How to get bright white backgrounds?

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kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

ray_parkhurst wrote:
Fri Dec 02, 2022 10:01 am
J_Rogers wrote:
Fri Dec 02, 2022 9:50 am
ray_parkhurst wrote:
Wed Nov 30, 2022 5:16 pm
There is an excellent and surprisingly high-end You Tube video that shows how to do this calibration. Unfortunately I looked but could not find it. I'll look again but you might try searching as well for axial lighting in coin photography with backlight. I'm terrible at finding such things so I'll bet you can find it before I can...
Is this it?
https://www.youtube.com/watch?v=jvPiMaLy2RE
Yes indeed, thanks for finding it!
Ah yeah this one is great, I watched it more than a few times when trying axial lighting at various points. Is it thought that the acrylic cylinder is doing anything more than allowing light to pass through it, or is there some "unique" effect it is producing e.g. the internal reflections of the light result in a more evenly lit background than if you had just used a flat sheet of acrylic?

Would love to get my hands on one of his RAW images to check the edges. I suspect he's having better luck than me just due to the size and shape of modern coins compared to ancients but it would be interesting to know in general whether he's getting any light from the background blending in with the coin's edge.
- Cam

ray_parkhurst
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Re: How to get bright white backgrounds?

Post by ray_parkhurst »

The acrylic pedestal acts like a height extension pedestal, but without the fear of the coin falling off. I suppose it would be better for edge reflections if the outer cylinder of the pedestal was rough sanded and painted black to limit reflections back toward the coin edge.

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

ray_parkhurst wrote:
Fri Dec 02, 2022 3:29 pm
The acrylic pedestal acts like a height extension pedestal, but without the fear of the coin falling off. I suppose it would be better for edge reflections if the outer cylinder of the pedestal was rough sanded and painted black to limit reflections back toward the coin edge.
I was more curious about if it was doing anything special optically, but sounds like it probably isn't?

Had a bit of a mess around today with an 6.5W LED COB I salvaged from a spare recessed ceiling light while I wait for some parts to arrive next week. For a very ad hoc setup, it actually did better than I expected. I had the LED sitting at the bottom of the rotation stage (which has a 25mm aperture), though not perfectly flat. I briefly experimented with a few setups, such as using the 35-degree lens that was part of the ceiling light, placing diffusion gel at the top of the rotation stage, using clear plastic, and so on. I didn't have much time so this was a very quick and rough test.

What seemed to work best was having the diffusion gel on the rotation stage (about 25-30mm above the LED), and then the LED lens inverted on top of this. The diffusion gels seemed to produce a more uniform background, without it the sections of the lens were very visible in the background and it was somewhat dark between the sections. Inverting the lens seemed to help too, either because it narrowed the light from the LED better or because it provided a more narrow illuminated background than using the face of the lens. Placing the coin on top of the inverted lens worked decently but it did have some bleed into the edges of the coin. Adding a 15mm spacer between inverted lens and coin seemed to remove that light bleed almost entirely, though, again, I just did a very quick test before having to pack up.

All in all, it seems promising. Once the Fresnel lenses and LED COBs I ordered get here, I'll look at 3D printing some new parts to incorporate the new components and give them a proper test.
e1.jpg
e2.jpg
e3.jpg
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Had another quick play-around today. The iterative process of trying different setups is taking awhile as some things aren't practical to test without printing new parts. I made a new coin platform with a few holes in it to accept some of the lenses I bought. The fresnel lenses seemed to be the most promising. I'm still experimenting with the distances between light source and subject for the lens and even placing diffusion gel between the light source and the lens to help make the background a more consistent white so you can't see the lens sections.

However, what seemed to work best was placing the fresnel lens upside down and about 25mm from the coin - at full brightness I get a very white background with good contrast around the edges. I tried a test stack to see how it would hold up under pixel-peeping. Some positives are that it does work without reflecting light onto the edges and creating bright spots, the downside is that I run into the same issue I had here. Increasing the threshold of the DMAP from 60% to 70% did help a lot, except in one area but that could be fixed manually. The issue with the specular highlights on the very edge is also manageable with the high threshold, the width of the affected area around the coin shrinks from 15-20px wide to about 5px wide. But I'd still like to do better and perhaps reducing the brightness of the LED to find the optimal balance between background brightness and edge contrast will help even more.

Full image for context. Note I wasn't aiming to take a proper photo here (hence underexposed areas on the coin), just to test the edges.
full_1.jpg
Comparison of threshold values: 60% on left vs. 70% on right
compare_1.jpg
Test setup with a new coin platform. Fresnel lens mounted upside-down, about 15mm from the LED light inside the rotation stage, coin on a pedestal about 25mm above the fresnel lens. Coin platform has an aperture of 32mm, compared to a fresnel lens diameter of 50mm.
stand_1.jpg
stand_2.jpg
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

I printed a new platform with a larger aperture of 37mm and made a few other small improvements to it. I also printed a "shroud" for the fresnel lens, which doesn't do much except cover it up so light doesn't escape externally and reflect of things and also to align it with the rotation stage and allow for easy mounting of the platform to the stage. I can't stress enough how great a tool 3D printers are for this hobby!
setup_1.jpg
setup_2.jpg
setup_23.jpg
I also realised it is probably better if I follow in the footsteps of this thread and treat this as an optical comparator problem as that is basically what I'm attempting to solve and makes it much easier for testing when I'm not mixing key lights with the backlight.

