How to get bright white backgrounds?

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kaleun96
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How to get bright white backgrounds?

Post by kaleun96 »

Hello again, I've been more focused on coin photography the past little while than insects and other macro but I've been dealing with an issue for awhile that I think you lot might be able to help me with.

My setup in short: I do 1x mag focus stacks of coins, namely ancient coins, which are typically higher relief, thicker, and far more imperfect than modern coins. In particular, the edges of the coins can be up to 6 or 7mm thick and have all sorts of weird shapes (such as tapered edges, random bits jutting out, and so on). I now shoot these coins on a white background with the intention of fully removing the background to get a transparent PNG, so the coin can then be placed on different backgrounds as I please.

My current method, using a custom 3D printed "ring light" that holds two Godox TT350 flashes, is to either use a white bit of plastic or a mirror as the background. The mirror is great and results in a perfectly white background as it reflects the white plastic from my "ring light" (which is attached to the lens) back to the lens and separates the coin from the background perfectly. There are a few limitations with this, however: I can't tilt the coin; the mirror tends to produce a slight "halo" around the edge ,which appears as a purply pixelated area on the very edges, only a few pixels wide, in the focus stack; and I think the mirror is perhaps diffusing the image slightly from stray reflections (or I need better flocking).

The issue with using just a white bit of plastic is that the background is always under-exposed relative to the coin as I have the coin lifted off the surface by 30mm or so to reduce shadows on the background. As the background is underexposed, it blends slightly with the typically gray to light-gray edges of the silver coins. This makes background separation much more difficult and time-consuming than with the mirror.

I've tried various alternatives such as an illuminated background and even retroreflectors (tape, bicycle reflectors, and my own 3D printed versions). The issue with separately illuminating the background is that it reflects a small amount of light on to the coin's edge, making small parts of the edge bright enough that it blends in with the white background. Retroreflectors barely worked better than a plain background, perhaps because they direct light *exactly* back to where it came from (i.e. the ring light) and not into the lens itself?

I've also tried illuminating the edges of the coin separately, by reflecting light from the ring light onto the edge using a 3D printed "cone" with 45 degree angle. This can work pretty well but it means I have to then use inner shadows in Photoshop to help darken the edge again, as I don't like the look of having bright edges against a dark background. Reflecting light onto the edge of the coin can also be difficult as some coins are much thicker than others so you need to adjust the height of the reflector else you can reflect light onto the relief of the coin and lose all the shadows and introduce unwanted highlights.

Any ideas on what I can do to help get a bright background while still having the coin properly exposed, and without reflecting light from the background back onto the coin's edges?

For context, this is a good example of the kind of edge illumination I like and also how thick and uneven edges on some ancient coins can be:
Image
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Here's some photos to provide more context.

Photo 1: coin against white background with no edge reflector
Photo 2: coin against white background with edge reflector
Photo 3: 3D printed ring light (still need to flock the inner "tube" but it's removeable and I haven't been using it for these photos)

Even with the edge reflector in Photo 2, it almost makes the issue worse as now the edge is more of a similar contrast to the background. Without the reflector, the edge is darker and easier to separate but it's a bit too dark for my liking in some areas and some of the very small areas (not visible in this single frame of the stack) are actually too bright and cause havoc with the selection tools in PS/Lightroom.
ex.jpg
ex2.jpg
ex3.jpg
- Cam

JayMcClellan
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Re: How to get bright white backgrounds?

Post by JayMcClellan »

I'll answer not as an expert in macro photography but as an expert in industrial machine vision systems. I would use independently-adjustable light sources for the foreground and background. In industrial applications we'd often use fiber-optic light panels but you could use a light table or simple diffuser with a strobe or continuous light source behind it. You can leave the background light turned off and adjust the foreground illumination for the best possible exposure of the coin, ideally with the background uniformly dim. Then you can bring up the background light to whatever brightness you want. Or conversely, if you have a non-adjustable background source (e.g. a simple light table) then you could start with just that and adjust the camera for best background exposure, then bring up the foreground illumination as desired. Maybe you've tried that already, but at any rate I think it will help to have more direct control of the balance between foreground and background.

