceiling would be white with only a small opening for the lens, because it is the ceiling that is going to be reflected.

If the lens opening is too much of a distraction (a dark circle reflection) you could resort to using an offset opening and a shift lens; but I do not think the surface is so reflective/critical as to require that.

Example diagram:enter image description here

To hide the access strip in the top of the tunnel you could insert white posterboard between the sidewalls to create a domed ceiling; which would actually create more even/unified lighting. But that assumes that the inserts would be compatible with the airflow.

![enter image description here

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  • \$\begingroup\$ Hello, thank you so much for the explication! My biggest problem is that with the team that build the tunnel we cannot find white colour for the broom/brush part (You can see from the first photo the black part in the middle of the ceiling), so it can't be all white inside the box... \$\endgroup\$
    – Catharina
    May 24, 2023 at 7:39
  • \$\begingroup\$ @Catharina, I would use white panel overlays inside the box to mask the areas not occupied by the camera lens. At least in the areas that will be reflected in the camera's field of view. Assuming the airflow is pretty low velocity you could probably just bend large pieces of posterboard between the side walls; creating a domed ceiling which would be even better/more even. I updated the post with a diagram of that as well. \$\endgroup\$ May 24, 2023 at 17:07
  • \$\begingroup\$ Thank you so much!! I will try this out, I also updated my post with a photo of the wind tunnel interior \$\endgroup\$
    – Catharina
    May 25, 2023 at 8:54
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If they can be dyed:

  • Color them with with a dye that fluoresces in blue light.
  • Illuminate the wind tunnel with blue light (if blue LED strips are not available, put blue cellophane filters over the LED strips).
  • Put a yellow or orange filter over the camera lens.

The seeds should stand out very clearly, since incident blue light is eliminated by the camera filter. The seeds would be bright specks on a very dark background.

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  • \$\begingroup\$ This is an excellent lateral thinking solution. It doesn't require adding extra light and reflectors, so much as using the right kind of light. Excellent. \$\endgroup\$
    – scottbb
    May 25, 2023 at 1:45
  • \$\begingroup\$ This is indeed an excellent idea, however, we cannot dye the seeds because the seeds come directly from a plant... \$\endgroup\$
    – Catharina
    May 25, 2023 at 8:59
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Probably there is not much that can be done.

The main problem is the reflective glossiness of the carpet. If this carpet is a transparent film, over a glossy Formica or similar, you could replace it with a matt one. I feel the carpet is "satin" at most.

Of course, it is not a perfect mirror, but a glossy surface photographs the surroundings, not only the surface itself.

Seeing the interior image what you could try is to try to twist the led lights in a 45° angle or more (A), so less light goes to the ceiling (B), then make it darker.

But one more thing on the image of the seeds. I feel that they are out of focus. Use a laser beam, a simple laser pointer will do (C), to have a clear marking on where the floor is (D). Then you can switch to manual focus. But keep confirming you do not move the focus ring with those brushes.

enter image description here

One additional thing. You probably can increase the exposure of your image. Your image tries to interpret the white floor as middle gray.

I feel your camera is handheld with some support on the box, try to accomplish these settings.

  1. A speed of about 1/100 - 1/60 (If it is really steady you could go as low as 1/30)

  2. You need the sharpest image you can. Try using f8.

  3. Depending on how noisy the camera is try ISO-400

If the image is still too dark, try to add an additional led strip.

If you can leave the camera without touching and with no vibration, try a remote shutter and you can use a slower speed.

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    \$\begingroup\$ Thank you for the suggestions! I actually already increase the the exposure (indeed mostly lower the shutte and let the ISO around 100-200) and I used MF as well because the AF is unable to detect the seeds and capture the photo, but the image of the seeds is more or less like that, I guess because the seeds are really small but I have to take it from the distance to cover a big area. Unfortunately the carpet is not a transparent film so nothing I can do with the glossy carpet... \$\endgroup\$
    – Catharina
    May 30, 2023 at 11:34

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