I think that approach works well, here are some tests:
Image 1: LED at full brightness (left) and low brightness (right). There's some hint at reflections off the thread edges in the center of the right image, outside the center it looks perfect but of course the background is too dark.
comparator_1.jpg
Image 2: Diffusion gel 15mm above the lens and thread placed on gel (left), and a plastic object blocking the center of the fresnel lens (right). Naturally the diffusion gel works horribly, this was more of a control to emphasise the things I don't want to see. The right image, using a blocker for the center, highlights the specular reflections i'm getting in the center of the image that aren't coming from light produced from the center.
comparator_2.jpg
Image 3: a small LED lens placed in the center, which I imagine is mostly reducing the center brightness by dispersing light over a wider area. This is the best result if my goal was to remove the background but is still far from perfect, less than ideal in setup, and introduces some new reflections to the center parts of the image.
comparator_3.jpg
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Here's a close-up of the thread profile from the center of Image 1 (right), not too shabby if I just needed to measure the thread dimensions or characteristics of the thread if I had machined or printed it myself.
thread_profile.jpg
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Just an update on this for anyone interested.

The fresnel lens approach worked pretty well but I ended up going in a different direction for two reasons:
1. I was still getting this weird fringing around the edge of the coin in the final stack, similar to what the mirror produced, probably partly related to me needing the LED to be so bright so as to make the background an even colour and hide the ridges/sections of the fresnel lens. Perhaps at a lower brightness it wouldn't be as much of an issue
2. The fresnel lens / parallel light rays approach also means I couldn't tilt the coin in my setup, since I use a goniometer that tilts both the platform with LED and fresnel lens as well as the coin.

So I ended up going back to the diffused lighting approach but increasing the distance between coin and diffused background. I'm also using a wider diameter LED COB to help with creating an even lighting on the diffusion gel, and placed the gel about 12mm above the LED light, with the coin being another 40mm or so above that. The "aperture" in the coin platform is about 40mm, less than the 50mm of the LED housing, but I've also printed off some inserts that can reduce this diameter from 40mm to 36mm, 32mm, and 28mm for smaller coins. With everything setup right, I get a tiny amount of diffused light from the LED hitting some parts of the coin edge but not enough to cause much of an issue in removing the background, and more importantly I can tilt my setup and it still works pretty well.

Some quick photos for reference:
20221219_202523_edit.jpg
20221219_202447_edit.jpg
20221227_164837_edit.jpg
And here's a completed stack of a coin I shot recently with this setup:
2022-12-19-21.53.55 ZS DMap.jpg
- Cam

Lou Jost
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Re: How to get bright white backgrounds?

Post by Lou Jost »

That looks very good. It is so nice to see people report back on their results after these discussions! Thanks.

Bob-O-Rama
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Re: How to get bright white backgrounds?

Post by Bob-O-Rama »

( The suggestion about using collimated rear illumination is likely the best solution. )

It may also be possible to use chromakey to enable your masks to be selected more reliably. You could distinguish between the chromakey background and white-ish specular highlights from the coins edges. Also there is no magic to the key color being green, as I could envision some patina being similar its. I mean you are essentially doing this now, except your keying color is white.

Otherwise, as someone who occasionally dabbles in the numismatic arts, those are great coin photos with a real "3D" feel. I've seen a lot worse when it comes to cataloging or auction photography, and while the subject is simple, its really difficult to get them to look that good.

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Bob-O-Rama wrote:
Fri Jan 20, 2023 12:43 pm
( The suggestion about using collimated rear illumination is likely the best solution. )

It may also be possible to use chromakey to enable your masks to be selected more reliably. You could distinguish between the chromakey background and white-ish specular highlights from the coins edges. Also there is no magic to the key color being green, as I could envision some patina being similar its. I mean you are essentially doing this now, except your keying color is white.

Otherwise, as someone who occasionally dabbles in the numismatic arts, those are great coin photos with a real "3D" feel. I've seen a lot worse when it comes to cataloging or auction photography, and while the subject is simple, its really difficult to get them to look that good.
Thanks, nice to hear that from a fellow collector! I try to keep an eye on maintaining the "3D feel" as that's something focus stacking tends to take away from the coin, particularly when you replace the background with something plain. Ancient coins in particular tend to look most attractive when they're shot with a macro lens at an angle so part of the coin is out of focus - it just seems to convey better how it would look in-hand. But it's a trade-off, can't have it all.

But on the topic of the collimated method, while it no doubt works for some applications, in this case I think it wasn't the best option in the end. Mainly because my setup requires rotating the coin and the light simultaneously, so the collimated light points away from the lens as soon as the coin is tilted. If I had a different setup that wouldn't be an issue though. The specular highlights on the edge are also another issue and I don't think it can be solved easily in post but it could be a trade-off I'd be willing to make if not for the limitation with tilting.
- Cam

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