I understand what you said about undesired reflections off the coin's edge, and you perhaps can't eliminate that entirely but I think you could reduce it by masking off the background so the bright area is no larger than absolutely required, i.e. just large enough to fill the field of view or even just a bit bigger than the coin if you don't mind filling in the rest of the background in Photoshop. I'm not sure you'd want to go this far but you could also consider trying collimated illumination behind the coin so that the light rays are parallel to the optical axis, or nearly so, which should substantially reduce the reflections off the edges.

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

JayMcClellan wrote:
Mon Nov 28, 2022 8:42 am
I'll answer not as an expert in macro photography but as an expert in industrial machine vision systems. I would use independently-adjustable light sources for the foreground and background. In industrial applications we'd often use fiber-optic light panels but you could use a light table or simple diffuser with a strobe or continuous light source behind it. You can leave the background light turned off and adjust the foreground illumination for the best possible exposure of the coin, ideally with the background uniformly dim. Then you can bring up the background light to whatever brightness you want. Or conversely, if you have a non-adjustable background source (e.g. a simple light table) then you could start with just that and adjust the camera for best background exposure, then bring up the foreground illumination as desired. Maybe you've tried that already, but at any rate I think it will help to have more direct control of the balance between foreground and background.

I understand what you said about undesired reflections off the coin's edge, and you perhaps can't eliminate that entirely but I think you could reduce it by masking off the background so the bright area is no larger than absolutely required, i.e. just large enough to fill the field of view or even just a bit bigger than the coin if you don't mind filling in the rest of the background in Photoshop. I'm not sure you'd want to go this far but you could also consider trying collimated illumination behind the coin so that the light rays are parallel to the optical axis, or nearly so, which should substantially reduce the reflections off the edges.
Thanks for replying, I think your experience in industrial machine vision is really exactly what I need here so I appreciate the thoughts!

I hadn't thought of collimation but that's an interesting idea. I guess a collimator lens might be out of the question as I have a centre "pedestal" that holds the coin off of the background and I'd need to either attach it to the lens or drill through it, neither of which sound ideal. I also can't support the coin from the side without a re-design of my coin-holding and diffuser setup (see pic below), so it wouldn't be the preferred solution but is an option.

Do you think a honeycomb grid would help align the light rays? Though in my experience it comes with a fairly significant drop in brightness. The retroreflector idea is perhaps similar to the collimator solution so may still have some promise but I may need something better than a bicycle reflector or a 3D printed part. A Fresnel lens or parabolic reflector could be options too, though for the Fresnel lens i'm not sure if there's a minimum distance between the light source and lens required and for the Parabolic reflector it seems the changing distance between light source (attached to the camera) and background/parabolic lens would be too great to get approximately parallel light rays reflecting back in all scenarios (i.e. at different focusing distances).

Though instead of relying on the ring flash to provide the light for the parabolic reflector, I could use a dedicated light source. According to this calculator, a parabolic lens with a diameter of 50mm and a height d of 12.5mm would need the light source to be placed 12.5mm away from the point V, which is potentially do-able. But that probably assumes the light source is coming from a specific point in the centre of the reflector, if I used something like a small LED COB, light would be coming from both the centre and away from the centre, resulting in stray light (which may not be a problem but would need testing to confirm).

Re: your comment about masking off the background - this does work but the trouble is the coins I shoot vary between 8mm to 40mm in diameter so it requires constantly changing the background to match. In some basic testing I have previously done with using a shroud around the coin, I found the shroud had to be quite close to the diameter of the coin to sufficiently reduce the stray reflections from the illuminated background hitting the coin's edge. So I wouldn't be able to have, say, just 3 sizes of "shrouds" to fit the range of my coins, it would probably be closer to 8 to 10 and there's the added issue of ancient coins not always being circular. To address the first part of this problem, I did 3D print an aperture/iris so that the masking off was adjustable and this did work reasonably well, just not for the non-circular coins.
ex4.jpg
- Cam

rjlittlefield
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Re: How to get bright white backgrounds?

Post by rjlittlefield »

Would it be feasible to shoot the coin twice, using two different lighting ratios for coin versus background, and then construct a computational mask from the difference?

--Rik

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

rjlittlefield wrote:
Mon Nov 28, 2022 10:20 am
Would it be feasible to shoot the coin twice, using two different lighting ratios for coin versus background, and then construct a computational mask from the difference?

--Rik
Interesting idea, do you imagine it would be necessary to repeat most of the stack for the second focus stack, or just enough to capture the edges in-focus? My worry would be that if I only did a truncated stack for the second part that the focus stacking software may align it different compared to the first stack. I guess you'd be the right person to answer that question :D

That idea is a little similar to something I just started doing recently to address the issue of bright spots on the edges blending with the backgrounds, which is to use an intersecting mask between the "background" and "object" in Lightroom as both of these masks tend to pick-up the troublesome areas of the coin's edges. I can then darken these parts selectively and export to Photoshop, where the selection tool there has an easier time at properly detecting the edges.

I should also probably mention what my expectations are here so it's clear what I'm aiming for. Generally I'm aiming for a one-click perfect selection of the background in Photoshop, which I can manage using the mirror background. I then typically smooth the selection edge and expand it by 3-5px into the coin's edge. When things go right, this kind of selection as shown below is generally what I can get around 99% of the edge after dragging the "quick selection" tool around the circumference. This one was shot with the mirror background, the pure white areas is where I had just done a rough background removal in Lightroom as I find it easier to do photo adjustments with a mostly-white background.
Attachments
edge selection.jpg
- Cam

blekenbleu
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Re: How to get bright white backgrounds?

Post by blekenbleu »

I would try substantially increasing distance from coin to background.
  • easier to independently illuminate fore and background to obtain "barely" white without excessive flare
  • background would be more out of focus and smoother
  • with a deep lens hood, rays from background would be more nearly parallel
Metaphot, Optiphot 1, 66; AO 10, 120, and EPIStar 2571
https://blekenbleu.github.io/microscope

JayMcClellan
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Re: How to get bright white backgrounds?

Post by JayMcClellan »

You could make a collimator with a cheap Fresnel lens drilled to hold a support pedestal. It needs a very small light source (e.g. a strobe covered in black tape with a small hole) placed right at its focal length away in order to get parallel rays. The focal length is easy to measure by focusing an image of the sun onto a non-flammable surface. If the physical distance is impractical in a vertical orientation, a 45-degree mirror under the lens would let you mount the light source off to the side - or you could put the whole collimator off to the side of the mirror and support the coin on clear acrylic for example. You can buy LED pinspot lights that are essentially collimated sources (within about 6 degrees) but I think they would be way too bright for your purpose and most aren't dimmable.

The suggestions from Rik and blekenbleu also sound like good ideas. Again from my industrial experience, I'd say first do everything you can to optimize the image quality before trying to improve it with image processing.

Lou Jost
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Re: How to get bright white backgrounds?

Post by Lou Jost »

This thread has produced some very clever suggestions! Now I wish I had this problem so I could try these solutions....

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Yeah some really great ideas already! I can give blekenbleu's suggestion a go first, though I can't use a lens hood as I have my custom ring flash attached to it and there's only so much I can raise the coin off of the platform before it becomes a bit impractical and the greater the height, the more chance of the coin falling on to the metal plates/tubes of my frame than onto the plastic platform below it.

I had done some tests earlier this year with LED ring COBs, about 30-40mm in diameter, and from that experience it did seem that I needed them at quite a high power to make a difference and at high power they get very hot very quickly and start melting PLA parts. Having an off-axis light setup would help there as JayMcClellan suggests. Taking a quick look online it seems you can find Fresnel lenses in the 40-70mm diameter range but the lowest focal length about 28mm, which is more than I would've liked but I think there could be room to fit one in the rotation stage.

I have a Newport M-481-A below the coin platform and it has a 25mm threaded hole in the centre that is about 30mm deep. I could put an LED cob inside there, spaced correctly with the help of the threaded locking rings that come with the stage, mount the coin platform above that (the white part in my earlier picture), and have the fresnel lens sit in the base of the coin platform with a hole drilled for the pedestal.

The only worry I would have then is whether the coin pedestal would get in the way too much. If I really need a pin-point light source and something like a 20mm diameter LED COB would be too wide for the Fresnel lens to work correctly, then I need to use a much smaller LED COB and put the pedestal on the other side of the Fresnel lens.
- Cam

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

Found some more Fresnel lenses that could be promising. I'm not familiar with data sheets for them so apologies if I get anything wrong. I've found a European supplier of the models in this datasheet (e.g. PL117440): https://download.khatod.com/documents/p ... 930576.pdf

They're all 50mm in diameter with 3.67mm thick glass. Looking at PL117440 as an example, it has an extended height of 3.5mm + L, where L = 6.15mm for this model. So I assume that means a focal distance only 9.65mm and on the first page it says the LED height can be a max of 6mm, which would place it only 3.65mm from the lens. I wonder if that's just the acceptable margin of error in placement of the light source from the lens and for optimal results you would still want to place it 9.65mm away.

Those are all very do-able numbers for my setup, the only remaining question then would be the tolerance of the width of the light source to still achieve (mostly) parallel light rays. These lenses seem to be cheap enough that it's worth experimenting with either way though.
- Cam

JayMcClellan
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Re: How to get bright white backgrounds?

Post by JayMcClellan »

I suggest getting a very cheap Fresnel (or convex) lens to experiment with. A plastic reading magnifier may be entirely adequate for your application because you're not trying to focus an image with it, and the rays don't need to be exactly parallel to greatly reduce the edge reflections. For the same reason your light source doesn't need to be a pinpoint, just "small". A magnifier may have a longer focal length than you'd prefer, meaning the light source must be farther away, but that also means you can get away with a bigger light source.

kaleun96
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Re: How to get bright white backgrounds?

Post by kaleun96 »

I've placed an order for some various Fresnel lenses from this European store: https://optomarket.net/

It seemed quite affordable, four lenses of 50mm in diameter and one holder for about 22 euro + shipping. Definitely in the price range that demands some messing about with, regardless of whether they end up working out for me or not.
- Cam

J_Rogers
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Re: How to get bright white backgrounds?

Post by J_Rogers »

Collimated background illumination wouldn't be all that difficult to implement with your setup. You would just need to collimate a beam larger than your coin and then feed it through a beam splitter from the side.

Although not the greatest picture - this should give you an idea. For your setup imagine it in the vertical orientation. Beam splitter is on the far left.
illumination-method.jpg

rjlittlefield
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Re: How to get bright white backgrounds?

Post by rjlittlefield »

kaleun96 wrote:
Mon Nov 28, 2022 10:35 am
rjlittlefield wrote:
Mon Nov 28, 2022 10:20 am
Would it be feasible to shoot the coin twice, using two different lighting ratios for coin versus background, and then construct a computational mask from the difference?
Interesting idea, do you imagine it would be necessary to repeat most of the stack for the second focus stack, or just enough to capture the edges in-focus? My worry would be that if I only did a truncated stack for the second part that the focus stacking software may align it different compared to the first stack. I guess you'd be the right person to answer that question :D
With default settings, Zerene Stacker establishes alignment from whichever end of the stack has the narrowest field. That can be either the foreground frame or the background frame, depending on what optics and focusing method you're using. If you're stepping focus "by rail", changing just the lens-to-subject distance, then typically alignment will start with the background frame. In that case you could shoot just the back portion of the stack, including enough depth to catch the edge of the coin in focus. But if your optics and focusing method causes Zerene Stacker to start at the front, then you would have to shoot the whole stack again.

There are also combinations of settings that will make Zerene Stacker always start at the back, even if by default it would start at the front. If the field is full of detail then that approach will usually produce degraded edges that have to cropped off. But with a central subject that is isolated on a bright background, I'm guessing there would be no problem in forcing alignment to start at the back.

BTW, I agree with Jay that the first line of attack should be to optimize the illumination. Computational cleanup is what we do if the other stuff doesn't work.

--Rik

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